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Date:   Wed, 7 Dec 2022 17:48:58 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Like Xu <like.xu.linux@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: Avoid ternary operator by directly
 referring to counters->type
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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.