Messages in this thread | | | Date | Wed, 14 Mar 2018 16:19:56 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V |
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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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