Re: [PATCH net-next v3] virtio-net: xsk: support tx wake up

[Menglong Dong <menglong.dong@linux.dev>]

On 2026/6/22 06:31 Michael S. Tsirkin <mst@redhat.com> write:
> On Tue, Jun 16, 2026 at 07:59:12PM +0800, Menglong Dong wrote:
[...]
> >  
> > +	vring_size = virtqueue_get_vring_size(sq->vq);
> > +	need_wakeup = xsk_uses_need_wakeup(pool);
> > +
> > +	if (need_wakeup && vring_size == sq->vq->num_free)
> > +		xsk_set_tx_need_wakeup(pool);
> > +
> 
> why are we doing this here?
> the check after virtnet_xsk_xmit_batch not enough?
> I vaguely think it's some kind of race we are closing?
> Pls add a comment to explain.

Hi, Michael. Thanks for your review.

Yeah, it's for a race condition between user space and kernel
space. I added a comment in V2, which is too confusing, and
I removed it 😢. I'll make it more clear and add it in the V4. The
origin comment is:

 * If the sq->vq is empty, and the tx ring is empty, and the user
 * submit an entry to the tx ring after virtnet_xsk_xmit_batch() and
 * before xsk_set_tx_need_wakeup(), we will lose the chance to wake
 * up the tx napi, so we have to set the need_wakeup flag here.

And the logic is like this:

Kernel: tx NAPI is waked up from skb_xmit_done() ->
Kernel: sq->vq and xsk->tx_ring are both empty ->
Kernel: call virtnet_xsk_xmit_batch()

    User: submit a entry to the xsk->tx_ring
    User: check the wakeup flag
    User: wakeup flag is not set, skip send()

Kernel: call xsk_set_tx_need_wakeup(), because sq->vq is empty

If we don't send more data, the data in the xsk->tx_ring will
not be sent forever.

> 
> >  	sent = virtnet_xsk_xmit_batch(sq, pool, budget, &kicks);
> >  
> > +	if (need_wakeup) {
> > +		if (vring_size == sq->vq->num_free)
> > +			/* we can't wake up by ourself, and it should be done
> > +			 * by the user.
> > +			 */
> > +			xsk_set_tx_need_wakeup(pool);
> > +		else
> > +			/* we can wake up from skb_xmit_done() */
> > +			xsk_clear_tx_need_wakeup(pool);
> 
> But what if we don't have get tx napi so no wakeup in skb_xmit_done?

Sorry that I'm not sure what "get tx napi" means here ;(

There are entry in sq->vq, so skb_xmit_done() will be called after
the entries in the ring is consumed by the HOST, right?
Then, the corresponding sq->napi will be scheduled, as we ensure
that tx napi is always enabled, which means napi->weight is not
zero, in this commit:
1df5116a41a8 ("virtio_net: xsk: prevent disable tx napi")

Right?

Thanks!
Menglong Dong

> 
> 
> > +	}
> > +
> >  	if (!is_xdp_raw_buffer_queue(vi, sq - vi->sq))
> >  		check_sq_full_and_disable(vi, vi->dev, sq);
> >  
> > @@ -1470,9 +1488,6 @@ static bool virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
> >  	u64_stats_add(&sq->stats.xdp_tx,  sent);
> >  	u64_stats_update_end(&sq->stats.syncp);
> >  
> > -	if (xsk_uses_need_wakeup(pool))
> > -		xsk_set_tx_need_wakeup(pool);
> > -
> >  	return sent;
> >  }
> >  
> > -- 
> > 2.54.0
> 
> 
>