Re: [PATCH net 2/2] vhost-net: correctly flush batched packet before enabling notification

Jon Kohler
2025-09-12T15:25:06Z

> On Sep 12, 2025, at 4:50 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>
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> On Fri, Sep 12, 2025 at 04:26:58PM +0800, Jason Wang 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 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 may
>> trigger the TX watchdog in the guest.
>
> It's not that it might.
> Pls clarify that it *has been reported* to do exactly that,
> and add a link to the report.
>
>
>> Fixing this by stick the notificaiton enabling logic inside the loop
>> when nothing new is spotted and flush the batched before.
>>
>> Reported-by: Jon Kohler <jon@nutanix.com>
>> Cc: stable@vger.kernel.org
>> Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> So this is mostly a revert, but with
> vhost_tx_batch(net, nvq, sock, &msg);
> added in to avoid regressing performance.
>
> If you do not want to structure it like this (revert+optimization),
> then pls make that clear in the message.
>
>
>> ---
>> drivers/vhost/net.c | 33 +++++++++++++--------------------
>> 1 file changed, 13 insertions(+), 20 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 16e39f3ab956..3611b7537932 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 +780,18 @@ 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.
>> + /* Flush batched packets before enabling
>> + * virqtueue notification to reduce
>> + * unnecssary virtqueue kicks.
>
> typos: virtqueue, unnecessary
>
>> */
>> + vhost_tx_batch(net, nvq, sock, &msg);
>> + 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;
>> }

See my comment below, but how about something like this?
if (head == vq->num) {
/* Flush batched packets before enabling
* virtqueue notification to reduce
* unnecessary virtqueue kicks.
*/
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);
break;
}

>>
>> @@ -839,22 +847,7 @@ 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);

Note: the use of vhost_net_busy_poll_try_queue was intentional in my
patch as it was checking to see both conditionals.

Can we simply hoist my logic up instead?

>> }
>>
>> static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>> --
>> 2.34.1
>

Tested-by: Jon Kohler <jon@nutanix.com <mailto:jon@nutanix.com>>

Tried this out on a 6.16 host / guest that locked up with iotlb miss loop,
applied this patch and all was well.