| Message ID | 1235677340-3139-1-git-send-email-glommer@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thursday 26 February 2009, Glauber Costa wrote: > @@ -548,15 +548,13 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) > Â > Â static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock) > Â { > -Â Â Â Â Â Â Â static int version; > +Â Â Â Â Â Â Â int version = 1; > Â Â Â Â Â Â Â Â struct pvclock_wall_clock wc; > Â Â Â Â Â Â Â Â struct timespec now, sys, boot; > Â > Â Â Â Â Â Â Â Â if (!wall_clock) > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â return; > Â > -Â Â Â Â Â Â Â version++; > - > Â Â Â Â Â Â Â Â kvm_write_guest(kvm, wall_clock, &version, sizeof(version)); > Â > Â Â Â Â Â Â Â Â /* Doesn't this mean that kvm_write_guest now writes an uninitialized value to the guest? I think what you need here is a 'static atomic_t version;' so you can do an atomic_inc instead of the ++. Arnd <>< -- 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, Feb 26, 2009 at 08:50:26PM +0100, Arnd Bergmann wrote: > On Thursday 26 February 2009, Glauber Costa wrote: > > @@ -548,15 +548,13 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) > > Â > > Â static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock) > > Â { > > -Â Â Â Â Â Â Â static int version; > > +Â Â Â Â Â Â Â int version = 1; > > Â Â Â Â Â Â Â Â struct pvclock_wall_clock wc; > > Â Â Â Â Â Â Â Â struct timespec now, sys, boot; > > Â > > Â Â Â Â Â Â Â Â if (!wall_clock) > > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â return; > > Â > > -Â Â Â Â Â Â Â version++; > > - > > Â Â Â Â Â Â Â Â kvm_write_guest(kvm, wall_clock, &version, sizeof(version)); > > Â > > Â Â Â Â Â Â Â Â /* > > Doesn't this mean that kvm_write_guest now writes an uninitialized value > to the guest? No. If you look closely, it's now initialized to 1. > > I think what you need here is a 'static atomic_t version;' so you can > do an atomic_inc instead of the ++. I don't see the need for atomicity. This is just called once, at boot time. The only thing we're protecting here is one guest from another. The stack will do fine for this. -- 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 Friday 27 February 2009, Glauber Costa wrote: > > > Doesn't this mean that kvm_write_guest now writes an uninitialized value > > to the guest? > No. If you look closely, it's now initialized to 1. Right, I didn't see that change at first. Arnd <>< -- 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, Feb 26, 2009 at 02:42:20PM -0500, Glauber Costa wrote: > Matt T. Yourst noted that we're currently having a dumb > race for no reason in paravirtual wall clock. This is due > to the use of a static variable to hold the counting. > > This can race with multiple guests reading wallclock > at the same time, since the static variable value would > then be accessible to all callers. This wasn't noted > before because it is a rather rare scenario. > > Instead, just use a normal stack variable. This will > mean that each caller will have it's version written > separatedly. No need for a global counter. Applied, thanks. -- 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
Glauber Costa wrote: > Matt T. Yourst noted that we're currently having a dumb > race for no reason in paravirtual wall clock. This is due > to the use of a static variable to hold the counting. > > This can race with multiple guests reading wallclock > at the same time, since the static variable value would > then be accessible to all callers. This wasn't noted > before because it is a rather rare scenario. > > Instead, just use a normal stack variable. This will > mean that each caller will have it's version written > separatedly. No need for a global counter. > > Signed-off-by: Glauber Costa <glommer@redhat.com> > --- > arch/x86/kvm/x86.c | 4 +--- > 1 files changed, 1 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 2511708..d7236f6 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -548,15 +548,13 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) > > static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock) > { > - static int version; > + int version = 1; > struct pvclock_wall_clock wc; > struct timespec now, sys, boot; > > if (!wall_clock) > return; > > - version++; > - > kvm_write_guest(kvm, wall_clock, &version, sizeof(version)); > Suppose currently version == 2. guest: read version (2) guest: read sec host: write version (1) host: write sec host: write nsec host: write version (2) guest: read nsec guest: read version (2) So now we have inconsistent time (sec from old data, nsec from new data). We need to make version a per-vm value. Best to read it from guest memory, so nothing special needs to be done for live migration. Also use mutual exclusion in kvm_write_wall_clock() - sequence locks don't support multiple writers.
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 2511708..d7236f6 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -548,15 +548,13 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock) { - static int version; + int version = 1; struct pvclock_wall_clock wc; struct timespec now, sys, boot; if (!wall_clock) return; - version++; - kvm_write_guest(kvm, wall_clock, &version, sizeof(version)); /*
Matt T. Yourst noted that we're currently having a dumb race for no reason in paravirtual wall clock. This is due to the use of a static variable to hold the counting. This can race with multiple guests reading wallclock at the same time, since the static variable value would then be accessible to all callers. This wasn't noted before because it is a rather rare scenario. Instead, just use a normal stack variable. This will mean that each caller will have it's version written separatedly. No need for a global counter. Signed-off-by: Glauber Costa <glommer@redhat.com> --- arch/x86/kvm/x86.c | 4 +--- 1 files changed, 1 insertions(+), 3 deletions(-)