Re: [PATCH v3 09/10] KVM: arm/arm64: use vcpu requests for irq injection

Paolo Bonzini
2017-05-04T11:47:46Z

On 03/05/2017 18:06, Andrew Jones wrote:
> Don't use request-less VCPU kicks when injecting IRQs, as a VCPU
> kick meant to trigger the interrupt injection could be sent while
> the VCPU is outside guest mode, which means no IPI is sent, and
> after it has called kvm_vgic_flush_hwstate(), meaning it won't see
> the updated GIC state until its next exit some time later for some
> other reason. The receiving VCPU only needs to check this request
> in VCPU RUN to handle it. By checking it, if it's pending, a
> memory barrier will be issued that ensures all state is visible.
> We still create a vcpu_req_irq_pending() function (which is a nop),
> though, in order to allow us to use the standard request checking
> pattern.

I wonder if you aren't just papering over this race:

/*
* If there are no virtual interrupts active or pending for this
* VCPU, then there is no work to do and we can bail out without
* taking any lock. There is a potential race with someone injecting
* interrupts to the VCPU, but it is a benign race as the VCPU will
* either observe the new interrupt before or after doing this check,
* and introducing additional synchronization mechanism doesn't change
* this.
*/
if (list_empty(&vcpu->arch.vgic_cpu.ap_list_head))
return;

spin_lock(&vcpu->arch.vgic_cpu.ap_list_lock);
vgic_flush_lr_state(vcpu);
spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);

not being so "benign" after all. :) Maybe you can remove the if (list_empty()),
and have kvm_arch_vcpu_ioctl_run do this instead:

if (kvm_request_pending(vcpu)) {
if (kvm_check_request(KVM_REQ_SLEEP, vcpu))
vcpu_req_sleep(vcpu);
}

preempt_disable();

kvm_pmu_flush_hwstate(vcpu);
kvm_timer_flush_hwstate(vcpu);

if (kvm_check_request(KVM_REQ_IRQ_PENDING, vcpu))
kvm_vgic_flush_hwstate(vcpu);

?

Paolo

> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
> arch/arm/include/asm/kvm_host.h | 1 +
> arch/arm/kvm/arm.c | 12 ++++++++++++
> arch/arm64/include/asm/kvm_host.h | 1 +
> virt/kvm/arm/arch_timer.c | 1 +
> virt/kvm/arm/vgic/vgic.c | 9 +++++++--
> 5 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 41669578b3df..7bf90aaf2e87 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -47,6 +47,7 @@
>
> #define KVM_REQ_SLEEP \
> KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_NO_WAKEUP | KVM_REQUEST_WAIT)
> +#define KVM_REQ_IRQ_PENDING KVM_ARCH_REQ(1)
>
> u32 *kvm_vcpu_reg(struct kvm_vcpu *vcpu, u8 reg_num, u32 mode);
> int __attribute_const__ kvm_target_cpu(void);
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index d62e99885434..330064475914 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -581,6 +581,15 @@ static void vcpu_req_sleep(struct kvm_vcpu *vcpu)
> (!vcpu->arch.pause)));
> }
>
> +static void vcpu_req_irq_pending(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * Nothing to do here. kvm_check_request() already issued a memory
> + * barrier that pairs with kvm_make_request(), so all hardware state
> + * we need to flush should now be visible.
> + */
> +}
> +
> static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
> {
> return vcpu->arch.target >= 0;
> @@ -634,6 +643,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> if (kvm_request_pending(vcpu)) {
> if (kvm_check_request(KVM_REQ_SLEEP, vcpu))
> vcpu_req_sleep(vcpu);
> + if (kvm_check_request(KVM_REQ_IRQ_PENDING, vcpu))
> + vcpu_req_irq_pending(vcpu);
> }
>
> /*
> @@ -777,6 +788,7 @@ static int vcpu_interrupt_line(struct kvm_vcpu *vcpu, int number, bool level)
> * trigger a world-switch round on the running physical CPU to set the
> * virtual IRQ/FIQ fields in the HCR appropriately.
> */
> + kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> kvm_vcpu_kick(vcpu);
>
> return 0;
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 04c0f9d37386..2c33fef945fe 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -44,6 +44,7 @@
>
> #define KVM_REQ_SLEEP \
> KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_NO_WAKEUP | KVM_REQUEST_WAIT)
> +#define KVM_REQ_IRQ_PENDING KVM_ARCH_REQ(1)
>
> int __attribute_const__ kvm_target_cpu(void);
> int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 5976609ef27c..469b43315c0a 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -95,6 +95,7 @@ static void kvm_timer_inject_irq_work(struct work_struct *work)
> * If the vcpu is blocked we want to wake it up so that it will see
> * the timer has expired when entering the guest.
> */
> + kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> kvm_vcpu_kick(vcpu);
> }
>
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index 3d0979c30721..bdd4b3a953b5 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -283,8 +283,10 @@ bool vgic_queue_irq_unlock(struct kvm *kvm, struct vgic_irq *irq)
> * won't see this one until it exits for some other
> * reason.
> */
> - if (vcpu)
> + if (vcpu) {
> + kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> kvm_vcpu_kick(vcpu);
> + }
> return false;
> }
>
> @@ -330,6 +332,7 @@ bool vgic_queue_irq_unlock(struct kvm *kvm, struct vgic_irq *irq)
> spin_unlock(&irq->irq_lock);
> spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);
>
> + kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> kvm_vcpu_kick(vcpu);
>
> return true;
> @@ -719,8 +722,10 @@ void vgic_kick_vcpus(struct kvm *kvm)
> * a good kick...
> */
> kvm_for_each_vcpu(c, vcpu, kvm) {
> - if (kvm_vgic_vcpu_pending_irq(vcpu))
> + if (kvm_vgic_vcpu_pending_irq(vcpu)) {
> + kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> kvm_vcpu_kick(vcpu);
> + }
> }
> }
>
>