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SubjectRe: [PATCH] KVM: x86: optimize PKU branching in kvm_load_{guest|host}_xsave_state
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On 3/26/22 02:37, Jon Kohler wrote:
>>> Flip the ordering of the || condition so that XFEATURE_MASK_PKRU is
>>> checked first, which when instrumented in our environment appeared
>>> to be always true and less overall work than kvm_read_cr4_bits.
>>
>> If it's always true, then it should be checked last, not first. And if
>
> Sean thanks for the review. This would be a left handed || short circuit, so
> wouldn’t we want always true to be first?

Yes.

>> Not that it really matters, since static_cpu_has() will patch out all the branches,
>> and in practice who cares about a JMP or NOP(s)? But...
>
> The reason I’ve been pursuing this is that the guest+host xsave adds up to
> a bit over ~1% as measured by perf top in an exit heavy workload. This is
> the first in a few patch we’ve drummed up to to get it back towards zero.
> I’ll send the rest out next week.

Can you add a testcase to x86/vmexit.c in kvm-unit-tests, too?

Thanks,

Paolo

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