| Message ID | 20191125123923.2000028-1-george.dunlap@citrix.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] x86: Don't increase ApicIdCoreSize past 7 | expand |
On 25.11.19 13:39, George Dunlap wrote: > Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM > guests") attempted to "fake up" a topology which would induce guest > operating systems to not treat vcpus as sibling hyperthreads. This > involved actually reporting hyperthreading as available, but giving > vcpus every other ApicId; which in turn led to doubling the ApicIds > per core by bumping the ApicIdCoreSize by one. In particular, Ryzen > 3xxx series processors, and reportedly EPYC "Rome" cpus -- have an > ApicIdCoreSize of 7; the "fake" topology increases this to 8. > > Unfortunately, Windows running on modern AMD hardware -- including > Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- > doesn't seem to cope with this value being higher than 7. (Linux > guests have so far continued to cope.) > > A "proper" fix is complicated and it's too late to fix it either for > 4.13, or to backport to supported branches. As a short-term fix, > limit this value to 7. > > This does mean that a Linux guest, booted on such a system without > this change, and then migrating to a system with this change, with > more than 64 vcpus, would see an apparent topology change. This is a > low enough risk in practice that enabling this limit unilaterally, to > allow other guests to boot without manual intervention, is worth it. > > Reported-by: Steven Haigh <netwiz@crc.id.au> > Reported-by: Andreas Kinzler <hfp@posteo.de> > Signed-off-by: George Dunlap <george.dunlap@citrix.com> Release-acked-by: Juergen Gross <jgross@suse.com> Juergen
On 25.11.2019 13:39, George Dunlap wrote: > Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM > guests") attempted to "fake up" a topology which would induce guest > operating systems to not treat vcpus as sibling hyperthreads. This > involved actually reporting hyperthreading as available, but giving > vcpus every other ApicId; which in turn led to doubling the ApicIds > per core by bumping the ApicIdCoreSize by one. In particular, Ryzen > 3xxx series processors, and reportedly EPYC "Rome" cpus -- have an > ApicIdCoreSize of 7; the "fake" topology increases this to 8. > > Unfortunately, Windows running on modern AMD hardware -- including > Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- > doesn't seem to cope with this value being higher than 7. (Linux > guests have so far continued to cope.) > > A "proper" fix is complicated and it's too late to fix it either for > 4.13, or to backport to supported branches. As a short-term fix, > limit this value to 7. > > This does mean that a Linux guest, booted on such a system without > this change, and then migrating to a system with this change, with > more than 64 vcpus, would see an apparent topology change. This is a > low enough risk in practice that enabling this limit unilaterally, to > allow other guests to boot without manual intervention, is worth it. > > Reported-by: Steven Haigh <netwiz@crc.id.au> > Reported-by: Andreas Kinzler <hfp@posteo.de> > Signed-off-by: George Dunlap <george.dunlap@citrix.com> Acked-by: Jan Beulich <jbeulich@suse.com> with ... > --- a/tools/libxc/xc_cpuid_x86.c > +++ b/tools/libxc/xc_cpuid_x86.c > @@ -616,10 +616,15 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, > * - going out of sync with leaf 1 EBX[23:16], > * - incrementing ApicIdCoreSize when it's zero (which changes the > * meaning of bits 7:0). > + * > + * UPDATE: I addition to avoiding overflow, some ... this becoming "UPDATE: In ...". Jan
On Mon, Nov 25, 2019 at 12:39:23PM +0000, George Dunlap wrote: > Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM > guests") attempted to "fake up" a topology which would induce guest > operating systems to not treat vcpus as sibling hyperthreads. This > involved actually reporting hyperthreading as available, but giving > vcpus every other ApicId; which in turn led to doubling the ApicIds > per core by bumping the ApicIdCoreSize by one. In particular, Ryzen > 3xxx series processors, and reportedly EPYC "Rome" cpus -- have an > ApicIdCoreSize of 7; the "fake" topology increases this to 8. > > Unfortunately, Windows running on modern AMD hardware -- including > Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- > doesn't seem to cope with this value being higher than 7. (Linux > guests have so far continued to cope.) > > A "proper" fix is complicated and it's too late to fix it either for > 4.13, or to backport to supported branches. As a short-term fix, > limit this value to 7. > > This does mean that a Linux guest, booted on such a system without > this change, and then migrating to a system with this change, with > more than 64 vcpus, would see an apparent topology change. This is a > low enough risk in practice that enabling this limit unilaterally, to > allow other guests to boot without manual intervention, is worth it. > > Reported-by: Steven Haigh <netwiz@crc.id.au> > Reported-by: Andreas Kinzler <hfp@posteo.de> > Signed-off-by: George Dunlap <george.dunlap@citrix.com> > --- > CC: Ian Jackson <ian.jackson@citrix.com> > CC: Wei Liu <wl@xen.org> > CC: Andrew Cooper <andrew.cooper3@citrix.com> > CC: Jan Beulich <jbeulich@suse.com> > CC: Juergen Gross <jgross@suse.com> > --- > tools/libxc/xc_cpuid_x86.c | 7 ++++++- I will defer this to x86 maintainers. Seeing that you already have an Ack from Jan, feel free to add mine if necessary. Wei.
On 11/25/19 12:49 PM, Jan Beulich wrote: > On 25.11.2019 13:39, George Dunlap wrote: >> Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM >> guests") attempted to "fake up" a topology which would induce guest >> operating systems to not treat vcpus as sibling hyperthreads. This >> involved actually reporting hyperthreading as available, but giving >> vcpus every other ApicId; which in turn led to doubling the ApicIds >> per core by bumping the ApicIdCoreSize by one. In particular, Ryzen >> 3xxx series processors, and reportedly EPYC "Rome" cpus -- have an >> ApicIdCoreSize of 7; the "fake" topology increases this to 8. >> >> Unfortunately, Windows running on modern AMD hardware -- including >> Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- >> doesn't seem to cope with this value being higher than 7. (Linux >> guests have so far continued to cope.) >> >> A "proper" fix is complicated and it's too late to fix it either for >> 4.13, or to backport to supported branches. As a short-term fix, >> limit this value to 7. >> >> This does mean that a Linux guest, booted on such a system without >> this change, and then migrating to a system with this change, with >> more than 64 vcpus, would see an apparent topology change. This is a >> low enough risk in practice that enabling this limit unilaterally, to >> allow other guests to boot without manual intervention, is worth it. >> >> Reported-by: Steven Haigh <netwiz@crc.id.au> >> Reported-by: Andreas Kinzler <hfp@posteo.de> >> Signed-off-by: George Dunlap <george.dunlap@citrix.com> > > Acked-by: Jan Beulich <jbeulich@suse.com> > with ... > >> --- a/tools/libxc/xc_cpuid_x86.c >> +++ b/tools/libxc/xc_cpuid_x86.c >> @@ -616,10 +616,15 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, >> * - going out of sync with leaf 1 EBX[23:16], >> * - incrementing ApicIdCoreSize when it's zero (which changes the >> * meaning of bits 7:0). >> + * >> + * UPDATE: I addition to avoiding overflow, some > > ... this becoming "UPDATE: In ...". Gah... Sorry, meant to apply this change on check-in, but screwed it up (accidentally edited the wrong buffer). Let me know if you want a follow-up patch to fix it. -George
diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c index 312c481f1e..519d6d8bd0 100644 --- a/tools/libxc/xc_cpuid_x86.c +++ b/tools/libxc/xc_cpuid_x86.c @@ -616,10 +616,15 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, * - going out of sync with leaf 1 EBX[23:16], * - incrementing ApicIdCoreSize when it's zero (which changes the * meaning of bits 7:0). + * + * UPDATE: I addition to avoiding overflow, some + * proprietary operating systems have trouble with + * apic_id_size values greater than 7. Limit the value to + * 7 for now. */ if ( p->extd.nc < 0x7f ) { - if ( p->extd.apic_id_size != 0 && p->extd.apic_id_size != 0xf ) + if ( p->extd.apic_id_size != 0 && p->extd.apic_id_size < 0x7 ) p->extd.apic_id_size++; p->extd.nc = (p->extd.nc << 1) | 1;
Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM guests") attempted to "fake up" a topology which would induce guest operating systems to not treat vcpus as sibling hyperthreads. This involved actually reporting hyperthreading as available, but giving vcpus every other ApicId; which in turn led to doubling the ApicIds per core by bumping the ApicIdCoreSize by one. In particular, Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- have an ApicIdCoreSize of 7; the "fake" topology increases this to 8. Unfortunately, Windows running on modern AMD hardware -- including Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- doesn't seem to cope with this value being higher than 7. (Linux guests have so far continued to cope.) A "proper" fix is complicated and it's too late to fix it either for 4.13, or to backport to supported branches. As a short-term fix, limit this value to 7. This does mean that a Linux guest, booted on such a system without this change, and then migrating to a system with this change, with more than 64 vcpus, would see an apparent topology change. This is a low enough risk in practice that enabling this limit unilaterally, to allow other guests to boot without manual intervention, is worth it. Reported-by: Steven Haigh <netwiz@crc.id.au> Reported-by: Andreas Kinzler <hfp@posteo.de> Signed-off-by: George Dunlap <george.dunlap@citrix.com> --- CC: Ian Jackson <ian.jackson@citrix.com> CC: Wei Liu <wl@xen.org> CC: Andrew Cooper <andrew.cooper3@citrix.com> CC: Jan Beulich <jbeulich@suse.com> CC: Juergen Gross <jgross@suse.com> --- tools/libxc/xc_cpuid_x86.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)