Re: [PATCH vhost 3/3] vhost-net: flush batched before enabling notifications

[Eugenio Perez Martin]

On Wed, Sep 17, 2025 at 8:31 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
> sendmsg") tries to defer the notification enabling by moving the logic
> out of the loop after the vhost_tx_batch() when nothing new is spotted.
> This caused unexpected side effects as the new logic is reused for
> several other error conditions.
>
> A previous patch reverted 8c2e6b26ffe2. Now, bring the performance
> back up by flushing batched buffers before enabling notifications.
>

Acked-by: Eugenio Pérez <eperezma@xxxxxxxxxx>

> Reported-by: Jon Kohler <jon@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>  drivers/vhost/net.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 57efd5c55f89..35ded4330431 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -780,6 +780,11 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)

The same optimization can be done in handle_tx_zerocopy, should it be
marked as TODO?

I guess a lot of logic could be reused from one function to the other
or, ideally, merging both handle_tx_zerocopy and handle_tx_copy.

But it is better to do it on top.

>                         break;
>                 /* Nothing new?  Wait for eventfd to tell us they refilled. */
>                 if (head == vq->num) {
> +                       /* Flush batched packets to handle pending RX
> +                        * work (if busyloop_intr is set) and to avoid
> +                        * unnecessary virtqueue kicks.
> +                        */
> +                       vhost_tx_batch(net, nvq, sock, &msg);
>                         if (unlikely(busyloop_intr)) {
>                                 vhost_poll_queue(&vq->poll);
>                         } else if (unlikely(vhost_enable_notify(&net->dev,
> --
> 2.34.1
>