Re: [PATCH 1/2] Revert "vhost-user: Monitor slave channel in vhost_user_read()"

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 1/19/23 18:24, Greg Kurz wrote:
> This reverts commit db8a3772e300c1a656331a92da0785d81667dc81.
> 
> Motivation : this is breaking vhost-user with DPDK as reported in [0].
> 
> Received unexpected msg type. Expected 22 received 40
> Fail to update device iotlb
> Received unexpected msg type. Expected 40 received 22
> Received unexpected msg type. Expected 22 received 11
> Fail to update device iotlb
> Received unexpected msg type. Expected 11 received 22
> vhost VQ 1 ring restore failed: -71: Protocol error (71)
> Received unexpected msg type. Expected 22 received 11
> Fail to update device iotlb
> Received unexpected msg type. Expected 11 received 22
> vhost VQ 0 ring restore failed: -71: Protocol error (71)
> unable to start vhost net: 71: falling back on userspace virtio
> 
> The failing sequence that leads to the first error is :
> - QEMU sends a VHOST_USER_GET_STATUS (40) request to DPDK on the master
>    socket
> - QEMU starts a nested event loop in order to wait for the
>    VHOST_USER_GET_STATUS response and to be able to process messages from
>    the slave channel
> - DPDK sends a couple of legitimate IOTLB miss messages on the slave
>    channel
> - QEMU processes each IOTLB request and sends VHOST_USER_IOTLB_MSG (22)
>    updates on the master socket
> - QEMU assumes to receive a response for the latest VHOST_USER_IOTLB_MSG
>    but it gets the response for the VHOST_USER_GET_STATUS instead
> 
> The subsequent errors have the same root cause : the nested event loop
> breaks the order by design. It lures QEMU to expect responses to the
> latest message sent on the master socket to arrive first.
> 
> Since this was only needed for DAX enablement which is still not merged
> upstream, just drop the code for now. A working solution will have to
> be merged later on. Likely protect the master socket with a mutex
> and service the slave channel with a separate thread, as discussed with
> Maxime in the mail thread below.
> 
> [0] https://lore.kernel.org/qemu-devel/43145ede-89dc-280e-b953-6a2b436de395@redhat.com/
> 
> Reported-by: Yanghang Liu <yanghliu@redhat.com>
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2155173
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>   hw/virtio/vhost-user.c | 35 +++--------------------------------
>   1 file changed, 3 insertions(+), 32 deletions(-)
> 

Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Thanks,
Maxime