Re: [PATCH RFC 2/2] vhost: support urgent descriptors

[Jason Wang <jasowang@redhat.com>]

On 09/20/2014 06:00 PM, Paolo Bonzini wrote:
> Il 19/09/2014 09:10, Jason Wang ha scritto:
>>>>  
>>>> -	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
>>>> +	if (vq->urgent || !vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
>> So the urgent descriptor only work when event index was not enabled?
>> This seems suboptimal, we may still want to benefit from event index
>> even if urgent descriptor is used. Looks like we need return true here
>> when vq->urgent is true?
> Its ||, not &&.
>
> Without event index, all descriptors are treated as urgent.
>
> Paolo
>

The problem is if vq->urgent is true, the patch checks
VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in
virtqueue_enable_cb() regardless of event index feature and cleared
unconditionally in virtqueue_disable_cb(). So virtqueue_enable_cb() was
used to not only publish a new event index but also enable the urgent
descriptor. And virtqueue_disable_cb() disabled all interrupts including
the urgent descriptor. Guest won't get urgent interrupts by just adding
virtqueue_add_outbuf_urgent() since what it needs is to enable and
disable interrupt for !urgent descriptor.

Btw, not sure "urgent" is a suitable name, since interrupt is often slow
in kvm guest. And in fact virtio-net will probably use "urgent"
descriptor for those packets (e.g stream packets who can be delayed a
little bit to batch more bytes from userspace) who was not urgent
compared to other packets.