Re: [PATCH net] virtio-net: correctly enable callback during start_xmit

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Mon, 12 Dec 2022 04:25:22 -0500, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Mon, Dec 12, 2022 at 05:10:29PM +0800, Jason Wang wrote:
> > Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
> > virtqueue callback via the following statement:
> >
> >         do {
> >            ......
> > 	} while (use_napi && kick &&
> >                unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> >
> > This will cause a missing call to virtqueue_enable_cb_delayed() when
> > kick is false. Fixing this by removing the checking of the kick from
> > the condition to make sure callback is enabled correctly.
> >
> > Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> > The patch is needed for -stable.
>
> stable rules don't allow for theoretical fixes. Was a problem observed?
>
> > ---
> >  drivers/net/virtio_net.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 86e52454b5b5..44d7daf0267b 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1834,8 +1834,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> >
> >  		free_old_xmit_skbs(sq, false);
> >
> > -	} while (use_napi && kick &&
> > -	       unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> > +	} while (use_napi &&
> > +		 unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> >
>
> A bit more explanation pls.  kick simply means !netdev_xmit_more -
> if it's false we know there will be another packet, then transmissing
> that packet will invoke virtqueue_enable_cb_delayed. No?

It's just that there may be a next packet, but in fact there may not be.
For example, the vq is full, and the driver stops the queue.

Thanks.

>
>
>
>
>
> >  	/* timestamp packet in software */
> >  	skb_tx_timestamp(skb);
> > --
> > 2.25.1
>
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