Subject: [PATCH] KVM: arm/arm64: Kick VCPUs when queueing already pending IRQs

From: [email-masked] (Christoffer Dall)
Date: Fri, 28 Oct 2016 10:35:00 +0200

On Thu, Oct 27, 2016 at 03:08:13PM +0000, Shih-Wei Li wrote:
> In cases like IPI, we could be queueing an interrupt for a VCPU
> that is already running and is not about to exit, because the
> VCPU has entered the VM with the interrupt pending and would
> not trap on EOI'ing that interrupt. This could result to delays
> in interrupt deliveries or even loss of interrupts.
> To guarantee prompt interrupt injection, here we have to try to
> kick the VCPU.
> 
> Signed-off-by: Shih-Wei Li <[email-masked]>
> ---
> 
> I've tested the code with an IPI test built on kvm-unit-test, which
> measures the cycles spent between one VCPU sending IPI to a target
> VCPU that busy loops in the VM, until the target VCPU ACKs and EOIs
> the IPI. The patch here can improve the performance in such case by
> more than 5000 cycles.
> 
>  virt/kvm/arm/vgic/vgic.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index b419a11..07cf239 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -273,6 +273,17 @@ retry:
>  		 * no more work for us to do.
>  		 */
>  		spin_unlock(&irq->irq_lock);
> +
> +		/*
> +		 * If the VCPU is not NULL, we could be queueing an
> +		 * edge-triggered interrupt for a VCPU which is already
> +		 * running and is not about to exit, because the VCPU has
> +		 * entered the VM with the interrupt pending and it wouldn't
> +		 * trap on EOI. To ensure prompt delivery of that interrupt,
> +		 * we have to try to kick the VCPU.
> +		 */

Perhaps the following comment is a better description:

		/*
		 * We have to kick the VCPU here, because we could be queueing
		 * an edge-triggered interrupt for which we get no EOI
		 * maintenance interrupt.  In that case, while the IRQ is
		 * already on the VCPU's AP list, the VCPU could have EOI'ed the
		 * original interrupt and won't see this one until it exits for
		 * some other reason.
		 */

> +		if (vcpu)
> +			kvm_vcpu_kick(vcpu);
>  		return false;
>  	}

Otherwise:

Reviewed-by: Christoffer Dall <[email-masked]>