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 <mlevitsk@xxxxxxxxxx>
>
> 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 <seanjc@xxxxxxxxxx>
> ---
> 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);
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