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([2001:b07:6468:f312:7de8:5d90:2370:d1ac]) by smtp.gmail.com with ESMTPSA id y3sm47966512wrm.46.2020.03.14.04.31.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 14 Mar 2020 04:31:29 -0700 (PDT) Subject: Re: [PATCH] KVM: X86: avoid meaningless kvm_apicv_activated() check To: Vitaly Kuznetsov , linmiaohe Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org, rkrcmar@redhat.com, sean.j.christopherson@intel.com, wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com References: <1582597279-32297-1-git-send-email-linmiaohe@huawei.com> <87d0a2n8g9.fsf@vitty.brq.redhat.com> From: Paolo Bonzini Message-ID: <7c0aedea-36dc-bd84-b7ba-1aa6d1cceb11@redhat.com> Date: Sat, 14 Mar 2020 12:31:28 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <87d0a2n8g9.fsf@vitty.brq.redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25/02/20 13:43, Vitaly Kuznetsov wrote: > If I'm not mistaken, the logic this function was supposed to implement > is: change the requested bit to the requested state and, if > kvm_apicv_activated() changed (we set the first bit or cleared the > last), proceed with KVM_REQ_APICV_UPDATE. What if we re-write it like > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 2103101eca78..b97b8ff4a789 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8027,19 +8027,19 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_update_apicv); > */ > void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit) > { > + bool apicv_was_activated = kvm_apicv_activated(kvm); > + > 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; > - } > + if (activate) > + clear_bit(bit, &kvm->arch.apicv_inhibit_reasons); > + else > + set_bit(bit, &kvm->arch.apicv_inhibit_reasons); > + > + if (kvm_apicv_activated(kvm) == apicv_was_activated) > + return; Yes, I got to the same conclusion before seeing you message. Another possibility is to use cmpxchg, which I slightly prefer because if there are multiple concurrent updates it has some possibilities of avoiding the atomic operation and consequent cacheline bouncing. I've sent a patch. Paolo