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Tue, 23 Sep 2025 09:23:56 -0700 (PDT) Received: from [192.168.10.48] ([176.206.127.188]) by smtp.googlemail.com with ESMTPSA id a640c23a62f3a-b29912fb4d2sm744651166b.14.2025.09.23.09.23.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Sep 2025 09:23:56 -0700 (PDT) Message-ID: <4f8b0d49-f81c-4716-a60f-2a0d7042badd@redhat.com> Date: Tue, 23 Sep 2025 18:23:54 +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 From: Paolo Bonzini 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> <7c7a5a75-a786-4a05-a836-4368582ca4c2@redhat.com> 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: <7c7a5a75-a786-4a05-a836-4368582ca4c2@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/23/25 17:58, Paolo Bonzini wrote: > - 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. Important thing that I forgot to mention: the above only covers the race case. There's also the case where KVM_REQ_APF_READY has been cleared already, and for that one the call to kvm_check_async_pf_completion() is *also* needed in kvm_set_msr_common(). Paolo > > 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 > >