Re: [PATCH v4] KVM: x86: Fix KVM_GET_CPUID2 ioctl to return cpuid entries count

[Jim Mattson]

On Tue, May 4, 2021 at 2:26 AM Alexander Graf <graf@xxxxxxxxxx> wrote:
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> On 04.05.21 10:21, Paolo Bonzini wrote:
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> > On 04/05/21 10:15, Denis V. Lunev wrote:
> >> As far as I understand only some testing within kernel now.
> >> Though we have plans to expose it for QAPI as the series
> >> in QEMU
> >>    [PATCH 1/2] qapi: fix error handling for x-vz-query-cpu-model-cpuid
> >>    [PATCH 2/2] qapi: blacklisted x-vz-query-cpu-model-cpuid in tests
> >> is not coming in a good way.
> >> The idea was to avoid manual code rework in QEMU and
> >> expose collected model at least for debug.
> >
> > KVM_GET_CPUID2 as a VM ioctl cannot expose the whole truth about CPUID
> > either, since it doesn't handle the TSX_CTRL_CPUID_CLEAR bit.  Given
> > that QEMU doesn't need KVM_GET_CPUID2; it only needs to save whatever it
> > passed to KVM_SET_CPUID2.
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> What if we instead deflect CPUID into user space so it can emulate it in
> whatever way it likes? Is the performance difference going to be
> relevant? Are people still using cpuid as barrier these days?

What else would they use (in ring 3 code)? Sure, serialize is coming
in Sapphire Rapids, but it will be 20+ years before kvm drops support
for CPUs without serialize.

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> Alex
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