On Mon, 12 Mar 2018, Vitaly Kuznetsov wrote: > Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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
