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

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Message-ID: <Y44gbvm5Zb7a1Sbj@google.com>
Date:   Mon, 5 Dec 2022 16:46:38 +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 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.

> 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.