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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 22, 2014 at 05:55:23PM +0800, Jason Wang wrote:
> On 09/22/2014 02:55 PM, Michael S. Tsirkin wrote:
> > On Mon, Sep 22, 2014 at 11:30:23AM +0800, Jason Wang wrote:
> >> 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().
> > The reverse actually, right?
> 
> Ah, right.
> >
> >> 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.
> > Right, we want a new API that advances event index but does not
> > set VRING_AVAIL_F_NO_INTERRUPT.
> > IMO still want to set VRING_AVAIL_F_NO_INTERRUPT when handling tx
> > interrupts, to avoid interrupt storms.
> 
> I see, so urgent descriptor needs to be disabled in this case. But vhost
> parts need a little big changes, we could not just check
> VRING_AVAIL_F_NO_INTERRUPT when vq->urgent is true. If event index is
> enabled, we still need to check used event to make sure the current tx
> delayed interrupt works.
> 
> But just re-using VRING_AVAIL_F_NO_INTERRUPT for urgent descriptor may
> not work in some case. I see codes of virtqueue_get_buf() that may
> breaks this:
> 
>         /* If we expect an interrupt for the next entry, tell
> host                                                                                                       
> 
>          * by writing event index and flush out the write
> before                                                                                                         
> 
>          * the read in the next get_buf call. */
>         if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
>                 vring_used_event(&vq->vring) = vq->last_used_idx;
>                 virtio_mb(vq->weak_barriers);
>         }


Right, we need to change this path to use a private flag.

> Consider if only urgent descriptor is enabled, this will publish used
> event which in fact enable lots of unnecessary interrupt. In fact I
> don't quite understand how the above lines is used. Virtio-net stop the
> queue before enable the tx interrupt in start_xmit(), so the above lines
> will not run at all.

True - this code is for drivers which process VQ without
disabling interrupts first. No rule says this is not allowed.

> >> 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.
> >>
> > Yes but we are asking for an interrupt before event index is reached
> > because something is waiting for the packet to be transmitted.
> > I couldn't come up with a better name.
> >
> 
> Ok.