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SubjectRe: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V
On Mon, 12 Mar 2018, Vitaly Kuznetsov wrote:
> Thomas Gleixner <tglx@linutronix.de> writes:
> > On Fri, 9 Mar 2018, Vitaly Kuznetsov wrote:
> >> Static key is being used to avoid performance penalty for non-Hyper-V
> >> deployments. Tests show we add around 3 (three) CPU cycles on each
> >> VMEXIT (1077.5 cycles before, 1080.7 cycles after for the same CPUID
> >> loop on bare metal). We can probably avoid one test/jmp in vmx_vcpu_run()
> >> but I don't see a clean way to use static key in assembly.
> >
> > STATIC_JUMP_IF_TRUE, STATIC_JUMP_IF_FALSE are your friends.
> >
>
> Thanks for the tip,
>
> with a single kernel user of these APIs it was easy to miss :-)
>
> Unfortunately, these APIs are only present if HAVE_JUMP_LABEL and
> (afaiu) we still care about KVM on !HAVE_JUMP_LABEL builds. It would be
> nice if we can make them behave the same way static_branch_likely() and
> friends do: compile into something else when !HAVE_JUMP_LABEL so we can
> avoid nasty #ifdefs in C code.

What's the reason for !jump label builds of a recent kernel? Old compilers?

Thanks,

tglx


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