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([2001:b07:6468:f312:5e2c:eb9a:a8b6:fd3e]) by smtp.gmail.com with ESMTPSA id v15sm1980057wmj.39.2021.08.03.02.11.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Aug 2021 02:11:14 -0700 (PDT) To: Maxim Levitsky , kvm@vger.kernel.org Cc: Wanpeng Li , Thomas Gleixner , Joerg Roedel , Borislav Petkov , Sean Christopherson , Jim Mattson , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Suravee Suthikulpanit , Vitaly Kuznetsov , Ingo Molnar , "H. Peter Anvin" References: <20210802183329.2309921-1-mlevitsk@redhat.com> <20210802183329.2309921-10-mlevitsk@redhat.com> From: Paolo Bonzini Subject: Re: [PATCH v3 09/12] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use Message-ID: Date: Tue, 3 Aug 2021 11:11:13 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210802183329.2309921-10-mlevitsk@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/08/21 20:33, Maxim Levitsky wrote: > + if (!auto_eoi_old && auto_eoi_new) { > + if (atomic_inc_return(&hv->synic_auto_eoi_used) == 1) > + kvm_request_apicv_update(vcpu->kvm, false, > + APICV_INHIBIT_REASON_HYPERV); > + } else if (!auto_eoi_new && auto_eoi_old) { > + if (atomic_dec_return(&hv->synic_auto_eoi_used) == 0) > + kvm_request_apicv_update(vcpu->kvm, true, > + APICV_INHIBIT_REASON_HYPERV); Hmm no, Reviewed-by rescinded. This is racy, you cannot use atomics. The check for zero needs to happen within the lock. The easiest way is to have a __kvm_request_apicv_update function that leaves it to the caller to take the lock. Then synic_update_vector can do if (!!auto_eoi_old == !!auto_eoi_new) return; mutex_lock(&kvm->apicv_update_lock); bool was_active = hv->synic_auto_eoi_used; if (auto_eoi_new) hv->synic_auto_eoi_used++; else hv->synic_auto_eoi_used--; if (!!hv->synic_auto_eoi_used != !!was_active) __kvm_request_apicv_update(vcpu->kvm, !!hv->synic_auto_eoi_used, APICV_INHIBIT_REASON_HYPERV); mutex_unlock(&kvm->apicv_update_lock); Please add a note to synic_auto_eoi_used that it is protected by apicv_update_lock. Thanks, Paolo