On Mon, Dec 05, 2022, Like Xu wrote: > From: Like Xu <likexu@xxxxxxxxxxx> > > In either case, the counters will point to fixed or gp pmc array, and > taking advantage of the C pointer, it's reasonable to use an almost known > mem load operation directly without disturbing the branch predictor. The compiler is extremely unlikely to generate a branch for this, e.g. gcc-12 uses setne and clang-14 shifts "fixed" by 30. FWIW, clang is also clever enough to use a cmov to load the address of counters, i.e. the happy path will have no taken branches for either type of counter. > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx> > --- > arch/x86/kvm/vmx/pmu_intel.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c > index e5cec07ca8d9..28b0a784f6e9 100644 > --- a/arch/x86/kvm/vmx/pmu_intel.c > +++ b/arch/x86/kvm/vmx/pmu_intel.c > @@ -142,7 +142,7 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu, > } > if (idx >= num_counters) > return NULL; > - *mask &= pmu->counter_bitmask[fixed ? KVM_PMC_FIXED : KVM_PMC_GP]; > + *mask &= pmu->counter_bitmask[counters->type]; In terms of readability, I have a slight preference for the current code as I don't have to look at counters->type to understand its possible values.
