On Fri, 2021-10-08 at 19:11 -0700, Sean Christopherson wrote: > 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.
Since vcpu->cpu can change any moment anyway, adding READ_ONCE I think can't really fix anything but I do agree that it makes this more readable. Reviewed-by: Maxim Levitsky <[email protected]> > > 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 <[email protected]> > --- > arch/x86/kvm/svm/avic.c | 13 ++++++++++--- > 1 file changed, 10 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c > index 8052d92069e0..208c5c71e827 100644 > --- a/arch/x86/kvm/svm/avic.c > +++ b/arch/x86/kvm/svm/avic.c > @@ -675,10 +675,17 @@ 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) > + */ > + if (cpu != get_cpu()) > + wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu)); > put_cpu(); > } else > kvm_vcpu_wake_up(vcpu); _______________________________________________ kvmarm mailing list [email protected] https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
