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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html