From 91b01895071770ed0c256869d0f94d69a2fb8ecf Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 8 Oct 2021 19:11:55 -0700 Subject: [PATCH] KVM: SVM: Ensure target pCPU is read once when signalling AVIC doorbell Ensure vcpu->cpu is read once when signalling the AVIC doorbell. If the compiler rereads the field and the vCPU is migrated between the check and writing the doorbell, KVM would signal the wrong physical CPU. Functionally, signalling the wrong CPU in this case is not an issue as task migration means the vCPU has exited and will pick up any pending interrupts on the next VMRUN. Add the READ_ONCE() purely to clean up the code. Opportunistically add a comment explaining the task migration behavior, and rename cpuid=>cpu to avoid conflating the CPU number with KVM's more common usage of CPUID. Signed-off-by: Sean Christopherson Message-Id: <20211009021236.4122790-3-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/svm/avic.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c index b7200595cbd4..0e5b49294086 100644 --- a/arch/x86/kvm/svm/avic.c +++ b/arch/x86/kvm/svm/avic.c @@ -675,10 +675,18 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec) smp_mb__after_atomic(); if (avic_vcpu_is_running(vcpu)) { - int cpuid = vcpu->cpu; + int cpu = READ_ONCE(vcpu->cpu); - if (cpuid != get_cpu()) - wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpuid)); + /* + * Note, the vCPU could get migrated to a different pCPU at any + * point, which could result in signalling the wrong/previous + * pCPU. But if that happens the vCPU is guaranteed to do a + * VMRUN (after being migrated) and thus will process pending + * interrupts, i.e. a doorbell is not needed (and the spurious + * one is harmless). + */ + if (cpu != get_cpu()) + wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu)); put_cpu(); } else kvm_vcpu_wake_up(vcpu); -- GitLab