Re: [PATCH 2/3] net/virtio_user: fix wrong sequence of messages

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Fri, Aug 05, 2016 at 11:36:42AM +0000, Jianfeng Tan wrote:
> When virtio_user is used with VPP's native vhost user, it cannot
> send/receive any packets.
> 
> The root cause is that vpp-vhost-user translates the message
> VHOST_USER_SET_FEATURES as puting this device into init state,
> aka, zero all related structures. However, previous code
> puts this message at last in the whole initialization process,
> which leads to all previous information are zeroed.
> 
> To fix this issue, we rearrange the sequence of those messages.
>   - step 0, send VHOST_USER_SET_VRING_CALL so that vhost allocates
>     virtqueue structures;

Yes, it is. However, it's not that right to do that (you see there is
a FIXME in vhost_user_set_vring_call()).

That means it need be fixed: we should not rely on fact that it's the
first per-vring message we will get in the current QEMU implementation
as the truth.

That also means, naming a function like virtio_user_create_queue() based
on above behaviour is wrong.

>   - step 1, send VHOST_USER_SET_FEATURES to confirm the features;
>   - step 2, send VHOST_USER_SET_MEM_TABLE to share mem regions;
>   - step 3, send VHOST_USER_SET_VRING_NUM, VHOST_USER_SET_VRING_BASE,
>     VHOST_USER_SET_VRING_ADDR, VHOST_USER_SET_VRING_KICK for each
>     queue;
>   - ...
> 
> Fixes: 37a7eb2ae816 ("net/virtio-user: add device emulation layer")
> 
> Reported-by: Zhihong Wang <zhihong.wang@intel.com>
> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
> ---
>  drivers/net/virtio/virtio_user/virtio_user_dev.c | 120 ++++++++++++++---------
>  1 file changed, 72 insertions(+), 48 deletions(-)

That's too much of code for a bug fix. I'm wondering how about just
moving VHOST_USER_GET_PROTOCOL_FEATURES ahead, to the begining of
virtio_user_start_device()? It should fix this issue.

	--yliu