[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH RFC] x86: Don't increase ApicIdCoreSize past 7
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@xxxxxxxxx> > Reported-by: Andreas Kinzler <hfp@xxxxxxxxx> > Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx> > --- > CC: Ian Jackson <ian.jackson@xxxxxxxxxx> > CC: Wei Liu <wl@xxxxxxx> > CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> > CC: Jan Beulich <jbeulich@xxxxxxxx> > CC: Juergen Gross <jgross@xxxxxxxx> > --- > 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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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