Re: [PATCH 12/13] KVM: x86: Replace late check_nested_events() hack with more precise fix

[Jim Mattson]

On Wed, Apr 22, 2020 at 7:26 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Add a separate hook for checking if interrupt injection is blocked and
> use the hook to handle the case where an interrupt arrives between
> check_nested_events() and the injection logic.  Drop the retry of
> check_nested_events() that hack-a-fixed the same condition.
>
> Blocking injection is also a bit of a hack, e.g. KVM should do exiting
> and non-exiting interrupt processing in a single pass, but it's a more
> precise hack.  The old comment is also misleading, e.g. KVM_REQ_EVENT is
> purely an optimization, setting it on every run loop (which KVM doesn't
> do) should not affect functionality, only performance.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/svm/svm.c          |  1 +
>  arch/x86/kvm/vmx/vmx.c          | 13 +++++++++++++
>  arch/x86/kvm/x86.c              | 22 ++++------------------
>  4 files changed, 19 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 787636acd648..16fdeddb4a65 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1140,6 +1140,7 @@ struct kvm_x86_ops {
>         void (*queue_exception)(struct kvm_vcpu *vcpu);
>         void (*cancel_injection)(struct kvm_vcpu *vcpu);
>         bool (*interrupt_allowed)(struct kvm_vcpu *vcpu);
> +       bool (*interrupt_injection_allowed)(struct kvm_vcpu *vcpu);
>         bool (*nmi_allowed)(struct kvm_vcpu *vcpu);
>         bool (*get_nmi_mask)(struct kvm_vcpu *vcpu);
>         void (*set_nmi_mask)(struct kvm_vcpu *vcpu, bool masked);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f21f734861dd..6d3ccbfc9e6a 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3993,6 +3993,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>         .queue_exception = svm_queue_exception,
>         .cancel_injection = svm_cancel_injection,
>         .interrupt_allowed = svm_interrupt_allowed,
> +       .interrupt_injection_allowed = svm_interrupt_allowed,
>         .nmi_allowed = svm_nmi_allowed,
>         .get_nmi_mask = svm_get_nmi_mask,
>         .set_nmi_mask = svm_set_nmi_mask,
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2f8cacb3aa9b..68b3748b5383 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4550,6 +4550,18 @@ static bool vmx_interrupt_allowed(struct kvm_vcpu *vcpu)
>         return !vmx_interrupt_blocked(vcpu);
>  }
>
> +static bool vmx_interrupt_injection_allowed(struct kvm_vcpu *vcpu)
> +{
> +       /*
> +        * An IRQ must not be injected into L2 if it's supposed to VM-Exit,
> +        * e.g. if the IRQ arrived asynchronously after checking nested events.
> +        */
> +       if (is_guest_mode(vcpu) && nested_exit_on_intr(vcpu))
> +               return false;
> +
> +       return vmx_interrupt_allowed(vcpu);
> +}
> +
>  static int vmx_set_tss_addr(struct kvm *kvm, unsigned int addr)
>  {
>         int ret;
> @@ -7823,6 +7835,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
>         .queue_exception = vmx_queue_exception,
>         .cancel_injection = vmx_cancel_injection,
>         .interrupt_allowed = vmx_interrupt_allowed,
> +       .interrupt_injection_allowed = vmx_interrupt_injection_allowed,
>         .nmi_allowed = vmx_nmi_allowed,
>         .get_nmi_mask = vmx_get_nmi_mask,
>         .set_nmi_mask = vmx_set_nmi_mask,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7c49a7dc601f..d9d6028a77e0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7755,24 +7755,10 @@ static int inject_pending_event(struct kvm_vcpu *vcpu)
>                 --vcpu->arch.nmi_pending;
>                 vcpu->arch.nmi_injected = true;
>                 kvm_x86_ops.set_nmi(vcpu);
> -       } else if (kvm_cpu_has_injectable_intr(vcpu)) {
> -               /*
> -                * Because interrupts can be injected asynchronously, we are
> -                * calling check_nested_events again here to avoid a race condition.
> -                * See https://lkml.org/lkml/2014/7/2/60 for discussion about this
> -                * proposal and current concerns.  Perhaps we should be setting
> -                * KVM_REQ_EVENT only on certain events and not unconditionally?
> -                */
> -               if (is_guest_mode(vcpu) && kvm_x86_ops.check_nested_events) {
> -                       r = kvm_x86_ops.check_nested_events(vcpu);
> -                       if (r != 0)
> -                               return r;
> -               }
> -               if (kvm_x86_ops.interrupt_allowed(vcpu)) {
> -                       kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu),
> -                                           false);
> -                       kvm_x86_ops.set_irq(vcpu);
> -               }
> +       } else if (kvm_cpu_has_injectable_intr(vcpu) &&
> +                  kvm_x86_ops.interrupt_injection_allowed(vcpu)) {
> +               kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu), false);
> +               kvm_x86_ops.set_irq(vcpu);
>         }
So, that's what this mess was all about! Well, this certainly looks better.

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>