Messages in this thread | | | Date | Sun, 27 Mar 2022 12:43:36 +0200 | Subject | Re: [PATCH] KVM: x86: optimize PKU branching in kvm_load_{guest|host}_xsave_state | From | Paolo Bonzini <> |
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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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