On Wed, Sep 17, 2025 at 8:31 AM Jason Wang <jasow...@redhat.com> wrote:
>
> This reverts commit 8c2e6b26ffe243be1e78f5a4bfb1a857d6e6f6d6. It 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 will
> bring side effects as the new logic would be reused for several other
> error conditions.
>
> One example is the IOTLB: when there's an IOTLB miss, get_tx_bufs()
> might return -EAGAIN and exit the loop and see there's still available
> buffers, so it will queue the tx work again until userspace feed the
> IOTLB entry correctly. This will slowdown the tx processing and
> trigger the TX watchdog in the guest as reported in
> https://lkml.org/lkml/2025/9/10/1596.
>
> To fix, revert the change. A follow up patch will being the performance
> back in a safe way.
>
> Reported-by: Jon Kohler <j...@nutanix.com>
> Cc: sta...@vger.kernel.org
> Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
> sendmsg")
Acked-by: Eugenio Pérez <epere...@redhat.com>
> Signed-off-by: Jason Wang <jasow...@redhat.com>
> Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
> ---
> drivers/vhost/net.c | 30 +++++++++---------------------
> 1 file changed, 9 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 16e39f3ab956..57efd5c55f89 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -765,11 +765,11 @@ static void handle_tx_copy(struct vhost_net *net,
> struct socket *sock)
> int err;
> int sent_pkts = 0;
> bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
> - bool busyloop_intr;
> bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
>
> do {
> - busyloop_intr = false;
> + bool busyloop_intr = false;
> +
> if (nvq->done_idx == VHOST_NET_BATCH)
> vhost_tx_batch(net, nvq, sock, &msg);
>
> @@ -780,10 +780,13 @@ static void handle_tx_copy(struct vhost_net *net,
> struct socket *sock)
> break;
> /* Nothing new? Wait for eventfd to tell us they refilled. */
> if (head == vq->num) {
> - /* Kicks are disabled at this point, break loop and
> - * process any remaining batched packets. Queue will
> - * be re-enabled afterwards.
> - */
> + if (unlikely(busyloop_intr)) {
> + vhost_poll_queue(&vq->poll);
> + } else if (unlikely(vhost_enable_notify(&net->dev,
> + vq))) {
> + vhost_disable_notify(&net->dev, vq);
> + continue;
> + }
> break;
> }
>
> @@ -839,22 +842,7 @@ static void handle_tx_copy(struct vhost_net *net, struct
> socket *sock)
> ++nvq->done_idx;
> } while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
>
> - /* Kicks are still disabled, dispatch any remaining batched msgs. */
> vhost_tx_batch(net, nvq, sock, &msg);
> -
> - if (unlikely(busyloop_intr))
> - /* If interrupted while doing busy polling, requeue the
> - * handler to be fair handle_rx as well as other tasks
> - * waiting on cpu.
> - */
> - vhost_poll_queue(&vq->poll);
> - else
> - /* All of our work has been completed; however, before
> - * leaving the TX handler, do one last check for work,
> - * and requeue handler if necessary. If there is no work,
> - * queue will be reenabled.
> - */
> - vhost_net_busy_poll_try_queue(net, vq);
> }
>
> static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> --
> 2.34.1
>
