[PATCH] virtio-serial-bus: Discard throttled VirtQueueElement when virtio-serial closed

[PATCH] virtio-serial-bus: Discard throttled VirtQueueElement when virtio-serial closed

[Sun Feng]

With commit d4c19cde("virtio-serial: add missing virtio_detach_element() call"),
when virtio serial is throttled and closed by host, port->elem should be discard with virtqueue_push,
otherwise virtqueue will not rewind, guest will blocked finally and cannot write anymore data.

It can be reproduced with following steps:
Create a vm with virtio-serial device through libvirt:

    <channel type='unix'>
      <source mode='bind' path='/tmp/test'/>
      <target type='virtio' name='com.test.channel.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>

Host run:
watch -n0.5 'timeout 0.5 nc -U /tmp/test'

Guest run:
hexdump -C -v /dev/zero > /dev/vport1p1

After sometime, guest can not write any data to serial.
We can see vq->last_avail_idx - vq->used_idx = 128 with gdb.

(gdb) p *(struct VirtQueue *) 0x565161c5c788
$4 = {vring = {num = 128, num_default = 128, align = 4096, desc = 4316049408, avail = 4316051456, used = 4316051776,
    caches = 0x7fc530067e60}, used_elems = 0x565161c88dc0, last_avail_idx = 7929, last_avail_wrap_counter = true,
  shadow_avail_idx = 7929, shadow_avail_wrap_counter = true, used_idx = 7801, used_wrap_counter = true, signalled_used = 7801,
  signalled_used_valid = true, notification = true, queue_index = 5, inuse = 0, vector = 1,
  handle_output = 0x56515da8aea0 <handle_output>, handle_aio_output = 0x0, vdev = 0x565161c54e30, guest_notifier = {rfd = 0,
    wfd = 0}, host_notifier = {rfd = 74, wfd = 74}, host_notifier_enabled = true, node = {le_next = 0x565161c5c6f0,
    le_prev = 0x565161c5c8a8}}

Fixes: d4c19cde("virtio-serial: add missing virtio_detach_element() call")
Signed-off-by: Sun Feng <loyou85@gmail.com>
---
 hw/char/virtio-serial-bus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
index dd619f0..30b00a4 100644
--- a/hw/char/virtio-serial-bus.c
+++ b/hw/char/virtio-serial-bus.c
@@ -151,7 +151,7 @@ static void discard_vq_data(VirtQueue *vq, VirtIODevice *vdev)
 static void discard_throttle_data(VirtIOSerialPort *port)
 {
     if (port->elem) {
-        virtqueue_detach_element(port->ovq, port->elem, 0);
+        virtqueue_push(port->ovq, port->elem, 0);
         g_free(port->elem);
         port->elem = NULL;
     }
-- 
2.7.4

Re: [Ping][PATCH] virtio-serial-bus: Discard throttled VirtQueueElement when virtio-serial closed

[Feng Sun]

Ping again.

On Tue, 10 Oct 2023 at 14:23, Sun Feng <loyou85@gmail.com> wrote:
>
> With commit d4c19cde("virtio-serial: add missing virtio_detach_element() call"),
> when virtio serial is throttled and closed by host, port->elem should be discard with virtqueue_push,
> otherwise virtqueue will not rewind, guest will blocked finally and cannot write anymore data.
>
> It can be reproduced with following steps:
> Create a vm with virtio-serial device through libvirt:
>
>     <channel type='unix'>
>       <source mode='bind' path='/tmp/test'/>
>       <target type='virtio' name='com.test.channel.0'/>
>       <address type='virtio-serial' controller='0' bus='0' port='1'/>
>     </channel>
>
> Host run:
> watch -n0.5 'timeout 0.5 nc -U /tmp/test'
>
> Guest run:
> hexdump -C -v /dev/zero > /dev/vport1p1
>
> After sometime, guest can not write any data to serial.
> We can see vq->last_avail_idx - vq->used_idx = 128 with gdb.
>
> (gdb) p *(struct VirtQueue *) 0x565161c5c788
> $4 = {vring = {num = 128, num_default = 128, align = 4096, desc = 4316049408, avail = 4316051456, used = 4316051776,
>     caches = 0x7fc530067e60}, used_elems = 0x565161c88dc0, last_avail_idx = 7929, last_avail_wrap_counter = true,
>   shadow_avail_idx = 7929, shadow_avail_wrap_counter = true, used_idx = 7801, used_wrap_counter = true, signalled_used = 7801,
>   signalled_used_valid = true, notification = true, queue_index = 5, inuse = 0, vector = 1,
>   handle_output = 0x56515da8aea0 <handle_output>, handle_aio_output = 0x0, vdev = 0x565161c54e30, guest_notifier = {rfd = 0,
>     wfd = 0}, host_notifier = {rfd = 74, wfd = 74}, host_notifier_enabled = true, node = {le_next = 0x565161c5c6f0,
>     le_prev = 0x565161c5c8a8}}
>
> Fixes: d4c19cde("virtio-serial: add missing virtio_detach_element() call")
> Signed-off-by: Sun Feng <loyou85@gmail.com>
> ---
>  hw/char/virtio-serial-bus.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
> index dd619f0..30b00a4 100644
> --- a/hw/char/virtio-serial-bus.c
> +++ b/hw/char/virtio-serial-bus.c
> @@ -151,7 +151,7 @@ static void discard_vq_data(VirtQueue *vq, VirtIODevice *vdev)
>  static void discard_throttle_data(VirtIOSerialPort *port)
>  {
>      if (port->elem) {
> -        virtqueue_detach_element(port->ovq, port->elem, 0);
> +        virtqueue_push(port->ovq, port->elem, 0);
>          g_free(port->elem);
>          port->elem = NULL;
>      }
> --
> 2.7.4
>