Subject: Re: [PATCH] virtio: Introduce a new kick interface virtqueue_kick_try()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 19 Oct 2021 06:37:00 -0400

On Tue, Oct 19, 2021 at 04:13:19PM +0800, Xuan Zhuo wrote:
> On Mon, 31 May 2021 14:34:16 +0800, Jason Wang <[email-masked]> wrote:
> >
> > ÔÚ 2021/5/19 ÏÂÎç7:47, Xuan Zhuo Ð´µÀ:
> > > Unlike virtqueue_kick(), virtqueue_kick_try() returns true only when the
> > > kick is successful. In virtio-net, we want to count the number of kicks.
> > > So we need an interface that can perceive whether the kick is actually
> > > executed.
> > >
> > > Signed-off-by: Xuan Zhuo <[email-masked]>
> >
> >
> > Acked-by: Jason Wang <[email-masked]>
> 
> Hi, this patch seems to have not been merged, is there something wrong with me?
> 
> Thanks.

The commit log does not make it clear, but this is just
code refactoring. Pls make it clearer in the log.
Also, if we add a new API like this as a cleanup,
it needs to be documented much better.

> >
> > Thanks
> >
> >
> > > ---
> > >   drivers/net/virtio_net.c     |  8 ++++----
> > >   drivers/virtio/virtio_ring.c | 20 ++++++++++++++++++++
> > >   include/linux/virtio.h       |  2 ++
> > >   3 files changed, 26 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 9b6a4a875c55..167697030cb6 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -617,7 +617,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> > >   	ret = nxmit;
> > >
> > >   	if (flags & XDP_XMIT_FLUSH) {
> > > -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq))
> > > +		if (virtqueue_kick_try(sq->vq))
> > >   			kicks = 1;
> > >   	}
> > >   out:
> > > @@ -1325,7 +1325,7 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
> > >   		if (err)
> > >   			break;
> > >   	} while (rq->vq->num_free);
> > > -	if (virtqueue_kick_prepare(rq->vq) && virtqueue_notify(rq->vq)) {
> > > +	if (virtqueue_kick_try(rq->vq)) {
> > >   		unsigned long flags;
> > >
> > >   		flags = u64_stats_update_begin_irqsave(&rq->stats.syncp);
> > > @@ -1533,7 +1533,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> > >
> > >   	if (xdp_xmit & VIRTIO_XDP_TX) {
> > >   		sq = virtnet_xdp_get_sq(vi);
> > > -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> > > +		if (virtqueue_kick_try(sq->vq)) {
> > >   			u64_stats_update_begin(&sq->stats.syncp);
> > >   			sq->stats.kicks++;
> > >   			u64_stats_update_end(&sq->stats.syncp);
> > > @@ -1710,7 +1710,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > >   	}
> > >
> > >   	if (kick || netif_xmit_stopped(txq)) {
> > > -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> > > +		if (virtqueue_kick_try(sq->vq)) {
> > >   			u64_stats_update_begin(&sq->stats.syncp);
> > >   			sq->stats.kicks++;
> > >   			u64_stats_update_end(&sq->stats.syncp);
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 71e16b53e9c1..1462be756875 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -1874,6 +1874,26 @@ bool virtqueue_kick(struct virtqueue *vq)
> > >   }
> > >   EXPORT_SYMBOL_GPL(virtqueue_kick);
> > >
> > > +/**
> > > + * virtqueue_kick_try - try update after add_buf
> > > + * @vq: the struct virtqueue
> > > + *
> > > + * After one or more virtqueue_add_* calls, invoke this to kick
> > > + * the other side.
> > > + *
> > > + * Caller must ensure we don't call this with other virtqueue
> > > + * operations at the same time (except where noted).
> > > + *
> > > + * Returns true if kick success, otherwise false.

on a successful kick?

> > > + */

I don't really understand what this is doing, the comment
doesn't seem to explain. Try implies it might fail to update.

virtqueue_kick seems to be documented the same:
 * Returns false if kick failed, otherwise true.

> > > +bool virtqueue_kick_try(struct virtqueue *vq)
> > > +{
> > > +	if (virtqueue_kick_prepare(vq) && virtqueue_notify(vq))
> > > +		return true;
> > > +	return false;
> > > +}
> > > +EXPORT_SYMBOL_GPL(virtqueue_kick_try);
> > > +
> > >   /**
> > >    * virtqueue_get_buf - get the next used buffer
> > >    * @_vq: the struct virtqueue we're talking about.
> > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > index b1894e0323fa..45cd6a0af24d 100644
> > > --- a/include/linux/virtio.h
> > > +++ b/include/linux/virtio.h
> > > @@ -59,6 +59,8 @@ int virtqueue_add_sgs(struct virtqueue *vq,
> > >
> > >   bool virtqueue_kick(struct virtqueue *vq);
> > >
> > > +bool virtqueue_kick_try(struct virtqueue *vq);
> > > +
> > >   bool virtqueue_kick_prepare(struct virtqueue *vq);
> > >
> > >   bool virtqueue_notify(struct virtqueue *vq);
> >