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 10:18:19 +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 6/12/2022 12:46 am, Sean Christopherson wrote:
> On Mon, Dec 05, 2022, Like Xu wrote:
>> From: Like Xu <likexu@...cent.com>
>>
>> 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.

If so, good news for users of the new tool chain. I assume our Linux project is also
to be commended when it comes to supporting legacy issues even if just a little.

> 
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
>>   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.
When someone tries to add a new type of pmc type, the code bugs up. And,
this one will make all usage of pmu->counter_bitmask[] more consistent.

Please reconsider this minor diff if it does no harm.