On Wed, Dec 07, 2022, Like Xu wrote:
> On 7/12/2022 1:19 am, Sean Christopherson wrote:
> > On Tue, Dec 06, 2022, Like Xu wrote:
> > > > > 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
>
> IMO, using counters->type directly just like pmc_bitmask() will add more readability
> and opportunistically helps some stale compilers behave better.
Anyone that cares about this level of micro-optimization absolutely should be
using a toolchain that's at or near the bleeding edge.
> > > > don't have to look at counters->type to understand its possible values.
> > > When someone tries to add a new type of pmc type, the code bugs up.
> >
> > Are there new types coming along? If so, I definitely would not object to refactoring
> > this code in the context of a series that adds a new type(s). But "fixing" this one
> > case is not sufficient to support a new type, e.g. intel_is_valid_rdpmc_ecx() also
> > needs to be updated. Actually, even this function would need additional updates
> > to perform a similar sanity check.
>
> True but this part of the change is semantically relevant, which should not
> be present in a harmless generic optimization like this one. Right ?
For modern compilers, it's not an optimization.
> > if (fixed) {
> > counters = pmu->fixed_counters;
> > num_counters = pmu->nr_arch_fixed_counters;
> > } else {
> > counters = pmu->gp_counters;
> > num_counters = pmu->nr_arch_gp_counters;
> > }
> > if (idx >= num_counters)
> > return NULL;
> >
> > > And, this one will make all usage of pmu->counter_bitmask[] more consistent.
> >
> > How's that? There's literally one instance of using ->type
> >
> > static inline u64 pmc_bitmask(struct kvm_pmc *pmc)
> > {
> > struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> >
> > return pmu->counter_bitmask[pmc->type];
> > }
> >
> > everything else is hardcoded. And using pmc->type there make perfect sense in
> > that case. But in intel_rdpmc_ecx_to_pmc(), there is already usage of "fixed",
> > so IMO switching to ->type makes that function somewhat inconsistent with itself.
>
> More, it's rare to see code like " [ a ? b : c] " in the world of both KVM and x86.
There are a few false positives here, but ternary operators are common.
$ git grep ? arch/x86/kvm | wc -l
292
If you're saying that indexing an array with a ternary operator is rare, then sure,
but only because there is almost never anything that fits such a pattern, not because
it's an inherently bad pattern.
> Good practice (branchless) should be scattered everywhere and not the other
> way around.
Once again, modern compilers will not generate branches for this code.
