Re: [PATCH] vhost-user: fix the issue of vhost deadlock in nested virtualization

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 26, 2024 at 06:07:37PM +0800, Hao Chen wrote:
> I run "dpdk-vdpa" and "qemur-L2" in "qemu-L1".
> 
> In a nested virtualization environment, "qemu-L2" vhost-user socket sends
> a "VHOST_USER_IOTLB_MSG" message to "dpdk-vdpa" and blocks waiting for
> "dpdk-vdpa" to process the message.
> If "dpdk-vdpa" doesn't complete the processing of the "VHOST_USER_IOTLB_MSG"
> message and sends a message that needs to be replied in another thread,
> such as "VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG", "dpdk-vdpa" will also
> block and wait for "qemu-L2" to process this message. However, "qemu-L2"
> vhost-user's socket is blocking while waiting for a reply from "dpdk-vdpa"
> after processing the message "VHOSTr_USER_IOTLB_MSG", and
> "VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG" will not be processed.
> In this case, both "dpdk-vdpa" and "qemu-L2" are blocked on the
> vhost read, resulting in a deadlock.
> 
> You can modify "VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG" or
> "VHOST_USER_IOTLB_MSG" to "no need reply" to fix this issue.
> There are too many messages in dpdk that are similar to
> "VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG", and I would prefer the latter.
> 
> Fixes: 24e34754eb78 ("vhost-user: factor out msg head and payload")
> 
> Signed-off-by: Hao Chen <chenh@yusur.tech>

I would be very worried that IOTLB becomes stale and
guest memory is corrupted if we just proceed without waiting.

Maxime what do you think? How would you address the issue?


> ---
>  hw/virtio/vhost-user.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index f214df804b..02caa94b6c 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -2371,20 +2371,14 @@ static int vhost_user_net_set_mtu(struct vhost_dev *dev, uint16_t mtu)
>  static int vhost_user_send_device_iotlb_msg(struct vhost_dev *dev,
>                                              struct vhost_iotlb_msg *imsg)
>  {
> -    int ret;
>      VhostUserMsg msg = {
>          .hdr.request = VHOST_USER_IOTLB_MSG,
>          .hdr.size = sizeof(msg.payload.iotlb),
> -        .hdr.flags = VHOST_USER_VERSION | VHOST_USER_NEED_REPLY_MASK,
> +        .hdr.flags = VHOST_USER_VERSION,
>          .payload.iotlb = *imsg,
>      };
>  
> -    ret = vhost_user_write(dev, &msg, NULL, 0);
> -    if (ret < 0) {
> -        return ret;
> -    }
> -
> -    return process_message_reply(dev, &msg);
> +    return vhost_user_write(dev, &msg, NULL, 0);
>  }
>  
>  
> -- 
> 2.27.0