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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id m17-20020a170902f21100b00189393ab02csm14805031plc.99.2022.12.07.09.49.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Dec 2022 09:49:02 -0800 (PST) Date: Wed, 7 Dec 2022 17:48:58 +0000 From: Sean Christopherson To: Like Xu Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] KVM: x86/pmu: Avoid ternary operator by directly referring to counters->type Message-ID: References: <20221205113718.1487-1-likexu@tencent.com> <38b2a836-f9a4-23e4-107b-61efc74638a4@gmail.com> <7db2bc22-99b8-96f3-66f3-d1695e2e82c1@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7db2bc22-99b8-96f3-66f3-d1695e2e82c1@gmail.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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.