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Tue, 23 Sep 2025 12:28:59 -0700 (PDT) Received: from [192.168.10.48] ([176.206.127.188]) by smtp.googlemail.com with ESMTPSA id a640c23a62f3a-b2adc17af3asm676542166b.6.2025.09.23.12.28.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Sep 2025 12:28:58 -0700 (PDT) Message-ID: <23f11dc1-4fd1-4286-a69a-3892a869ed33@redhat.com> Date: Tue, 23 Sep 2025 21:28:57 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/3] KVM: x86: Fix a semi theoretical bug in kvm_arch_async_page_present_queued To: Sean Christopherson Cc: Maxim Levitsky , kvm@vger.kernel.org, Dave Hansen , "H. Peter Anvin" , Ingo Molnar , Thomas Gleixner , x86@kernel.org, Borislav Petkov , linux-kernel@vger.kernel.org References: <20250813192313.132431-1-mlevitsk@redhat.com> <20250813192313.132431-3-mlevitsk@redhat.com> <7c7a5a75-a786-4a05-a836-4368582ca4c2@redhat.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/23/25 20:55, Sean Christopherson wrote: > On Tue, Sep 23, 2025, Paolo Bonzini wrote: >> On 8/13/25 21:23, Maxim Levitsky wrote: >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index 9018d56b4b0a..3d45a4cd08a4 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -13459,9 +13459,14 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu, >>> void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu) >>> { >>> - kvm_make_request(KVM_REQ_APF_READY, vcpu); >>> - if (!vcpu->arch.apf.pageready_pending) >>> + /* Pairs with smp_store_release in vcpu_enter_guest. */ >>> + bool in_guest_mode = (smp_load_acquire(&vcpu->mode) == IN_GUEST_MODE); >>> + bool page_ready_pending = READ_ONCE(vcpu->arch.apf.pageready_pending); >>> + >>> + if (!in_guest_mode || !page_ready_pending) { >>> + kvm_make_request(KVM_REQ_APF_READY, vcpu); >>> kvm_vcpu_kick(vcpu); >>> + } >> >> Unlike Sean, I think the race exists in abstract and is not benign > > How is it not benign? I never said the race doesn't exist, I said that consuming > a stale vcpu->arch.apf.pageready_pending in kvm_arch_async_page_present_queued() > is benign. In principle there is a possibility that a KVM_REQ_APF_READY is missed. Just by the reading of the specs, without a smp__mb_after_atomic() this is broken: kvm_make_request(KVM_REQ_APF_READY, vcpu); if (!vcpu->arch.apf.pageready_pending) kvm_vcpu_kick(vcpu); It won't happen because set_bit() is written with asm("memory"), because x86 set_bit() does prevent reordering at the processor level, etc. In other words the race is only avoided by the fact that compiler reorderings are prevented even in cases that memory-barriers.txt does not promise. Paolo