On 10/19/2022 11:15 PM, Sean Christopherson wrote: > For all the shortlogs, "expose it to guest" is technically wrong. Adding > recognition in kvm/cpuid.c advertises KVM support to host userspace. Whether or > not a feature is exposed to the guest is up to the userspace VMM. > > On Wed, Oct 19, 2022, Jiaxi Chen wrote: >> CMPccXADD is a new set of instructions in the latest Intel platform Sierra >> Forest. It includes a semaphore operation that can compare and add the > > In general, avoid pronouns in changelogs, it's not clear what "it" refers to here. > > And for all of these changelogs, please explicitly state that there are no VMX > controls for these instructions, assuming that's actually true. From a KVM > perspective, that's far more interesting than the details of the instruction(s). > >> operands if condition is met, which can improve database performance. >> >> The bit definition: >> CPUID.(EAX=7,ECX=1):EAX[bit 7] >> >> This patch enables this CPUID in the kernel feature bits and expose it to >> guest OS. > > Same thing here, KVM doesn't decide whether or not to expose the feature to the > guest. > >> Signed-off-by: Jiaxi Chen <jiaxi.chen@xxxxxxxxxxxxxxx> >> --- >> arch/x86/include/asm/cpufeatures.h | 1 + >> arch/x86/kvm/cpuid.c | 2 +- >> 2 files changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h >> index ef4775c6db01..445626cb5779 100644 >> --- a/arch/x86/include/asm/cpufeatures.h >> +++ b/arch/x86/include/asm/cpufeatures.h >> @@ -308,6 +308,7 @@ >> /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */ >> #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */ >> #define X86_FEATURE_AVX512_BF16 (12*32+ 5) /* AVX512 BFLOAT16 instructions */ >> +#define X86_FEATURE_CMPCCXADD (12*32+ 7) /* CMPccXADD instructions */ > > Boris, > > What do you think about moving CPUID_7_1_EAX to be a KVM-only leaf too? AFAICT, > KVM passthrough is the only reason the existing features are defined. Boris, Since CPUID_7_1_EAX has only 5 features now, it is a big waste, should we move it to KVM-only leaf as Sean suggested. What's your opinion about this? -- Regards, Jiaxi
