Subject: Re: [RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 13:41:30 +0300

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 <[email-masked]>
> >> Cc: Michael S. Tsirkin <[email-masked]>
> >> Signed-off-by: Jason Wang <[email-masked]>
> >> ---
> >>  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
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > the body of a message to [email-masked]
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/