2018-03-12 15:19+0100, Vitaly Kuznetsov: > Thomas Gleixner <t...@linutronix.de> 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 :-)
Indeed, I had no idea. > 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)? Please take a look into making a macro that uses STATIC_JUMP_IF_FALSE or reads the value from provided static_key and does a test-jump, depending on HAVE_JUMP_LABEL. It doesn't need to be suited for general use, just something that moves the ugliness away from vmx_vcpu_run. (Although having it in jump_label.h would be great. I think the main obstacle is clobbering of flags.) If it were still looking horrible, I'm ok with the series as-is, thanks.