[PATCH] hw/char/virtio-serial-bus: fix: Unpop throttled VirtQueueElement to queue before discard vq data

["AIERPATIJIANG1 [艾尔帕提江·阿布都赛买提]" <AIERPATIJIANG1@kingsoft.com>]

Ports enter a "throttled" state when writing to the chardev would block.
The current output VirtQueueElement is kept around until the chardev
becomes writable again.
 
Because closing the virtio serial device does not reset the queue, we cannot
directly discard this element,  otherwise the control variables of the front
and back ends of the queue are inconsistent such as used_index. We should unpop the
VirtQueueElement to queue, let discard_vq_data process it.
 
The test environment:
kernel: linux-5.12
Qemu command:
Qemu-system-x86 -machine pc,accel=kvm \
    -cpu host,host-phys-bits \
    -smp 4 \
    -m 4G \
    -kernel ./kernel \
    -display none \
    -nodefaults \
    -serial mon:stdio \
    -append "panic=1 no_timer_check noreplace-smp rootflags=data=ordered rootfstype=ext4 console=ttyS0 reboot=k root=/dev/vda1 rw" \
    -drive id=os,file=./disk,if=none \
    -device virtio-blk-pci,drive=os \
    -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x4 \
    -chardev socket,id=charchannel0,path=/tmp/char-dev-test,server,nowait \
  -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0
 
full up virtio queue after VM started:
Cat /large-file > /dev/vport1p1
 
Host side:
Open and close character device sockets repeatedly
 
After awhile we can’t write any request to /dev/vport1p1 at VM side, VM kernel soft lockup at drivers/char/virtio_console.c: __send_to_port
 
 
Signed-off-by: Arafatms <aierpatijiang1@kingsoft.com>
 
diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
index dd6bc27b3b..36236defdf 100644
--- a/hw/char/virtio-serial-bus.c
+++ b/hw/char/virtio-serial-bus.c
@@ -150,8 +150,12 @@ static void discard_vq_data(VirtQueue *vq, VirtIODevice *vdev)
 
static void discard_throttle_data(VirtIOSerialPort *port)
{
+    if (!virtio_queue_ready(port->ovq)) {
+        return;
+    }
+
     if (port->elem) {
-        virtqueue_detach_element(port->ovq, port->elem, 0);
+        virtqueue_unpop(port->ovq, port->elem, 0);
         g_free(port->elem);
         port->elem = NULL;
     }