On Tue, Sep 16, 2025 at 1:19 PM Michael S. Tsirkin mst@redhat.com wrote:
On Tue, Sep 16, 2025 at 10:37:35AM +0800, Jason Wang wrote:
On Tue, Sep 16, 2025 at 12:03 AM Michael S. Tsirkin mst@redhat.com wrote:
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.
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
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. */vhost_tx_batch(net, nvq, sock, &msg);So why don't we do this in the "else" branch"? If we are busy polling then we are not enabling kicks, so is there a reason to flush?
It should be functional equivalent:
do { if (head == vq->num) { vhost_tx_batch(); if (unlikely(busyloop_intr)) { vhost_poll_queue() } else if () { vhost_disable_notify(&net->dev, vq); continue; } return; }
vs
do { if (head == vq->num) { if (unlikely(busyloop_intr)) { vhost_poll_queue() } else if () { vhost_tx_batch(); vhost_disable_notify(&net->dev, vq); continue; } break; }
vhost_tx_batch(); return;
Thanks
But this is not what the code comment says:
/* Flush batched packets before enabling * virqtueue notification to reduce * unnecssary virtqueue kicks.So I ask - of we queued more polling, why do we need to flush batched packets? We might get more in the next polling round, this is what polling is designed to do.
The reason is there could be a rx work when busyloop_intr is true, so we need to flush.
Thanks
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 +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);}
static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
2.34.1