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

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Message-ID: <7db2bc22-99b8-96f3-66f3-d1695e2e82c1@gmail.com>
Date:   Wed, 7 Dec 2022 16:44:11 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.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 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.

>>> 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 ?

> 
> 	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.
Good practice (branchless) should be scattered everywhere and not the other way 
around.

I have absolutely no objection to your "slight preference". Thanks for your time 
in reviewing this.