lkml.org 
[lkml]   [2021]   [Dec]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [PATCH 3/6] KVM: SVM: fix AVIC race of host->guest IPI delivery vs AVIC inhibition
    From
    On 12/9/21 12:54, Maxim Levitsky wrote:
    > If svm_deliver_avic_intr is called just after the target vcpu's AVIC got
    > inhibited, it might read a stale value of vcpu->arch.apicv_active
    > which can lead to the target vCPU not noticing the interrupt.
    >
    > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
    > ---
    > arch/x86/kvm/svm/avic.c | 16 +++++++++++++---
    > 1 file changed, 13 insertions(+), 3 deletions(-)
    >
    > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
    > index 859ad2dc50f1..8c1b934bfa9b 100644
    > --- a/arch/x86/kvm/svm/avic.c
    > +++ b/arch/x86/kvm/svm/avic.c
    > @@ -691,6 +691,15 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
    > * automatically process AVIC interrupts at VMRUN.
    > */
    > if (vcpu->mode == IN_GUEST_MODE) {
    > +
    > + /*
    > + * At this point we had read the vcpu->arch.apicv_active == true
    > + * and the vcpu->mode == IN_GUEST_MODE.
    > + * Since we have a memory barrier after setting IN_GUEST_MODE,
    > + * it ensures that AVIC inhibition is complete and thus
    > + * the target is really running with AVIC enabled.
    > + */
    > +
    > int cpu = READ_ONCE(vcpu->cpu);

    I don't think it's correct. The vCPU has apicv_active written (in
    kvm_vcpu_update_apicv) before vcpu->mode.

    For the acquire/release pair to work properly you need to 1) read
    apicv_active *after* vcpu->mode here 2) use store_release and
    load_acquire for vcpu->mode, respectively in vcpu_enter_guest and here.

    Paolo

    > /*
    > @@ -706,10 +715,11 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
    > put_cpu();
    > } else {
    > /*
    > - * Wake the vCPU if it was blocking. KVM will then detect the
    > - * pending IRQ when checking if the vCPU has a wake event.
    > + * Kick the target vCPU otherwise, to make sure
    > + * it processes the interrupt even if its AVIC is inhibited.
    > */
    > - kvm_vcpu_wake_up(vcpu);
    > + kvm_make_request(KVM_REQ_EVENT, vcpu);
    > + kvm_vcpu_kick(vcpu);
    > }
    >
    > return 0;
    >

    \
     
     \ /
      Last update: 2021-12-09 15:12    [W:4.637 / U:0.476 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site