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([2001:b07:6468:f312:5e2c:eb9a:a8b6:fd3e]) by smtp.googlemail.com with ESMTPSA id gt18sm7974997ejc.46.2021.11.29.09.53.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Nov 2021 09:53:53 -0800 (PST) Sender: Paolo Bonzini Message-ID: <496c2fc6-26b0-9b5d-32f4-2f9e9dd6a064@redhat.com> Date: Mon, 29 Nov 2021 18:53:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v2 11/43] KVM: Don't block+unblock when halt-polling is successful Content-Language: en-US To: Sean Christopherson , Maxim Levitsky Cc: Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , James Morse , Alexandru Elisei , Suzuki K Poulose , Atish Patra , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, David Matlack , Oliver Upton , Jing Zhang References: <20211009021236.4122790-1-seanjc@google.com> <20211009021236.4122790-12-seanjc@google.com> <4e883728e3e5201a94eb46b56315afca5e95ad9c.camel@redhat.com> From: Paolo Bonzini In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/29/21 18:25, Sean Christopherson wrote: > If a posted interrupt arrives after KVM has done its final search through the vIRR, > but before avic_update_iommu_vcpu_affinity() is called, the posted interrupt will > be set in the vIRR without triggering a host IRQ to wake the vCPU via the GA log. > > I.e. KVM is missing an equivalent to VMX's posted interrupt check for an outstanding > notification after switching to the wakeup vector. BTW Maxim reported that it can break even without assigned devices. > For now, the least awful approach is sadly to keep the vcpu_(un)blocking() hooks. I agree that the hooks cannot be dropped but the bug is reproducible with this patch, where the hooks are still there. With the hooks in place, you have: kvm_vcpu_blocking(vcpu) avic_set_running(vcpu, false) avic_vcpu_put(vcpu) avic_update_iommu_vcpu_affinity() WRITE_ONCE(...) // clear IS_RUNNING bit set_current_state() smp_mb() kvm_vcpu_check_block() return kvm_arch_vcpu_runnable() || ... return kvm_vcpu_has_events() || ... return kvm_cpu_has_interrupt() || ... return kvm_apic_has_interrupt() || ... return apic_has_interrupt_for_ppr() apic_find_highest_irr() scan vIRR This covers the barrier between the write of is_running and the read of vIRR, and the other side should be correct as well. in particular, reads of is_running always come after an atomic write to vIRR, and hence after an implicit full memory barrier. svm_deliver_avic_intr() has an smp_mb__after_atomic() after writing IRR; avic_kick_target_vcpus() even has an explicit barrier in srcu_read_lock(), between the microcode's write to vIRR and its own call to avic_vcpu_is_running(). Still it does seem to be a race that happens when IS_RUNNING=true but vcpu->mode == OUTSIDE_GUEST_MODE. This patch makes the race easier to trigger because it moves IS_RUNNING=false later. Paolo