| Message ID | 20150325110814.GE21522@potion.brq.redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
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2015-03-25 12:08+0100, Radim Kr?má?: > Reverting the patch protects us from any migration, but I don't think we > need to care about changing VCPUs as long as we read a consistent data > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > matter if we return a value not fit for this VCPU.) > > I think we could drop the second __getcpu if our kvmclock was being > handled better; maybe with a patch like the one below: The second __getcpu is not neccessary, but I forgot about rdtsc. We need to either use rtdscp, know the host has synchronized tsc, or monitor VCPU migrations. Only the last one works everywhere. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: > 2015-03-25 12:08+0100, Radim Kr?má?: > > Reverting the patch protects us from any migration, but I don't think we > > need to care about changing VCPUs as long as we read a consistent data > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > > matter if we return a value not fit for this VCPU.) > > > > I think we could drop the second __getcpu if our kvmclock was being > > handled better; maybe with a patch like the one below: > > The second __getcpu is not neccessary, but I forgot about rdtsc. > We need to either use rtdscp, know the host has synchronized tsc, or > monitor VCPU migrations. Only the last one works everywhere. The vdso code is only used if host has synchronized tsc. But you have to handle the case where host goes from synchronized tsc to unsynchronized tsc (see the clocksource notifier in the host side). -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: > > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: > > 2015-03-25 12:08+0100, Radim Kr?má?: > > > Reverting the patch protects us from any migration, but I don't think we > > > need to care about changing VCPUs as long as we read a consistent data > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > > > matter if we return a value not fit for this VCPU.) > > > > > > I think we could drop the second __getcpu if our kvmclock was being > > > handled better; maybe with a patch like the one below: > > > > The second __getcpu is not neccessary, but I forgot about rdtsc. > > We need to either use rtdscp, know the host has synchronized tsc, or > > monitor VCPU migrations. Only the last one works everywhere. > > The vdso code is only used if host has synchronized tsc. > > But you have to handle the case where host goes from synchronized tsc to > unsynchronized tsc (see the clocksource notifier in the host side). > Can't we change the host to freeze all vcpus and clear the stable bit on all of them if this happens? This would simplify and speed up vclock_gettime. --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: > On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: > > > > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: > > > 2015-03-25 12:08+0100, Radim Kr?má?: > > > > Reverting the patch protects us from any migration, but I don't think we > > > > need to care about changing VCPUs as long as we read a consistent data > > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > > > > matter if we return a value not fit for this VCPU.) > > > > > > > > I think we could drop the second __getcpu if our kvmclock was being > > > > handled better; maybe with a patch like the one below: > > > > > > The second __getcpu is not neccessary, but I forgot about rdtsc. > > > We need to either use rtdscp, know the host has synchronized tsc, or > > > monitor VCPU migrations. Only the last one works everywhere. > > > > The vdso code is only used if host has synchronized tsc. > > > > But you have to handle the case where host goes from synchronized tsc to > > unsynchronized tsc (see the clocksource notifier in the host side). > > > > Can't we change the host to freeze all vcpus and clear the stable bit > on all of them if this happens? This would simplify and speed up > vclock_gettime. > > --Andy Seems interesting to do on 512-vcpus, but sure, could be done. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Mar 25, 2015 at 3:41 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: >> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: >> > >> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: >> > > 2015-03-25 12:08+0100, Radim Kr?má?: >> > > > Reverting the patch protects us from any migration, but I don't think we >> > > > need to care about changing VCPUs as long as we read a consistent data >> > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't >> > > > matter if we return a value not fit for this VCPU.) >> > > > >> > > > I think we could drop the second __getcpu if our kvmclock was being >> > > > handled better; maybe with a patch like the one below: >> > > >> > > The second __getcpu is not neccessary, but I forgot about rdtsc. >> > > We need to either use rtdscp, know the host has synchronized tsc, or >> > > monitor VCPU migrations. Only the last one works everywhere. >> > >> > The vdso code is only used if host has synchronized tsc. >> > >> > But you have to handle the case where host goes from synchronized tsc to >> > unsynchronized tsc (see the clocksource notifier in the host side). >> > >> >> Can't we change the host to freeze all vcpus and clear the stable bit >> on all of them if this happens? This would simplify and speed up >> vclock_gettime. >> >> --Andy > > Seems interesting to do on 512-vcpus, but sure, could be done. > If you have a 512-vcpu system that switches between stable and unstable more than once per migration, then I expect that you have serious problems and this is the least of your worries. Personally, I'd *much* rather we just made vcpu 0's pvti authoritative if we're stable. If nothing else, I'm not even remotely convinced that the current scheme gives monotonic timing due to skew between when the updates happen on different vcpus. --Andy
On Wed, Mar 25, 2015 at 03:48:02PM -0700, Andy Lutomirski wrote: > On Wed, Mar 25, 2015 at 3:41 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > > On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: > >> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: > >> > > >> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: > >> > > 2015-03-25 12:08+0100, Radim Kr?má?: > >> > > > Reverting the patch protects us from any migration, but I don't think we > >> > > > need to care about changing VCPUs as long as we read a consistent data > >> > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > >> > > > matter if we return a value not fit for this VCPU.) > >> > > > > >> > > > I think we could drop the second __getcpu if our kvmclock was being > >> > > > handled better; maybe with a patch like the one below: > >> > > > >> > > The second __getcpu is not neccessary, but I forgot about rdtsc. > >> > > We need to either use rtdscp, know the host has synchronized tsc, or > >> > > monitor VCPU migrations. Only the last one works everywhere. > >> > > >> > The vdso code is only used if host has synchronized tsc. > >> > > >> > But you have to handle the case where host goes from synchronized tsc to > >> > unsynchronized tsc (see the clocksource notifier in the host side). > >> > > >> > >> Can't we change the host to freeze all vcpus and clear the stable bit > >> on all of them if this happens? This would simplify and speed up > >> vclock_gettime. > >> > >> --Andy > > > > Seems interesting to do on 512-vcpus, but sure, could be done. > > > > If you have a 512-vcpu system that switches between stable and > unstable more than once per migration, then I expect that you have > serious problems and this is the least of your worries. > > Personally, I'd *much* rather we just made vcpu 0's pvti authoritative > if we're stable. If nothing else, I'm not even remotely convinced > that the current scheme gives monotonic timing due to skew between > when the updates happen on different vcpus. Can you write down the problem ? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Mar 25, 2015 at 4:13 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > On Wed, Mar 25, 2015 at 03:48:02PM -0700, Andy Lutomirski wrote: >> On Wed, Mar 25, 2015 at 3:41 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> > On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: >> >> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: >> >> > >> >> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: >> >> > > 2015-03-25 12:08+0100, Radim Kr?má?: >> >> > > > Reverting the patch protects us from any migration, but I don't think we >> >> > > > need to care about changing VCPUs as long as we read a consistent data >> >> > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't >> >> > > > matter if we return a value not fit for this VCPU.) >> >> > > > >> >> > > > I think we could drop the second __getcpu if our kvmclock was being >> >> > > > handled better; maybe with a patch like the one below: >> >> > > >> >> > > The second __getcpu is not neccessary, but I forgot about rdtsc. >> >> > > We need to either use rtdscp, know the host has synchronized tsc, or >> >> > > monitor VCPU migrations. Only the last one works everywhere. >> >> > >> >> > The vdso code is only used if host has synchronized tsc. >> >> > >> >> > But you have to handle the case where host goes from synchronized tsc to >> >> > unsynchronized tsc (see the clocksource notifier in the host side). >> >> > >> >> >> >> Can't we change the host to freeze all vcpus and clear the stable bit >> >> on all of them if this happens? This would simplify and speed up >> >> vclock_gettime. >> >> >> >> --Andy >> > >> > Seems interesting to do on 512-vcpus, but sure, could be done. >> > >> >> If you have a 512-vcpu system that switches between stable and >> unstable more than once per migration, then I expect that you have >> serious problems and this is the least of your worries. >> >> Personally, I'd *much* rather we just made vcpu 0's pvti authoritative >> if we're stable. If nothing else, I'm not even remotely convinced >> that the current scheme gives monotonic timing due to skew between >> when the updates happen on different vcpus. > > Can you write down the problem ? > I can try. Suppose we start out with all vcpus agreeing on their pvti and perfect invariant TSCs. Now the host updates its frequency (due to NTP or whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. They'll disagree on the time, and one of them will be ahead until vcpu 1's pvti gets updated. --Andy
On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: > On Wed, Mar 25, 2015 at 4:13 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > > On Wed, Mar 25, 2015 at 03:48:02PM -0700, Andy Lutomirski wrote: > >> On Wed, Mar 25, 2015 at 3:41 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > >> > On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: > >> >> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: > >> >> > > >> >> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: > >> >> > > 2015-03-25 12:08+0100, Radim Kr?má?: > >> >> > > > Reverting the patch protects us from any migration, but I don't think we > >> >> > > > need to care about changing VCPUs as long as we read a consistent data > >> >> > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > >> >> > > > matter if we return a value not fit for this VCPU.) > >> >> > > > > >> >> > > > I think we could drop the second __getcpu if our kvmclock was being > >> >> > > > handled better; maybe with a patch like the one below: > >> >> > > > >> >> > > The second __getcpu is not neccessary, but I forgot about rdtsc. > >> >> > > We need to either use rtdscp, know the host has synchronized tsc, or > >> >> > > monitor VCPU migrations. Only the last one works everywhere. > >> >> > > >> >> > The vdso code is only used if host has synchronized tsc. > >> >> > > >> >> > But you have to handle the case where host goes from synchronized tsc to > >> >> > unsynchronized tsc (see the clocksource notifier in the host side). > >> >> > > >> >> > >> >> Can't we change the host to freeze all vcpus and clear the stable bit > >> >> on all of them if this happens? This would simplify and speed up > >> >> vclock_gettime. > >> >> > >> >> --Andy > >> > > >> > Seems interesting to do on 512-vcpus, but sure, could be done. > >> > > >> > >> If you have a 512-vcpu system that switches between stable and > >> unstable more than once per migration, then I expect that you have > >> serious problems and this is the least of your worries. > >> > >> Personally, I'd *much* rather we just made vcpu 0's pvti authoritative > >> if we're stable. If nothing else, I'm not even remotely convinced > >> that the current scheme gives monotonic timing due to skew between > >> when the updates happen on different vcpus. > > > > Can you write down the problem ? > > > > I can try. > > Suppose we start out with all vcpus agreeing on their pvti and perfect > invariant TSCs. Now the host updates its frequency (due to NTP or > whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's > pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. > They'll disagree on the time, and one of them will be ahead until vcpu > 1's pvti gets updated. The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs to be visible at one time, for all vcpus. * That is, when timespec0 != timespec1, M < N. Unfortunately that is * not * always the case (the difference between two distinct xtime instances * might be smaller then the difference between corresponding TSC reads, * when updating guest vcpus pvclock areas). * * To avoid that problem, do not allow visibility of distinct * system_timestamp/tsc_timestamp values simultaneously: use a master * copy of host monotonic time values. Update that master copy * in lockstep. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Mar 25, 2015 at 4:08 AM, Radim Kr?má? <rkrcmar@redhat.com> wrote: > 2015-03-24 15:33-0700, Andy Lutomirski: >> On Tue, Mar 24, 2015 at 8:34 AM, Radim Kr?má? <rkrcmar@redhat.com> wrote: >> > What is the problem? >> >> The kvmclock spec says that the host will increment a version field to >> an odd number, then update stuff, then increment it to an even number. >> The host is buggy and doesn't do this, and the result is observable >> when one vcpu reads another vcpu's kvmclock data. >> >> Since there's no good way for a guest kernel to keep its vdso from >> reading a different vcpu's kvmclock data, this is a real corner-case >> bug. This patch allows the vdso to retry when this happens. I don't >> think it's a great solution, but it should mostly work. > > Great explanation, thank you. > > Reverting the patch protects us from any migration, but I don't think we > need to care about changing VCPUs as long as we read a consistent data > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > matter if we return a value not fit for this VCPU.) > > I think we could drop the second __getcpu if our kvmclock was being > handled better; maybe with a patch like the one below: > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index cc2c759f69a3..8658599e0024 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -1658,12 +1658,24 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) > &guest_hv_clock, sizeof(guest_hv_clock)))) > return 0; > > - /* > - * The interface expects us to write an even number signaling that the > - * update is finished. Since the guest won't see the intermediate > - * state, we just increase by 2 at the end. > + /* A guest can read other VCPU's kvmclock; specification says that > + * version is odd if data is being modified and even after it is > + * consistent. > + * We write three times to be sure. > + * 1) update version to odd number > + * 2) write modified data (version is still odd) > + * 3) update version to even number > + * > + * TODO: optimize > + * - only two writes should be enough -- version is first > + * - the second write could update just version > */ > - vcpu->hv_clock.version = guest_hv_clock.version + 2; > + guest_hv_clock.version += 1; > + kvm_write_guest_cached(v->kvm, &vcpu->pv_time, > + &guest_hv_clock, > + sizeof(guest_hv_clock)); > + > + vcpu->hv_clock.version = guest_hv_clock.version; > > /* retain PVCLOCK_GUEST_STOPPED if set in guest copy */ > pvclock_flags = (guest_hv_clock.flags & PVCLOCK_GUEST_STOPPED); > @@ -1684,6 +1696,11 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) > kvm_write_guest_cached(v->kvm, &vcpu->pv_time, > &vcpu->hv_clock, > sizeof(vcpu->hv_clock)); > + > + vcpu->hv_clock.version += 1; > + kvm_write_guest_cached(v->kvm, &vcpu->pv_time, > + &vcpu->hv_clock, > + sizeof(vcpu->hv_clock)); > return 0; > } > The trouble with this is that kvm_write_guest_cached seems to correspond roughly to a "rep movs" variant, and those are weakly ordered. As a result, I don't really know whether they have well-defined semantics wrt concurrent reads. What we really want is just "mov". --Andy
On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: >> On Wed, Mar 25, 2015 at 4:13 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> > On Wed, Mar 25, 2015 at 03:48:02PM -0700, Andy Lutomirski wrote: >> >> On Wed, Mar 25, 2015 at 3:41 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> >> > On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: >> >> >> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote: >> >> >> > >> >> >> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Kr?má? wrote: >> >> >> > > 2015-03-25 12:08+0100, Radim Kr?má?: >> >> >> > > > Reverting the patch protects us from any migration, but I don't think we >> >> >> > > > need to care about changing VCPUs as long as we read a consistent data >> >> >> > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't >> >> >> > > > matter if we return a value not fit for this VCPU.) >> >> >> > > > >> >> >> > > > I think we could drop the second __getcpu if our kvmclock was being >> >> >> > > > handled better; maybe with a patch like the one below: >> >> >> > > >> >> >> > > The second __getcpu is not neccessary, but I forgot about rdtsc. >> >> >> > > We need to either use rtdscp, know the host has synchronized tsc, or >> >> >> > > monitor VCPU migrations. Only the last one works everywhere. >> >> >> > >> >> >> > The vdso code is only used if host has synchronized tsc. >> >> >> > >> >> >> > But you have to handle the case where host goes from synchronized tsc to >> >> >> > unsynchronized tsc (see the clocksource notifier in the host side). >> >> >> > >> >> >> >> >> >> Can't we change the host to freeze all vcpus and clear the stable bit >> >> >> on all of them if this happens? This would simplify and speed up >> >> >> vclock_gettime. >> >> >> >> >> >> --Andy >> >> > >> >> > Seems interesting to do on 512-vcpus, but sure, could be done. >> >> > >> >> >> >> If you have a 512-vcpu system that switches between stable and >> >> unstable more than once per migration, then I expect that you have >> >> serious problems and this is the least of your worries. >> >> >> >> Personally, I'd *much* rather we just made vcpu 0's pvti authoritative >> >> if we're stable. If nothing else, I'm not even remotely convinced >> >> that the current scheme gives monotonic timing due to skew between >> >> when the updates happen on different vcpus. >> > >> > Can you write down the problem ? >> > >> >> I can try. >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect >> invariant TSCs. Now the host updates its frequency (due to NTP or >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. >> They'll disagree on the time, and one of them will be ahead until vcpu >> 1's pvti gets updated. > > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs > to be visible at one time, for all vcpus. > > > * That is, when timespec0 != timespec1, M < N. Unfortunately that is > * not > * always the case (the difference between two distinct xtime instances > * might be smaller then the difference between corresponding TSC reads, > * when updating guest vcpus pvclock areas). > * > * To avoid that problem, do not allow visibility of distinct > * system_timestamp/tsc_timestamp values simultaneously: use a master > * copy of host monotonic time values. Update that master copy > * in lockstep. > > [resend without HTML] Yuck. So we have per cpu timing data, but the protocol is only usable for monotonic timing because we forcibly freeze all vcpus when we update the nominally per cpu data. The obvious guest implementations are still unnecessarily slow, though. It would be nice if the guest could get away without using any getcpu operation at all. Even if we fixed the host to increment version as advertised, I think we can't avoid two getcpu ops. We need one before rdtsc to figure out which pvti to look at, and we need another to make sure that we were actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help -- we need to check version before rdtsc, and we don't know what version to check until we do a getcpu.). The migration hook has the same issue -- we need to check the migration count, then confirm we're on that cpu, then check the migration count again, and we can't do that until we know what cpu we're on. If, on the other hand, we could rely on having all of these things in sync, then this complication goes away, and we go down from two getcpu ops to zero. --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
2015-03-26 11:47-0700, Andy Lutomirski: > On Wed, Mar 25, 2015 at 4:08 AM, Radim Kr?má? <rkrcmar@redhat.com> wrote: > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > + /* A guest can read other VCPU's kvmclock; specification says that > > + * version is odd if data is being modified and even after it is > > + * consistent. > > + * We write three times to be sure. > > + * 1) update version to odd number > > + * 2) write modified data (version is still odd) > > + * 3) update version to even number > > + * > > + * TODO: optimize > > + * - only two writes should be enough -- version is first > > + * - the second write could update just version > > */ > > The trouble with this is that kvm_write_guest_cached seems to > correspond roughly to a "rep movs" variant, and those are weakly > ordered. As a result, I don't really know whether they have > well-defined semantics wrt concurrent reads. What we really want is > just "mov". Ah, so the first optimization TODO is not possible, but stores are weakly ordered only within one rep movs. We're safe if compiler outputs three mov-like instructions. (Btw. does current hardware reorder string stores?) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
2015-03-26 11:51-0700, Andy Lutomirski: > On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: > >> Suppose we start out with all vcpus agreeing on their pvti and perfect > >> invariant TSCs. Now the host updates its frequency (due to NTP or > >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's > >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. > >> They'll disagree on the time, and one of them will be ahead until vcpu > >> 1's pvti gets updated. > > > > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs > > to be visible at one time, for all vcpus. > > > > > > * That is, when timespec0 != timespec1, M < N. Unfortunately that is > > * not > > * always the case (the difference between two distinct xtime instances > > * might be smaller then the difference between corresponding TSC reads, > > * when updating guest vcpus pvclock areas). > > * > > * To avoid that problem, do not allow visibility of distinct > > * system_timestamp/tsc_timestamp values simultaneously: use a master > > * copy of host monotonic time values. Update that master copy > > * in lockstep. > > Yuck. So we have per cpu timing data, but the protocol is only usable > for monotonic timing because we forcibly freeze all vcpus when we > update the nominally per cpu data. > > The obvious guest implementations are still unnecessarily slow, > though. It would be nice if the guest could get away without using > any getcpu operation at all. > > Even if we fixed the host to increment version as advertised, I think > we can't avoid two getcpu ops. We need one before rdtsc to figure out > which pvti to look at, Yes. > and we need another to make sure that we were > actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help > -- we need to check version before rdtsc, and we don't know what > version to check until we do a getcpu.). Exactly, reading cpuid after rdtsc doesn't do that though, we could have migrated back between those reads. rtdscp would allow us to check that we read tsc of pvti's cpu. (It doesn't get rid of that first read.) > The migration hook has the > same issue -- we need to check the migration count, then confirm we're > on that cpu, then check the migration count again, and we can't do > that until we know what cpu we're on. True; the revert has a bug -- we need to check cpuid for the second time before rdtsc. (Migration hook is there just because we don't know which cpu executed rdtsc.) > If, on the other hand, we could rely on having all of these things in > sync, then this complication goes away, and we go down from two getcpu > ops to zero. (Yeah, we should look what are the drawbacks of doing it differently.) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 26/03/2015 21:10, Radim Kr?má? wrote: > 2015-03-26 11:47-0700, Andy Lutomirski: >> On Wed, Mar 25, 2015 at 4:08 AM, Radim Kr?má? <rkrcmar@redhat.com> wrote: >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> + /* A guest can read other VCPU's kvmclock; specification says that >>> + * version is odd if data is being modified and even after it is >>> + * consistent. >>> + * We write three times to be sure. >>> + * 1) update version to odd number >>> + * 2) write modified data (version is still odd) >>> + * 3) update version to even number >>> + * >>> + * TODO: optimize >>> + * - only two writes should be enough -- version is first >>> + * - the second write could update just version >>> */ >> >> The trouble with this is that kvm_write_guest_cached seems to >> correspond roughly to a "rep movs" variant, and those are weakly >> ordered. As a result, I don't really know whether they have >> well-defined semantics wrt concurrent reads. What we really want is >> just "mov". > > Ah, so the first optimization TODO is not possible, but stores are > weakly ordered only within one rep movs. We're safe if compiler > outputs three mov-like instructions. > > (Btw. does current hardware reorder string stores?) It probably does so if they hit multiple cache lines. Within a cache line, probably not. We can add kvm_map/unmap_guest_cached and then use __put_user. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@redhat.com> wrote: > 2015-03-26 11:51-0700, Andy Lutomirski: >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect >> >> invariant TSCs. Now the host updates its frequency (due to NTP or >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. >> >> They'll disagree on the time, and one of them will be ahead until vcpu >> >> 1's pvti gets updated. >> > >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs >> > to be visible at one time, for all vcpus. >> > >> > >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is >> > * not >> > * always the case (the difference between two distinct xtime instances >> > * might be smaller then the difference between corresponding TSC reads, >> > * when updating guest vcpus pvclock areas). >> > * >> > * To avoid that problem, do not allow visibility of distinct >> > * system_timestamp/tsc_timestamp values simultaneously: use a master >> > * copy of host monotonic time values. Update that master copy >> > * in lockstep. >> >> Yuck. So we have per cpu timing data, but the protocol is only usable >> for monotonic timing because we forcibly freeze all vcpus when we >> update the nominally per cpu data. >> >> The obvious guest implementations are still unnecessarily slow, >> though. It would be nice if the guest could get away without using >> any getcpu operation at all. >> >> Even if we fixed the host to increment version as advertised, I think >> we can't avoid two getcpu ops. We need one before rdtsc to figure out >> which pvti to look at, > > Yes. > >> and we need another to make sure that we were >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help >> -- we need to check version before rdtsc, and we don't know what >> version to check until we do a getcpu.). > > Exactly, reading cpuid after rdtsc doesn't do that though, we could have > migrated back between those reads. > rtdscp would allow us to check that we read tsc of pvti's cpu. > (It doesn't get rid of that first read.) > >> The migration hook has the >> same issue -- we need to check the migration count, then confirm we're >> on that cpu, then check the migration count again, and we can't do >> that until we know what cpu we're on. > > True; the revert has a bug -- we need to check cpuid for the second > time before rdtsc. (Migration hook is there just because we don't know > which cpu executed rdtsc.) One way or another, I'm planning on completely rewriting the vdso code. An early draft is here: https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 but I can't finish it until the KVM side shakes out. I think there are at least two ways that would work: a) If KVM incremented version as advertised: cpu = getcpu(); pvti = pvti for cpu; ver1 = pvti->version; check stable bit; rdtsc_barrier, rdtsc, read scale, shift, etc. if (getcpu() != cpu) retry; if (pvti->version != ver1) retry; I think this is safe because, we're guaranteed that there was an interval (between the two version reads) in which the vcpu we think we're on was running and the kvmclock data was valid and marked stable, and we know that the tsc we read came from that interval. Note: rdtscp isn't needed. If we're stable, is makes no difference which cpu's tsc we actually read. b) If version remains buggy but we use this migrations_from hack: cpu = getcpu(); pvti = pvti for cpu; m1 = pvti->migrations_from; barrier(); ver1 = pvti->version; check stable bit; rdtsc_barrier, rdtsc, read scale, shift, etc. if (getcpu() != cpu) retry; if (pvti->version != ver1) retry; /* probably not really needed */ barrier(); if (pvti->migrations_from != m1) retry; This is just like (a), except that we're using a guest kernel hack to ensure that no one migrated off the vcpu during the version-protected critical section and that we were, in fact, on that vcpu at some point during that critical section. Once we've ensured that we were on pvti's associated vcpu for the entire time we were reading it, then we are protected by the existing versioning in the host. > >> If, on the other hand, we could rely on having all of these things in >> sync, then this complication goes away, and we go down from two getcpu >> ops to zero. > > (Yeah, we should look what are the drawbacks of doing it differently.) If the versioning were fixed, I think we could almost get away with: pvti = pvti for vcpu 0; ver1 = pvti->version; check stable bit; rdtsc_barrier, rdtsc, read scale, shift, etc. if (pvti->version != ver1) retry; This guarantees that the tsc came from an interval in which vcpu0's kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely stable in that interval, then we'd be fine, but there's a race window in which the kvmclock is *not* stable and vcpu 0 wasn't running. Why doesn't KVM just update all of the kvmclock data at once? (For that matter, why is the pvti in guest memory at all? Wouldn't this all be simpler if the kvmclock data were host-allocated so the host could write it directly and maybe even share it between guests?) --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
[much snippage] On Thu, Mar 26, 2015 at 1:58 PM, Andy Lutomirski <luto@amacapital.net> wrote: > > If the versioning were fixed, I think we could almost get away with: > > pvti = pvti for vcpu 0; > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (pvti->version != ver1) retry; > > This guarantees that the tsc came from an interval in which vcpu0's > kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely > stable in that interval, then we'd be fine, but there's a race window > in which the kvmclock is *not* stable and vcpu 0 wasn't running. Rik pointed out that this could actually be okay. Apparently vcpu 0 is somewhat special, and it may actually be impossible to switch from stable to unstable which a vcpu other than 0 is running and vcpu0 hasn't updated its kvmclock data. --Andy > > Why doesn't KVM just update all of the kvmclock data at once? (For > that matter, why is the pvti in guest memory at all? Wouldn't this > all be simpler if the kvmclock data were host-allocated so the host > could write it directly and maybe even share it between guests?) > > --Andy
On Thu, Mar 26, 2015 at 01:58:25PM -0700, Andy Lutomirski wrote: > On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@redhat.com> wrote: > > 2015-03-26 11:51-0700, Andy Lutomirski: > >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: > >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect > >> >> invariant TSCs. Now the host updates its frequency (due to NTP or > >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's > >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. > >> >> They'll disagree on the time, and one of them will be ahead until vcpu > >> >> 1's pvti gets updated. > >> > > >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs > >> > to be visible at one time, for all vcpus. > >> > > >> > > >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is > >> > * not > >> > * always the case (the difference between two distinct xtime instances > >> > * might be smaller then the difference between corresponding TSC reads, > >> > * when updating guest vcpus pvclock areas). > >> > * > >> > * To avoid that problem, do not allow visibility of distinct > >> > * system_timestamp/tsc_timestamp values simultaneously: use a master > >> > * copy of host monotonic time values. Update that master copy > >> > * in lockstep. > >> > >> Yuck. So we have per cpu timing data, but the protocol is only usable > >> for monotonic timing because we forcibly freeze all vcpus when we > >> update the nominally per cpu data. > >> > >> The obvious guest implementations are still unnecessarily slow, > >> though. It would be nice if the guest could get away without using > >> any getcpu operation at all. > >> > >> Even if we fixed the host to increment version as advertised, I think > >> we can't avoid two getcpu ops. We need one before rdtsc to figure out > >> which pvti to look at, > > > > Yes. > > > >> and we need another to make sure that we were > >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help > >> -- we need to check version before rdtsc, and we don't know what > >> version to check until we do a getcpu.). > > > > Exactly, reading cpuid after rdtsc doesn't do that though, we could have > > migrated back between those reads. > > rtdscp would allow us to check that we read tsc of pvti's cpu. > > (It doesn't get rid of that first read.) > > > >> The migration hook has the > >> same issue -- we need to check the migration count, then confirm we're > >> on that cpu, then check the migration count again, and we can't do > >> that until we know what cpu we're on. > > > > True; the revert has a bug -- we need to check cpuid for the second > > time before rdtsc. (Migration hook is there just because we don't know > > which cpu executed rdtsc.) > > One way or another, I'm planning on completely rewriting the vdso > code. An early draft is here: > > https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 > > but I can't finish it until the KVM side shakes out. > > I think there are at least two ways that would work: > > a) If KVM incremented version as advertised: All for it. > cpu = getcpu(); > pvti = pvti for cpu; > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (getcpu() != cpu) retry; > if (pvti->version != ver1) retry; > > I think this is safe because, we're guaranteed that there was an > interval (between the two version reads) in which the vcpu we think > we're on was running and the kvmclock data was valid and marked > stable, and we know that the tsc we read came from that interval. > > Note: rdtscp isn't needed. If we're stable, is makes no difference > which cpu's tsc we actually read. Yes, can't see a problem with that. > b) If version remains buggy but we use this migrations_from hack: There is no reason for version to remain buggy. > cpu = getcpu(); > pvti = pvti for cpu; > m1 = pvti->migrations_from; > barrier(); > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (getcpu() != cpu) retry; > if (pvti->version != ver1) retry; /* probably not really needed */ > > barrier(); > if (pvti->migrations_from != m1) retry; > > This is just like (a), except that we're using a guest kernel hack to > ensure that no one migrated off the vcpu during the version-protected > critical section and that we were, in fact, on that vcpu at some point > during that critical section. Once we've ensured that we were on > pvti's associated vcpu for the entire time we were reading it, then we > are protected by the existing versioning in the host. > > > > >> If, on the other hand, we could rely on having all of these things in > >> sync, then this complication goes away, and we go down from two getcpu > >> ops to zero. > > > > (Yeah, we should look what are the drawbacks of doing it differently.) > > If the versioning were fixed, I think we could almost get away with: > > pvti = pvti for vcpu 0; > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (pvti->version != ver1) retry; > > This guarantees that the tsc came from an interval in which vcpu0's > kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely > stable in that interval, then we'd be fine, but there's a race window > in which the kvmclock is *not* stable and vcpu 0 wasn't running. What is that window again ? Have no objections against using vcpu0's pvti (cacheline should be read-only 99.9% of time). > Why doesn't KVM just update all of the kvmclock data at once? Because it has not been necessary -- updating kvmclock data on vcpu entry was the previous method, so that was reused. > (For > that matter, why is the pvti in guest memory at all? Wouldn't this > all be simpler if the kvmclock data were host-allocated so the host > could write it directly and maybe even share it between guests?) And use a 4K TLB entry for that kvmclock area rather than sharing one of kernel's 2MB (or 1GB) TLB entry? > > --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Mar 26, 2015 at 3:56 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > On Thu, Mar 26, 2015 at 01:58:25PM -0700, Andy Lutomirski wrote: >> On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@redhat.com> wrote: >> > 2015-03-26 11:51-0700, Andy Lutomirski: >> >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: >> >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect >> >> >> invariant TSCs. Now the host updates its frequency (due to NTP or >> >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's >> >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. >> >> >> They'll disagree on the time, and one of them will be ahead until vcpu >> >> >> 1's pvti gets updated. >> >> > >> >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs >> >> > to be visible at one time, for all vcpus. >> >> > >> >> > >> >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is >> >> > * not >> >> > * always the case (the difference between two distinct xtime instances >> >> > * might be smaller then the difference between corresponding TSC reads, >> >> > * when updating guest vcpus pvclock areas). >> >> > * >> >> > * To avoid that problem, do not allow visibility of distinct >> >> > * system_timestamp/tsc_timestamp values simultaneously: use a master >> >> > * copy of host monotonic time values. Update that master copy >> >> > * in lockstep. >> >> >> >> Yuck. So we have per cpu timing data, but the protocol is only usable >> >> for monotonic timing because we forcibly freeze all vcpus when we >> >> update the nominally per cpu data. >> >> >> >> The obvious guest implementations are still unnecessarily slow, >> >> though. It would be nice if the guest could get away without using >> >> any getcpu operation at all. >> >> >> >> Even if we fixed the host to increment version as advertised, I think >> >> we can't avoid two getcpu ops. We need one before rdtsc to figure out >> >> which pvti to look at, >> > >> > Yes. >> > >> >> and we need another to make sure that we were >> >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help >> >> -- we need to check version before rdtsc, and we don't know what >> >> version to check until we do a getcpu.). >> > >> > Exactly, reading cpuid after rdtsc doesn't do that though, we could have >> > migrated back between those reads. >> > rtdscp would allow us to check that we read tsc of pvti's cpu. >> > (It doesn't get rid of that first read.) >> > >> >> The migration hook has the >> >> same issue -- we need to check the migration count, then confirm we're >> >> on that cpu, then check the migration count again, and we can't do >> >> that until we know what cpu we're on. >> > >> > True; the revert has a bug -- we need to check cpuid for the second >> > time before rdtsc. (Migration hook is there just because we don't know >> > which cpu executed rdtsc.) >> >> One way or another, I'm planning on completely rewriting the vdso >> code. An early draft is here: >> >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 >> >> but I can't finish it until the KVM side shakes out. >> >> I think there are at least two ways that would work: >> >> a) If KVM incremented version as advertised: > > All for it. > >> cpu = getcpu(); >> pvti = pvti for cpu; >> >> ver1 = pvti->version; >> check stable bit; >> rdtsc_barrier, rdtsc, read scale, shift, etc. >> if (getcpu() != cpu) retry; >> if (pvti->version != ver1) retry; >> >> I think this is safe because, we're guaranteed that there was an >> interval (between the two version reads) in which the vcpu we think >> we're on was running and the kvmclock data was valid and marked >> stable, and we know that the tsc we read came from that interval. >> >> Note: rdtscp isn't needed. If we're stable, is makes no difference >> which cpu's tsc we actually read. > > Yes, can't see a problem with that. > >> b) If version remains buggy but we use this migrations_from hack: > > There is no reason for version to remain buggy. > >> cpu = getcpu(); >> pvti = pvti for cpu; >> m1 = pvti->migrations_from; >> barrier(); >> >> ver1 = pvti->version; >> check stable bit; >> rdtsc_barrier, rdtsc, read scale, shift, etc. >> if (getcpu() != cpu) retry; >> if (pvti->version != ver1) retry; /* probably not really needed */ >> >> barrier(); >> if (pvti->migrations_from != m1) retry; >> >> This is just like (a), except that we're using a guest kernel hack to >> ensure that no one migrated off the vcpu during the version-protected >> critical section and that we were, in fact, on that vcpu at some point >> during that critical section. Once we've ensured that we were on >> pvti's associated vcpu for the entire time we were reading it, then we >> are protected by the existing versioning in the host. >> >> > >> >> If, on the other hand, we could rely on having all of these things in >> >> sync, then this complication goes away, and we go down from two getcpu >> >> ops to zero. >> > >> > (Yeah, we should look what are the drawbacks of doing it differently.) >> >> If the versioning were fixed, I think we could almost get away with: >> >> pvti = pvti for vcpu 0; >> >> ver1 = pvti->version; >> check stable bit; >> rdtsc_barrier, rdtsc, read scale, shift, etc. >> if (pvti->version != ver1) retry; >> >> This guarantees that the tsc came from an interval in which vcpu0's >> kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely >> stable in that interval, then we'd be fine, but there's a race window >> in which the kvmclock is *not* stable and vcpu 0 wasn't running. > > What is that window again ? Have no objections against using vcpu0's > pvti (cacheline should be read-only 99.9% of time). This is based on my (mis?)understanding of the code. Here goes. Suppose we transition from stable to unstable. The host freezes all vcpus and set a flag for each vcpu that the kvmclock data needs updating. There could then be a window in which vcpu 1 runs vdso code and vcpu 0 hasn't updated its kvmclock data yet. I don't know whether this is actually possible. Rik suspects it isn't. > >> Why doesn't KVM just update all of the kvmclock data at once? > > Because it has not been necessary -- updating kvmclock data on vcpu > entry was the previous method, so that was reused. > >> (For >> that matter, why is the pvti in guest memory at all? Wouldn't this >> all be simpler if the kvmclock data were host-allocated so the host >> could write it directly and maybe even share it between guests?) > > And use a 4K TLB entry for that kvmclock area rather than > sharing one of kernel's 2MB (or 1GB) TLB entry? Exactly. I'd also move it into the vvar area instead of using the fixmap so 32-bit userspace could use it. I'm more than happy to handle the vdso side of all of this, but I'd like the host code to settle down first. I'm also not quite sure whether it's okay to cause the vdso timing code to regress on old hosts with new gusts. --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Mar 26, 2015 at 04:09:53PM -0700, Andy Lutomirski wrote: > On Thu, Mar 26, 2015 at 3:56 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > > On Thu, Mar 26, 2015 at 01:58:25PM -0700, Andy Lutomirski wrote: > >> On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@redhat.com> wrote: > >> > 2015-03-26 11:51-0700, Andy Lutomirski: > >> >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > >> >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: > >> >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect > >> >> >> invariant TSCs. Now the host updates its frequency (due to NTP or > >> >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's > >> >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. > >> >> >> They'll disagree on the time, and one of them will be ahead until vcpu > >> >> >> 1's pvti gets updated. > >> >> > > >> >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs > >> >> > to be visible at one time, for all vcpus. > >> >> > > >> >> > > >> >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is > >> >> > * not > >> >> > * always the case (the difference between two distinct xtime instances > >> >> > * might be smaller then the difference between corresponding TSC reads, > >> >> > * when updating guest vcpus pvclock areas). > >> >> > * > >> >> > * To avoid that problem, do not allow visibility of distinct > >> >> > * system_timestamp/tsc_timestamp values simultaneously: use a master > >> >> > * copy of host monotonic time values. Update that master copy > >> >> > * in lockstep. > >> >> > >> >> Yuck. So we have per cpu timing data, but the protocol is only usable > >> >> for monotonic timing because we forcibly freeze all vcpus when we > >> >> update the nominally per cpu data. > >> >> > >> >> The obvious guest implementations are still unnecessarily slow, > >> >> though. It would be nice if the guest could get away without using > >> >> any getcpu operation at all. > >> >> > >> >> Even if we fixed the host to increment version as advertised, I think > >> >> we can't avoid two getcpu ops. We need one before rdtsc to figure out > >> >> which pvti to look at, > >> > > >> > Yes. > >> > > >> >> and we need another to make sure that we were > >> >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help > >> >> -- we need to check version before rdtsc, and we don't know what > >> >> version to check until we do a getcpu.). > >> > > >> > Exactly, reading cpuid after rdtsc doesn't do that though, we could have > >> > migrated back between those reads. > >> > rtdscp would allow us to check that we read tsc of pvti's cpu. > >> > (It doesn't get rid of that first read.) > >> > > >> >> The migration hook has the > >> >> same issue -- we need to check the migration count, then confirm we're > >> >> on that cpu, then check the migration count again, and we can't do > >> >> that until we know what cpu we're on. > >> > > >> > True; the revert has a bug -- we need to check cpuid for the second > >> > time before rdtsc. (Migration hook is there just because we don't know > >> > which cpu executed rdtsc.) > >> > >> One way or another, I'm planning on completely rewriting the vdso > >> code. An early draft is here: > >> > >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 > >> > >> but I can't finish it until the KVM side shakes out. > >> > >> I think there are at least two ways that would work: > >> > >> a) If KVM incremented version as advertised: > > > > All for it. > > > >> cpu = getcpu(); > >> pvti = pvti for cpu; > >> > >> ver1 = pvti->version; > >> check stable bit; > >> rdtsc_barrier, rdtsc, read scale, shift, etc. > >> if (getcpu() != cpu) retry; > >> if (pvti->version != ver1) retry; > >> > >> I think this is safe because, we're guaranteed that there was an > >> interval (between the two version reads) in which the vcpu we think > >> we're on was running and the kvmclock data was valid and marked > >> stable, and we know that the tsc we read came from that interval. > >> > >> Note: rdtscp isn't needed. If we're stable, is makes no difference > >> which cpu's tsc we actually read. > > > > Yes, can't see a problem with that. > > > >> b) If version remains buggy but we use this migrations_from hack: > > > > There is no reason for version to remain buggy. > > > >> cpu = getcpu(); > >> pvti = pvti for cpu; > >> m1 = pvti->migrations_from; > >> barrier(); > >> > >> ver1 = pvti->version; > >> check stable bit; > >> rdtsc_barrier, rdtsc, read scale, shift, etc. > >> if (getcpu() != cpu) retry; > >> if (pvti->version != ver1) retry; /* probably not really needed */ > >> > >> barrier(); > >> if (pvti->migrations_from != m1) retry; > >> > >> This is just like (a), except that we're using a guest kernel hack to > >> ensure that no one migrated off the vcpu during the version-protected > >> critical section and that we were, in fact, on that vcpu at some point > >> during that critical section. Once we've ensured that we were on > >> pvti's associated vcpu for the entire time we were reading it, then we > >> are protected by the existing versioning in the host. > >> > >> > > >> >> If, on the other hand, we could rely on having all of these things in > >> >> sync, then this complication goes away, and we go down from two getcpu > >> >> ops to zero. > >> > > >> > (Yeah, we should look what are the drawbacks of doing it differently.) > >> > >> If the versioning were fixed, I think we could almost get away with: > >> > >> pvti = pvti for vcpu 0; > >> > >> ver1 = pvti->version; > >> check stable bit; > >> rdtsc_barrier, rdtsc, read scale, shift, etc. > >> if (pvti->version != ver1) retry; > >> > >> This guarantees that the tsc came from an interval in which vcpu0's > >> kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely > >> stable in that interval, then we'd be fine, but there's a race window > >> in which the kvmclock is *not* stable and vcpu 0 wasn't running. > > > > What is that window again ? Have no objections against using vcpu0's > > pvti (cacheline should be read-only 99.9% of time). > > This is based on my (mis?)understanding of the code. Here goes. > > Suppose we transition from stable to unstable. The host freezes all > vcpus and set a flag for each vcpu that the kvmclock data needs > updating. There could then be a window in which vcpu 1 runs vdso code > and vcpu 0 hasn't updated its kvmclock data yet. > > I don't know whether this is actually possible. Rik suspects it isn't. I don't see why its not possible. We can force vcpu0's kvmclock to be updated. Do you have an estimation of the performance gain of using vcpu0 pvti? > >> Why doesn't KVM just update all of the kvmclock data at once? > > > > Because it has not been necessary -- updating kvmclock data on vcpu > > entry was the previous method, so that was reused. > > > >> (For > >> that matter, why is the pvti in guest memory at all? Wouldn't this > >> all be simpler if the kvmclock data were host-allocated so the host > >> could write it directly and maybe even share it between guests?) > > > > And use a 4K TLB entry for that kvmclock area rather than > > sharing one of kernel's 2MB (or 1GB) TLB entry? > > Exactly. I'd also move it into the vvar area instead of using the > fixmap so 32-bit userspace could use it. Thats an obvious downside (an additional entry in the TLB occupied just for the kvmclock area?). > I'm more than happy to handle the vdso side of all of this, but I'd > like the host code to settle down first. > I'm also not quite sure whether it's okay to cause the vdso timing > code to regress on old hosts with new gusts. Must be backwards compatible. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Mar 26, 2015 at 4:22 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > On Thu, Mar 26, 2015 at 04:09:53PM -0700, Andy Lutomirski wrote: >> On Thu, Mar 26, 2015 at 3:56 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> > On Thu, Mar 26, 2015 at 01:58:25PM -0700, Andy Lutomirski wrote: >> >> On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@redhat.com> wrote: >> >> > 2015-03-26 11:51-0700, Andy Lutomirski: >> >> >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: >> >> >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: >> >> >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect >> >> >> >> invariant TSCs. Now the host updates its frequency (due to NTP or >> >> >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's >> >> >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. >> >> >> >> They'll disagree on the time, and one of them will be ahead until vcpu >> >> >> >> 1's pvti gets updated. >> >> >> > >> >> >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs >> >> >> > to be visible at one time, for all vcpus. >> >> >> > >> >> >> > >> >> >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is >> >> >> > * not >> >> >> > * always the case (the difference between two distinct xtime instances >> >> >> > * might be smaller then the difference between corresponding TSC reads, >> >> >> > * when updating guest vcpus pvclock areas). >> >> >> > * >> >> >> > * To avoid that problem, do not allow visibility of distinct >> >> >> > * system_timestamp/tsc_timestamp values simultaneously: use a master >> >> >> > * copy of host monotonic time values. Update that master copy >> >> >> > * in lockstep. >> >> >> >> >> >> Yuck. So we have per cpu timing data, but the protocol is only usable >> >> >> for monotonic timing because we forcibly freeze all vcpus when we >> >> >> update the nominally per cpu data. >> >> >> >> >> >> The obvious guest implementations are still unnecessarily slow, >> >> >> though. It would be nice if the guest could get away without using >> >> >> any getcpu operation at all. >> >> >> >> >> >> Even if we fixed the host to increment version as advertised, I think >> >> >> we can't avoid two getcpu ops. We need one before rdtsc to figure out >> >> >> which pvti to look at, >> >> > >> >> > Yes. >> >> > >> >> >> and we need another to make sure that we were >> >> >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help >> >> >> -- we need to check version before rdtsc, and we don't know what >> >> >> version to check until we do a getcpu.). >> >> > >> >> > Exactly, reading cpuid after rdtsc doesn't do that though, we could have >> >> > migrated back between those reads. >> >> > rtdscp would allow us to check that we read tsc of pvti's cpu. >> >> > (It doesn't get rid of that first read.) >> >> > >> >> >> The migration hook has the >> >> >> same issue -- we need to check the migration count, then confirm we're >> >> >> on that cpu, then check the migration count again, and we can't do >> >> >> that until we know what cpu we're on. >> >> > >> >> > True; the revert has a bug -- we need to check cpuid for the second >> >> > time before rdtsc. (Migration hook is there just because we don't know >> >> > which cpu executed rdtsc.) >> >> >> >> One way or another, I'm planning on completely rewriting the vdso >> >> code. An early draft is here: >> >> >> >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 >> >> >> >> but I can't finish it until the KVM side shakes out. >> >> >> >> I think there are at least two ways that would work: >> >> >> >> a) If KVM incremented version as advertised: >> > >> > All for it. >> > >> >> cpu = getcpu(); >> >> pvti = pvti for cpu; >> >> >> >> ver1 = pvti->version; >> >> check stable bit; >> >> rdtsc_barrier, rdtsc, read scale, shift, etc. >> >> if (getcpu() != cpu) retry; >> >> if (pvti->version != ver1) retry; >> >> >> >> I think this is safe because, we're guaranteed that there was an >> >> interval (between the two version reads) in which the vcpu we think >> >> we're on was running and the kvmclock data was valid and marked >> >> stable, and we know that the tsc we read came from that interval. >> >> >> >> Note: rdtscp isn't needed. If we're stable, is makes no difference >> >> which cpu's tsc we actually read. >> > >> > Yes, can't see a problem with that. >> > >> >> b) If version remains buggy but we use this migrations_from hack: >> > >> > There is no reason for version to remain buggy. >> > >> >> cpu = getcpu(); >> >> pvti = pvti for cpu; >> >> m1 = pvti->migrations_from; >> >> barrier(); >> >> >> >> ver1 = pvti->version; >> >> check stable bit; >> >> rdtsc_barrier, rdtsc, read scale, shift, etc. >> >> if (getcpu() != cpu) retry; >> >> if (pvti->version != ver1) retry; /* probably not really needed */ >> >> >> >> barrier(); >> >> if (pvti->migrations_from != m1) retry; >> >> >> >> This is just like (a), except that we're using a guest kernel hack to >> >> ensure that no one migrated off the vcpu during the version-protected >> >> critical section and that we were, in fact, on that vcpu at some point >> >> during that critical section. Once we've ensured that we were on >> >> pvti's associated vcpu for the entire time we were reading it, then we >> >> are protected by the existing versioning in the host. >> >> >> >> > >> >> >> If, on the other hand, we could rely on having all of these things in >> >> >> sync, then this complication goes away, and we go down from two getcpu >> >> >> ops to zero. >> >> > >> >> > (Yeah, we should look what are the drawbacks of doing it differently.) >> >> >> >> If the versioning were fixed, I think we could almost get away with: >> >> >> >> pvti = pvti for vcpu 0; >> >> >> >> ver1 = pvti->version; >> >> check stable bit; >> >> rdtsc_barrier, rdtsc, read scale, shift, etc. >> >> if (pvti->version != ver1) retry; >> >> >> >> This guarantees that the tsc came from an interval in which vcpu0's >> >> kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely >> >> stable in that interval, then we'd be fine, but there's a race window >> >> in which the kvmclock is *not* stable and vcpu 0 wasn't running. >> > >> > What is that window again ? Have no objections against using vcpu0's >> > pvti (cacheline should be read-only 99.9% of time). >> >> This is based on my (mis?)understanding of the code. Here goes. >> >> Suppose we transition from stable to unstable. The host freezes all >> vcpus and set a flag for each vcpu that the kvmclock data needs >> updating. There could then be a window in which vcpu 1 runs vdso code >> and vcpu 0 hasn't updated its kvmclock data yet. >> >> I don't know whether this is actually possible. Rik suspects it isn't. > > I don't see why its not possible. We can force vcpu0's kvmclock to be > updated. > > Do you have an estimation of the performance gain of using vcpu0 pvti? rdtscp is approximately 3 cycles worse than rdtsc_barrier(); rdtsc() on my machine, but that doesn't help much, since the current scheme requires that we get the cpu number before reading the time. The fastest way I know of to get the cpu number is 25-ish cycles, so this ends up being a large fraction of the overall cost. > >> >> Why doesn't KVM just update all of the kvmclock data at once? >> > >> > Because it has not been necessary -- updating kvmclock data on vcpu >> > entry was the previous method, so that was reused. >> > >> >> (For >> >> that matter, why is the pvti in guest memory at all? Wouldn't this >> >> all be simpler if the kvmclock data were host-allocated so the host >> >> could write it directly and maybe even share it between guests?) >> > >> > And use a 4K TLB entry for that kvmclock area rather than >> > sharing one of kernel's 2MB (or 1GB) TLB entry? >> >> Exactly. I'd also move it into the vvar area instead of using the >> fixmap so 32-bit userspace could use it. > > Thats an obvious downside (an additional entry in the TLB occupied just > for the kvmclock area?). True. But TLB misses are actually pretty cheap on new hardware, and it's still only one TLB entry per process. > >> I'm more than happy to handle the vdso side of all of this, but I'd >> like the host code to settle down first. >> I'm also not quite sure whether it's okay to cause the vdso timing >> code to regress on old hosts with new gusts. > > Must be backwards compatible. > Could we add a new feature bit that indicates that the host is updated? Then we could use the new code on new hosts and the old code on old hosts and eventually deprecate accelerated vdso timing on old hosts. --Andy
On Thu, Mar 26, 2015 at 04:28:37PM -0700, Andy Lutomirski wrote: > On Thu, Mar 26, 2015 at 4:22 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > > On Thu, Mar 26, 2015 at 04:09:53PM -0700, Andy Lutomirski wrote: > >> On Thu, Mar 26, 2015 at 3:56 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > >> > On Thu, Mar 26, 2015 at 01:58:25PM -0700, Andy Lutomirski wrote: > >> >> On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@redhat.com> wrote: > >> >> > 2015-03-26 11:51-0700, Andy Lutomirski: > >> >> >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > >> >> >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: > >> >> >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect > >> >> >> >> invariant TSCs. Now the host updates its frequency (due to NTP or > >> >> >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's > >> >> >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. > >> >> >> >> They'll disagree on the time, and one of them will be ahead until vcpu > >> >> >> >> 1's pvti gets updated. > >> >> >> > > >> >> >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs > >> >> >> > to be visible at one time, for all vcpus. > >> >> >> > > >> >> >> > > >> >> >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is > >> >> >> > * not > >> >> >> > * always the case (the difference between two distinct xtime instances > >> >> >> > * might be smaller then the difference between corresponding TSC reads, > >> >> >> > * when updating guest vcpus pvclock areas). > >> >> >> > * > >> >> >> > * To avoid that problem, do not allow visibility of distinct > >> >> >> > * system_timestamp/tsc_timestamp values simultaneously: use a master > >> >> >> > * copy of host monotonic time values. Update that master copy > >> >> >> > * in lockstep. > >> >> >> > >> >> >> Yuck. So we have per cpu timing data, but the protocol is only usable > >> >> >> for monotonic timing because we forcibly freeze all vcpus when we > >> >> >> update the nominally per cpu data. > >> >> >> > >> >> >> The obvious guest implementations are still unnecessarily slow, > >> >> >> though. It would be nice if the guest could get away without using > >> >> >> any getcpu operation at all. > >> >> >> > >> >> >> Even if we fixed the host to increment version as advertised, I think > >> >> >> we can't avoid two getcpu ops. We need one before rdtsc to figure out > >> >> >> which pvti to look at, > >> >> > > >> >> > Yes. > >> >> > > >> >> >> and we need another to make sure that we were > >> >> >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help > >> >> >> -- we need to check version before rdtsc, and we don't know what > >> >> >> version to check until we do a getcpu.). > >> >> > > >> >> > Exactly, reading cpuid after rdtsc doesn't do that though, we could have > >> >> > migrated back between those reads. > >> >> > rtdscp would allow us to check that we read tsc of pvti's cpu. > >> >> > (It doesn't get rid of that first read.) > >> >> > > >> >> >> The migration hook has the > >> >> >> same issue -- we need to check the migration count, then confirm we're > >> >> >> on that cpu, then check the migration count again, and we can't do > >> >> >> that until we know what cpu we're on. > >> >> > > >> >> > True; the revert has a bug -- we need to check cpuid for the second > >> >> > time before rdtsc. (Migration hook is there just because we don't know > >> >> > which cpu executed rdtsc.) > >> >> > >> >> One way or another, I'm planning on completely rewriting the vdso > >> >> code. An early draft is here: > >> >> > >> >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 > >> >> > >> >> but I can't finish it until the KVM side shakes out. > >> >> > >> >> I think there are at least two ways that would work: > >> >> > >> >> a) If KVM incremented version as advertised: > >> > > >> > All for it. > >> > > >> >> cpu = getcpu(); > >> >> pvti = pvti for cpu; > >> >> > >> >> ver1 = pvti->version; > >> >> check stable bit; > >> >> rdtsc_barrier, rdtsc, read scale, shift, etc. > >> >> if (getcpu() != cpu) retry; > >> >> if (pvti->version != ver1) retry; > >> >> > >> >> I think this is safe because, we're guaranteed that there was an > >> >> interval (between the two version reads) in which the vcpu we think > >> >> we're on was running and the kvmclock data was valid and marked > >> >> stable, and we know that the tsc we read came from that interval. > >> >> > >> >> Note: rdtscp isn't needed. If we're stable, is makes no difference > >> >> which cpu's tsc we actually read. > >> > > >> > Yes, can't see a problem with that. > >> > > >> >> b) If version remains buggy but we use this migrations_from hack: > >> > > >> > There is no reason for version to remain buggy. > >> > > >> >> cpu = getcpu(); > >> >> pvti = pvti for cpu; > >> >> m1 = pvti->migrations_from; > >> >> barrier(); > >> >> > >> >> ver1 = pvti->version; > >> >> check stable bit; > >> >> rdtsc_barrier, rdtsc, read scale, shift, etc. > >> >> if (getcpu() != cpu) retry; > >> >> if (pvti->version != ver1) retry; /* probably not really needed */ > >> >> > >> >> barrier(); > >> >> if (pvti->migrations_from != m1) retry; > >> >> > >> >> This is just like (a), except that we're using a guest kernel hack to > >> >> ensure that no one migrated off the vcpu during the version-protected > >> >> critical section and that we were, in fact, on that vcpu at some point > >> >> during that critical section. Once we've ensured that we were on > >> >> pvti's associated vcpu for the entire time we were reading it, then we > >> >> are protected by the existing versioning in the host. > >> >> > >> >> > > >> >> >> If, on the other hand, we could rely on having all of these things in > >> >> >> sync, then this complication goes away, and we go down from two getcpu > >> >> >> ops to zero. > >> >> > > >> >> > (Yeah, we should look what are the drawbacks of doing it differently.) > >> >> > >> >> If the versioning were fixed, I think we could almost get away with: > >> >> > >> >> pvti = pvti for vcpu 0; > >> >> > >> >> ver1 = pvti->version; > >> >> check stable bit; > >> >> rdtsc_barrier, rdtsc, read scale, shift, etc. > >> >> if (pvti->version != ver1) retry; > >> >> > >> >> This guarantees that the tsc came from an interval in which vcpu0's > >> >> kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely > >> >> stable in that interval, then we'd be fine, but there's a race window > >> >> in which the kvmclock is *not* stable and vcpu 0 wasn't running. > >> > > >> > What is that window again ? Have no objections against using vcpu0's > >> > pvti (cacheline should be read-only 99.9% of time). > >> > >> This is based on my (mis?)understanding of the code. Here goes. > >> > >> Suppose we transition from stable to unstable. The host freezes all > >> vcpus and set a flag for each vcpu that the kvmclock data needs > >> updating. There could then be a window in which vcpu 1 runs vdso code > >> and vcpu 0 hasn't updated its kvmclock data yet. > >> > >> I don't know whether this is actually possible. Rik suspects it isn't. > > > > I don't see why its not possible. We can force vcpu0's kvmclock to be > > updated. > > > > Do you have an estimation of the performance gain of using vcpu0 pvti? > > rdtscp is approximately 3 cycles worse than rdtsc_barrier(); rdtsc() > on my machine, but that doesn't help much, since the current scheme > requires that we get the cpu number before reading the time. The > fastest way I know of to get the cpu number is 25-ish cycles, so this > ends up being a large fraction of the overall cost. It should be possible to force vcpu0 kvmclock area to be updated before vm-entry, on the update of any other vcpuN kvmclock area. kvm_make_mclock_inprogress_request(kvm); /* no guest entries from this point */ pvclock_update_vm_gtod_copy(kvm); <---- HERE -------> kvm_for_each_vcpu(i, vcpu, kvm) kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu); /* guest entries allowed */ kvm_for_each_vcpu(i, vcpu, kvm) clear_bit(KVM_REQ_MCLOCK_INPROGRESS, &vcpu->requests); > >> >> Why doesn't KVM just update all of the kvmclock data at once? > >> > > >> > Because it has not been necessary -- updating kvmclock data on vcpu > >> > entry was the previous method, so that was reused. > >> > > >> >> (For > >> >> that matter, why is the pvti in guest memory at all? Wouldn't this > >> >> all be simpler if the kvmclock data were host-allocated so the host > >> >> could write it directly and maybe even share it between guests?) > >> > > >> > And use a 4K TLB entry for that kvmclock area rather than > >> > sharing one of kernel's 2MB (or 1GB) TLB entry? > >> > >> Exactly. I'd also move it into the vvar area instead of using the > >> fixmap so 32-bit userspace could use it. > > > > Thats an obvious downside (an additional entry in the TLB occupied just > > for the kvmclock area?). > > True. But TLB misses are actually pretty cheap on new hardware, and > it's still only one TLB entry per process. > >> I'm more than happy to handle the vdso side of all of this, but I'd > >> like the host code to settle down first. > >> I'm also not quite sure whether it's okay to cause the vdso timing > >> code to regress on old hosts with new gusts. > > > > Must be backwards compatible. > > > > Could we add a new feature bit that indicates that the host is > updated? Then we could use the new code on new hosts and the old code > on old hosts and eventually deprecate accelerated vdso timing on old > hosts. Makes sense. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index cc2c759f69a3..8658599e0024 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1658,12 +1658,24 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) &guest_hv_clock, sizeof(guest_hv_clock)))) return 0; - /* - * The interface expects us to write an even number signaling that the - * update is finished. Since the guest won't see the intermediate - * state, we just increase by 2 at the end. + /* A guest can read other VCPU's kvmclock; specification says that + * version is odd if data is being modified and even after it is + * consistent. + * We write three times to be sure. + * 1) update version to odd number + * 2) write modified data (version is still odd) + * 3) update version to even number + * + * TODO: optimize + * - only two writes should be enough -- version is first + * - the second write could update just version */ - vcpu->hv_clock.version = guest_hv_clock.version + 2; + guest_hv_clock.version += 1; + kvm_write_guest_cached(v->kvm, &vcpu->pv_time, + &guest_hv_clock, + sizeof(guest_hv_clock)); + + vcpu->hv_clock.version = guest_hv_clock.version; /* retain PVCLOCK_GUEST_STOPPED if set in guest copy */ pvclock_flags = (guest_hv_clock.flags & PVCLOCK_GUEST_STOPPED); @@ -1684,6 +1696,11 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) kvm_write_guest_cached(v->kvm, &vcpu->pv_time, &vcpu->hv_clock, sizeof(vcpu->hv_clock)); + + vcpu->hv_clock.version += 1; + kvm_write_guest_cached(v->kvm, &vcpu->pv_time, + &vcpu->hv_clock, + sizeof(vcpu->hv_clock)); return 0; }