linux-kernel - Re: [PATCH] KVM: x86/pmu: Avoid ternary operator by directly referring to counters->type

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Date:   Tue, 6 Dec 2022 17:19:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: Avoid ternary operator by directly
 referring to counters->type

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
> > 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.

	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.