Re: [Xen-devel] [PATCH RFC] x86: Don't increase ApicIdCoreSize past 7

[Jan Beulich <jbeulich@xxxxxxxx>]

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@xxxxxxxxx>
> Reported-by: Andreas Kinzler <hfp@xxxxxxxxx>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
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

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