[PATCH v3 6/6] virtio-net: don't handle mq request in userspace handler for vhost-vdpa

[Si-Wei Liu <si-wei.liu@oracle.com>]

virtio_queue_host_notifier_read() tends to read pending event
left behind on ioeventfd in the vhost_net_stop() path, and
attempts to handle outstanding kicks from userspace vq handler.
However, in the ctrl_vq handler, virtio_net_handle_mq() has a
recursive call into virtio_net_set_status(), which may lead to
segmentation fault as shown in below stack trace:

0  0x000055f800df1780 in qdev_get_parent_bus (dev=0x0) at ../hw/core/qdev.c:376
1  0x000055f800c68ad8 in virtio_bus_device_iommu_enabled (vdev=vdev@entry=0x0) at ../hw/virtio/virtio-bus.c:331
2  0x000055f800d70d7f in vhost_memory_unmap (dev=<optimized out>) at ../hw/virtio/vhost.c:318
3  0x000055f800d70d7f in vhost_memory_unmap (dev=<optimized out>, buffer=0x7fc19bec5240, len=2052, is_write=1, access_len=2052) at ../hw/virtio/vhost.c:336
4  0x000055f800d71867 in vhost_virtqueue_stop (dev=dev@entry=0x55f8037ccc30, vdev=vdev@entry=0x55f8044ec590, vq=0x55f8037cceb0, idx=0) at ../hw/virtio/vhost.c:1241
5  0x000055f800d7406c in vhost_dev_stop (hdev=hdev@entry=0x55f8037ccc30, vdev=vdev@entry=0x55f8044ec590) at ../hw/virtio/vhost.c:1839
6  0x000055f800bf00a7 in vhost_net_stop_one (net=0x55f8037ccc30, dev=0x55f8044ec590) at ../hw/net/vhost_net.c:315
7  0x000055f800bf0678 in vhost_net_stop (dev=dev@entry=0x55f8044ec590, ncs=0x55f80452bae0, data_queue_pairs=data_queue_pairs@entry=7, cvq=cvq@entry=1)
   at ../hw/net/vhost_net.c:423
8  0x000055f800d4e628 in virtio_net_set_status (status=<optimized out>, n=0x55f8044ec590) at ../hw/net/virtio-net.c:296
9  0x000055f800d4e628 in virtio_net_set_status (vdev=vdev@entry=0x55f8044ec590, status=15 '\017') at ../hw/net/virtio-net.c:370
10 0x000055f800d534d8 in virtio_net_handle_ctrl (iov_cnt=<optimized out>, iov=<optimized out>, cmd=0 '\000', n=0x55f8044ec590) at ../hw/net/virtio-net.c:1408
11 0x000055f800d534d8 in virtio_net_handle_ctrl (vdev=0x55f8044ec590, vq=0x7fc1a7e888d0) at ../hw/net/virtio-net.c:1452
12 0x000055f800d69f37 in virtio_queue_host_notifier_read (vq=0x7fc1a7e888d0) at ../hw/virtio/virtio.c:2331
13 0x000055f800d69f37 in virtio_queue_host_notifier_read (n=n@entry=0x7fc1a7e8894c) at ../hw/virtio/virtio.c:3575
14 0x000055f800c688e6 in virtio_bus_cleanup_host_notifier (bus=<optimized out>, n=n@entry=14) at ../hw/virtio/virtio-bus.c:312
15 0x000055f800d73106 in vhost_dev_disable_notifiers (hdev=hdev@entry=0x55f8035b51b0, vdev=vdev@entry=0x55f8044ec590)
   at ../../../include/hw/virtio/virtio-bus.h:35
16 0x000055f800bf00b2 in vhost_net_stop_one (net=0x55f8035b51b0, dev=0x55f8044ec590) at ../hw/net/vhost_net.c:316
17 0x000055f800bf0678 in vhost_net_stop (dev=dev@entry=0x55f8044ec590, ncs=0x55f80452bae0, data_queue_pairs=data_queue_pairs@entry=7, cvq=cvq@entry=1)
   at ../hw/net/vhost_net.c:423
18 0x000055f800d4e628 in virtio_net_set_status (status=<optimized out>, n=0x55f8044ec590) at ../hw/net/virtio-net.c:296
19 0x000055f800d4e628 in virtio_net_set_status (vdev=0x55f8044ec590, status=15 '\017') at ../hw/net/virtio-net.c:370
20 0x000055f800d6c4b2 in virtio_set_status (vdev=0x55f8044ec590, val=<optimized out>) at ../hw/virtio/virtio.c:1945
21 0x000055f800d11d9d in vm_state_notify (running=running@entry=false, state=state@entry=RUN_STATE_SHUTDOWN) at ../softmmu/runstate.c:333
22 0x000055f800d04e7a in do_vm_stop (state=state@entry=RUN_STATE_SHUTDOWN, send_stop=send_stop@entry=false) at ../softmmu/cpus.c:262
23 0x000055f800d04e99 in vm_shutdown () at ../softmmu/cpus.c:280
24 0x000055f800d126af in qemu_cleanup () at ../softmmu/runstate.c:812
25 0x000055f800ad5b13 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at ../softmmu/main.c:51

For now, temporarily disable handling MQ request from the ctrl_vq
userspace hanlder to avoid the recursive virtio_net_set_status()
call. Some rework is needed to allow changing the number of
queues without going through a full virtio_net_set_status cycle,
particularly for vhost-vdpa backend.

This patch will need to be reverted as soon as future patches of
having the change of #queues handled in userspace is merged.

Fixes: 402378407db ("vhost-vdpa: multiqueue support")
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
---
 hw/net/virtio-net.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index f0bb29c..e263116 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1381,6 +1381,7 @@ static int virtio_net_handle_mq(VirtIONet *n, uint8_t cmd,
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(n);
     uint16_t queue_pairs;
+    NetClientState *nc = qemu_get_queue(n->nic);
 
     virtio_net_disable_rss(n);
     if (cmd == VIRTIO_NET_CTRL_MQ_HASH_CONFIG) {
@@ -1412,6 +1413,18 @@ static int virtio_net_handle_mq(VirtIONet *n, uint8_t cmd,
         return VIRTIO_NET_ERR;
     }
 
+    /* Avoid changing the number of queue_pairs for vdpa device in
+     * userspace handler. A future fix is needed to handle the mq
+     * change in userspace handler with vhost-vdpa. Let's disable
+     * the mq handling from userspace for now and only allow get
+     * done through the kernel. Ripples may be seen when falling
+     * back to userspace, but without doing it qemu process would
+     * crash on a recursive entry to virtio_net_set_status().
+     */ 
+    if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
+        return VIRTIO_NET_ERR;
+    }
+
     n->curr_queue_pairs = queue_pairs;
     /* stop the backend before changing the number of queue_pairs to avoid handling a
      * disabled queue */
-- 
1.8.3.1