Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes: > On Fri, 9 Mar 2018, Vitaly Kuznetsov wrote: > >> Enlightened VMCS is just a structure in memory, the main benefit >> besides avoiding somewhat slower VMREAD/VMWRITE is using clean field >> mask: we tell the underlying hypervisor which fields were modified >> since VMEXIT so there's no need to inspect them all. >> >> Tight CPUID loop test shows significant speedup: >> Before: 18890 cycles >> After: 8304 cycles >> >> 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. That said I'd like to defer the question to KVM maintainers: Paolo, Radim, what would you like me to do? Use STATIC_JUMP_IF_TRUE/FALSE as they are, try to make them work for !HAVE_JUMP_LABEL and use them or maybe we can commit the series as-is and have it as a future optimization (e.g. when HAVE_JUMP_LABEL becomes mandatory)? -- Vitaly
