Re: [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/16/2015 12:39 PM, Andrew Cooper wrote:
> On 16/07/15 17:29, Jan Beulich wrote:
> > > > > On 16.07.15 at 17:50, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > Can't we set leaf 1's EBX[32:16] to 1?
(I obviously fat-fingered this --- I meant EBX[23:16])

> > I don't think we should partially overwrite the relevant parts of
> > CPUID output - either all or nothing (so that things at least
> > remain consistent).
Possibly more, but specifically for Dario's problem I think this could 
resolve that.
> Also, there are no masking/override MSRs for that feature leaf on any
> hardware I am aware of, and a PV guest using plain `cpuid` will not
> observe any attempt of Xen's to control the value.
True, but we already modify other CPUID bits (e.g. we clear 
X86_FEATURE_TOPOEXT) so it won't make things much worse. (And again, for 
this specific problem CPUID is queried by guest kernel which uses PV 
CPUID instruction).
And in general (both for PV and HVM) --- is there any reason to expose 
CPU topology at all? I can see it being useful if VCPUs are pinned but 
if they are not then it can make performance worse.
-boris


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