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Tue, 23 Sep 2025 08:58:05 -0700 (PDT) Received: from [192.168.10.48] ([176.206.127.188]) by smtp.googlemail.com with ESMTPSA id 4fb4d7f45d1cf-62fa5f27640sm10916316a12.39.2025.09.23.08.58.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Sep 2025 08:58:04 -0700 (PDT) Message-ID: <7c7a5a75-a786-4a05-a836-4368582ca4c2@redhat.com> Date: Tue, 23 Sep 2025 17:58:03 +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: Maxim Levitsky , kvm@vger.kernel.org Cc: Sean Christopherson , 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> 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: <20250813192313.132431-3-mlevitsk@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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, but there are already enough memory barriers to tame it. That said, in_guest_mode is a red herring. The way I look at it, is through the very common wake/sleep (or kick/check) pattern that has smp_mb() on both sides. The pair you are considering consists of this function (the "kick side"), and the wrmsr handler for MSR_KVM_ASYNC_PF_ACK (the "check side"). Let's see how the synchronization between the two sides happens: - here, you need to check whether to inject the interrupt. It looks like this: kvm_make_request(KVM_REQ_APF_READY, vcpu); smp_mb(); if (!READ_ONCE(page_ready_pending)) kvm_vcpu_kick(vcpu); - on the other side, after clearing page_ready_pending, there will be a check for a wakeup: WRITE_ONCE(page_ready_pending, false); smp_mb(); if (kvm_check_request(KVM_REQ_APF_READY, vcpu)) kvm_check_async_pf_completion(vcpu) except that the "if" is not in kvm_set_msr_common(); it will happen naturally as part of the first re-entry. So let's look at the changes you need to make, in order to make the code look like the above. - using READ_ONCE/WRITE_ONCE for pageready_pending never hurts - here in kvm_arch_async_page_present_queued(), a smp_mb__after_atomic() (compiler barrier on x86) is missing after kvm_make_request(): kvm_make_request(KVM_REQ_APF_READY, vcpu); /* * Tell vCPU to wake up before checking if they need an * interrupt. Pairs with any memory barrier between * the clearing of pageready_pending and vCPU entry. */ smp_mb__after_atomic(); if (!READ_ONCE(vcpu->arch.apf.pageready_pending)) kvm_vcpu_kick(vcpu); - in kvm_set_msr_common(), there are two possibilities. The easy one is to just use smp_store_mb() to clear vcpu->arch.apf.pageready_pending. The other would be a comment like this: WRITE_ONCE(vcpu->arch.apf.pageready_pending, false); /* * Ensure they know to wake this vCPU up, before the vCPU * next checks KVM_REQ_APF_READY. Use an existing memory * barrier between here and thenext kvm_request_pending(), * for example in vcpu_run(). */ /* smp_mb(); */ plus a memory barrier in common code like this: diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 706b6fd56d3c..e302c617e4b2 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -11236,6 +11236,11 @@ static int vcpu_run(struct kvm_vcpu *vcpu) if (r <= 0) break; + /* + * Provide a memory barrier between handle_exit and the + * kvm_request_pending() read in vcpu_enter_guest(). It + * pairs with any barrier after kvm_make_request(), for + * example in kvm_arch_async_page_present_queued(). + */ + smp_mb__before_atomic(); kvm_clear_request(KVM_REQ_UNBLOCK, vcpu); if (kvm_xen_has_pending_events(vcpu)) kvm_xen_inject_pending_events(vcpu); The only advantage of this second, more complex approach is that it shows *why* the race was not happening. The 50 clock cycles saved on an MSR write are not worth the extra complication, and on a quick grep I could not find other cases which rely on the same implicit barriers. So I'd say use smp_store_mb(), with a comment about the pairing with kvm_arch_async_page_present_queued(); and write in the commit message that the race wasn't happening thanks to unrelated memory barriers between handle_exit and the kvm_request_pending() read in vcpu_enter_guest. Thanks, Paolo