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    SubjectRe: [PATCH 1/6] x86: KVM: Enable CMPccXADD CPUID and expose it to guest
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    On 11/1/2022 11:07 PM, Sean Christopherson wrote:
    > On Tue, Nov 01, 2022, Jiaxi Chen wrote:
    >>
    >>
    >> On 10/27/2022 1:15 AM, Borislav Petkov wrote:
    >>> On Wed, Oct 26, 2022 at 11:40:31AM +0800, Jiaxi Chen wrote:
    >>>>> What do you think about moving CPUID_7_1_EAX to be a KVM-only leaf too? AFAICT,
    >>>>> KVM passthrough is the only reason the existing features are defined.
    >>>
    >>> Yap, looking at the patches which added those 2 feature flags upstream,
    >>> they don't look like some particular use was the goal but rather to
    >>> expose it to guests. Besides, AVX512 apps do their own CPUID detection.
    >>>
    >>>> Since CPUID_7_1_EAX has only 5 features now, it is a big waste,
    >>>> should we move it to KVM-only leaf as Sean suggested. What's your
    >>>> opinion about this?
    >>>
    >>> Yes, pls do.
    >>>
    >>> And when you do, make sure to undo what
    >>>
    >>> b302e4b176d0 ("x86/cpufeatures: Enumerate the new AVX512 BFLOAT16 instructions")
    >>>
    >>> added.
    >>>
    >>> Thx.
    >>>
    >> Hi Sean and Boris,
    >>
    >> Just realized moving CPUID_7_1_EAX to kvm-only leaf will not save space
    >> in enum cpuid_leafs[]. CPUID_7_1_EAX is indeed removed, but someone
    >> else, ie. CPUID_DUMMY needs to take the place, otherwise the cpuid_leafs
    >> array would be deranged. Therefore, the length of x86 cpuid leaves is
    >> not decreased.
    >
    > The order of "enum cpuid_leafs" is completely arbitrary.
    >
    > After replacing CPUID_7_1_EAX with CPUID_DUMMY, replace CPUID_DUMMY with the last
    > leaf, which is currently CPUID_8000_001F_EAX, and update the #defines accordingly.
    > Alternatively, Boris may prefer skipping the intermediate CPUID_DUMMY step and
    > just replace CPUID_7_1_EAX with CPUID_8000_001F_EAX straightaway.

    Yes, thanks for Sean's kind suggestion. I think use CPUID_DUMMY as the
    transition leaf will make the code logic and commit message clearer.
    Will change it in v2.
    --
    Regards,
    Jiaxi

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