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

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.


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