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

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

On 9/6/2016 4:20 PM, Yuanhan Liu wrote:
> On Tue, Sep 06, 2016 at 03:54:30PM +0800, Tan, Jianfeng wrote:
>> Hi Yuanhan,
>>
>>
>> On 9/6/2016 2:42 PM, Yuanhan Liu wrote:
>>> 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()).
>> I suppose you are specifying vhost_set_vring_call().
> Oh, I was talking about the new code: I have renamed it to
> 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.
>> It's actually a good catch. After a light thought, I think in DPDK vhost, we
>> may need to create those virtqueues once unix socket gets connected, just
>> like in vhost-net, virtqueues are created on char file open. Right?
> There is a difference: for vhost-net and tap mode, IIRC, it knows how
> many queues before doing setup.

No, from linux/drivers/vhost/net.c:vhost_net_open(), we can see that 
virtqueues are allocated according to VHOST_NET_VQ_MAX.
How about reconsidering previous suggestion to allocate vq once 
connection is established?
Never mind, above fix on the vhost side will not take effect on existing 
vpp-vhost implementations. We still need to fix it in the virtio side.

>   While for vhost-user, it doesn't. That
> means, we have to allocate and setup virtqueues reactively: just like
> what we have done in vhost_set_vring_call(). What doesn't look perfect
> is it assume SET_VRING_CALL is the first per-vring message we will get.

Yes, depending on the assumption that SET_VRING_CALL is the first 
per-vring message, looks like a bad implementation. As Stephen has 
suggested, it's more like a bug in vpp. If we treat it like that way, we 
will fix nothing here.


>>>>    - 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.
>> Why does VHOST_USER_GET_PROTOCOL_FEATURES care? Do you mean shifting
>> VHOST_USER_SET_FEATURES earlier?
> Oops, right, I meant SET_FEATURES. Sorry about confusion introduced by
> the silly auto-completion.

Still not working. VPP needs SET_VRING_CALL to create vq firstly.

Thanks,
Jianfeng

>
> 	--yliu