Re: [dpdk-dev] [PATCH v2] net/virtio: fix virtio-user init when using existing tap

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 9/28/2021 9:51 AM, David Marchand wrote:
> When attaching to an existing mono queue tap, the virtio-user was not
> reporting that the virtio device was not properly initialised which
> prevented from starting the port later.
> 
> $ ip tuntap add test mode tap
> $ dpdk-testpmd --vdev \
>   net_virtio_user0,iface=test,path=/dev/vhost-net,queues=2 -- -i
> 
> ...
> virtio_user_dev_init_mac(): (/dev/vhost-net) No valid MAC in devargs or
> device, use random
> vhost_kernel_open_tap(): TUNSETIFF failed: Invalid argument
> vhost_kernel_enable_queue_pair(): fail to open tap for vhost kernel
> virtio_user_start_device(): (/dev/vhost-net) Failed to start device
> ...
> Configuring Port 0 (socket 0)
> vhost_kernel_open_tap(): TUNSETIFF failed: Invalid argument
> vhost_kernel_enable_queue_pair(): fail to open tap for vhost kernel
> virtio_set_multiple_queues(): Multiqueue configured but send command
> failed, this is too late now...
> Fail to start port 0: Invalid argument
> Please stop the ports first
> Done
> 
> The virtio-user with vhost-kernel backend was going through a lot
> of complications to initialise tap fds only when using them.
> 
> For each qp enabled for the first time, a tapfd was created via
> TUNSETIFF with unneeded additional steps (see below) and then mapped to
> the right qp in the vhost-net backend.
> Unneeded steps (as long as it has been done once for the port):
> - tap features were queried while this is a constant on a running
>   system,
> - the device name in DPDK was updated,
> - the mac address of the tap was set,
> 
> On subsequent qps state change, the vhost-net backend fd mapping was
> updated and the associated queue/tapfd were disabled/enabled via
> TUNSETQUEUE.
> 
> Now, this patch simplifies the whole logic by keeping all tapfds opened
> and in enabled state (from the tap point of view) at all time.
> 
> Unused ioctl defines are removed.
> 
> Tap features are validated earlier to fail initialisation asap.
> Tap name discovery and mac address configuration are moved when
> configuring qp 0.
> 
> To support attaching to mono queue tap, the virtio-user driver now tries
> to attach in multi queue first, then fallbacks to mono queue.
> 
> Finally (but this is more for consistency), VIRTIO_NET_F_MQ feature is
> exposed only if the underlying tap supports multi queue.
> 
> Signed-off-by: David Marchand <david.marchand@redhat.com>

Do we want to backport this patch? If so can you please provide a fixes tag?