[PATCH] KVM: arm/arm64: Kick VCPUs when queueing already pending IRQs
Christoffer Dall
christoffer.dall at linaro.org
Fri Oct 28 01:35:00 PDT 2016
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 <shihwei at cs.columbia.edu>
> ---
>
> 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 <christoffer.dall at linaro.org>
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