Re: [PATCH] KVM: x86: optimize PKU branching in kvm_load_{guest|host}_xsave_state

[Paolo Bonzini]

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