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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id 31sm8703050wrr.5.2020.03.16.01.33.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Mar 2020 01:33:51 -0700 (PDT) From: Vitaly Kuznetsov To: Paolo Bonzini Cc: linmiaohe@huawei.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check In-Reply-To: <1584185480-3556-1-git-send-email-pbonzini@redhat.com> References: <1584185480-3556-1-git-send-email-pbonzini@redhat.com> Date: Mon, 16 Mar 2020 09:33:50 +0100 Message-ID: <878sk0n1g1.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Paolo Bonzini writes: > After test_and_set_bit() for kvm->arch.apicv_inhibit_reasons, we will > always get false when calling kvm_apicv_activated() because it's sure > apicv_inhibit_reasons do not equal to 0. > > What the code wants to do, is check whether APICv was *already* active > and if so skip the costly request; we can do this using cmpxchg. > > Reported-by: Miaohe Lin > Signed-off-by: Paolo Bonzini > --- > arch/x86/kvm/x86.c | 25 ++++++++++++++++--------- > 1 file changed, 16 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index a7cb85231330..49efa4529662 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8049,19 +8049,26 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu) > */ > void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit) > { > + unsigned long old, new, expected; > + > if (!kvm_x86_ops->check_apicv_inhibit_reasons || > !kvm_x86_ops->check_apicv_inhibit_reasons(bit)) > return; > > - if (activate) { > - if (!test_and_clear_bit(bit, &kvm->arch.apicv_inhibit_reasons) || > - !kvm_apicv_activated(kvm)) > - return; > - } else { > - if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) || > - kvm_apicv_activated(kvm)) > - return; > - } > + old = READ_ONCE(kvm->arch.apicv_inhibit_reasons); > + do { > + expected = new = old; > + if (activate) > + __clear_bit(bit, &new); > + else > + __set_bit(bit, &new); > + if (new == old) > + break; > + old = cmpxchg(&kvm->arch.apicv_inhibit_reasons, expected, new); > + } while (old != expected); 'expected' here is a bit confusing as it's not what we expect to get as the result but rather what we expect to see pre-change. I don't have a better suggestion though. > + > + if ((old == 0) == (new == 0)) > + return; This is a very laconic expression I personally find hard to read :-) /* Check if WE actually changed APICv state */ if ((!old && !new) || (old && new)) return; would be my preference (not strong though, I read yours several times and now I feel like I understand it just fine :-) > > trace_kvm_apicv_update_request(activate, bit); > if (kvm_x86_ops->pre_update_apicv_exec_ctrl) Reviewed-by: Vitaly Kuznetsov -- Vitaly