On Wed, Oct 15, 2014 at 06:19:15PM +0800, Jason Wang wrote:
> On 10/15/2014 05:28 PM, Michael S. Tsirkin wrote:
> > On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
> >> This patch introduces virtio_enable_cb_avail() to publish avail idx
> >> and used event. This could be used by batched buffer submitting to
> >> reduce the number of tx interrupts.
> >>
> >> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> >> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> >> ---
> >> drivers/virtio/virtio_ring.c | 22 ++++++++++++++++++++--
> >> include/linux/virtio.h | 2 ++
> >> 2 files changed, 22 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> >> index 1b3929f..d67fbf8 100644
> >> --- a/drivers/virtio/virtio_ring.c
> >> +++ b/drivers/virtio/virtio_ring.c
> >> @@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> >> * entry. Always do both to keep code simple. */
> >> vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >> /* Make sure used event never go backwards */
> >> - if (!vring_need_event(vring_used_event(&vq->vring),
> >> - vq->vring.avail->idx, last_used_idx))
> >> + if (vq->vring.avail->idx != vring_used_event(&vq->vring) &&
> >> + !vring_need_event(vring_used_event(&vq->vring),
> >> + vq->vring.avail->idx, last_used_idx)) {
> >> vring_used_event(&vq->vring) = last_used_idx;
> >> + }
> >> END_USE(vq);
> >> return last_used_idx;
> >> }
> >> EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> >>
> > I see you are also changing virtqueue_enable_cb_prepare, why?
>
> This is also used to prevent it from moving the used event backwards.
> This may happens when we handle tx napi after we publish avail idx as
> used event (virtqueue_enable_cb_avail() was called).
So it's wrong exactly in the same way.
But also, please document this stuff, don't put
unrelated changes in a patch called "introduce
virtqueue_enable_cb_avail".
> >
> >> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
> >> +{
> >> + struct vring_virtqueue *vq = to_vvq(_vq);
> >> + bool ret;
> >> +
> >> + START_USE(vq);
> >> + vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >> + vring_used_event(&vq->vring) = vq->vring.avail->idx;
> >> + ret = vring_need_event(vq->vring.avail->idx,
> >> + vq->last_used_idx, vq->vring.used->idx);
> >> + END_USE(vq);
> >> +
> >> + return ret;
> >> +}
> >> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
> >> +
> >> /**
> >> * virtqueue_poll - query pending used buffers
> >> * @vq: the struct virtqueue we're talking about.
> > Could not figure out what this does.
> > Please add documentation.
> >
>
> Sure, does something like below explain what does this function do?
>
> /**
>
> * virtqueue_enable_cb_avail - restart callbacks after
> disable_cb.
> * @vq: the struct virtqueue we're talking
> about.
> *
>
> * This re-enables callbacks but hints to the other side to
> delay
> * interrupts all of the available buffers have been processed;
So this is like virtqueue_enable_cb_delayed but even more
aggressive?
I think it's too agressive: it's better to wake up guest
after you are through most of buffers, but not all,
so guest and host can work in parallel.
> * it returns "false" if there are at least one pending buffer in the
> queue,
> * to detect a possible race between the driver checking for more
> work,
> * and enabling
> callbacks.
> *
>
> * Caller must ensure we don't call this with other
> virtqueue
> * operations at the same time (except where
> noted).
> */
>
> >> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> >> index b46671e..bfaf058 100644
> >> --- a/include/linux/virtio.h
> >> +++ b/include/linux/virtio.h
> >> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
> >>
> >> unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
> >>
> >> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
> >> +
> >> bool virtqueue_poll(struct virtqueue *vq, unsigned);
> >>
> >> bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> >> --
> >> 1.7.1
> > --
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