Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes: > 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? > To be honest I don't see any, we can start depending on HAVE_JUMP_LABEL for CONFIG_KVM I guess. -- Vitaly
