Re: [dpdk-dev] [PATCH 9/9] vhost: only use vDPA config workaround if needed

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

On 6/9/20 1:09 PM, Matan Azrad wrote:
> Hi Maxime
> 
> From: Maxime Coquelin
>> Hi Matan,
>>
>> On 6/8/20 11:19 AM, Matan Azrad wrote:
>>> Hi Maxime
>>>
>>> From: Maxime Coquelin:
>>>> Hi Matan,
>>>>
>>>> On 6/7/20 12:38 PM, Matan Azrad wrote:
>>>>> Hi Maxime
>>>>>
>>>>> Thanks for the huge work.
>>>>> Please see a suggestion inline.
>>>>>
>>>>> From: Maxime Coquelin:
>>>>>> Sent: Thursday, May 14, 2020 11:02 AM
>>>>>> To: xiaolong.ye@intel.com; Shahaf Shuler <shahafs@mellanox.com>;
>>>>>> Matan Azrad <matan@mellanox.com>; amorenoz@redhat.com;
>>>>>> xiao.w.wang@intel.com; Slava Ovsiienko
>> <viacheslavo@mellanox.com>;
>>>>>> dev@dpdk.org
>>>>>> Cc: jasowang@redhat.com; lulu@redhat.com; Maxime Coquelin
>>>>>> <maxime.coquelin@redhat.com>
>>>>>> Subject: [PATCH 9/9] vhost: only use vDPA config workaround if
>>>>>> needed
>>>>>>
>>>>>> Now that we have Virtio device status support, let's only use the
>>>>>> vDPA workaround if it is not supported.
>>>>>>
>>>>>> This patch also document why Virtio device status protocol feature
>>>>>> support is strongly advised.
>>>>>>
>>>>>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>>>>>> ---
>>>>>>  lib/librte_vhost/vhost_user.c | 16 ++++++++++++++--
>>>>>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/lib/librte_vhost/vhost_user.c
>>>>>> b/lib/librte_vhost/vhost_user.c index e5a44be58d..67e96a872a 100644
>>>>>> --- a/lib/librte_vhost/vhost_user.c
>>>>>> +++ b/lib/librte_vhost/vhost_user.c
>>>>>> @@ -2847,8 +2847,20 @@ vhost_user_msg_handler(int vid, int fd)
>>>>>>  	if (!vdpa_dev)
>>>>>>  		goto out;
>>>>>>
>>>>>> -	if (!(dev->flags & VIRTIO_DEV_VDPA_CONFIGURED) &&
>>>>>> -			request == VHOST_USER_SET_VRING_CALL) {
>>>>>> +	if (!(dev->flags & VIRTIO_DEV_VDPA_CONFIGURED)) {
>>>>>> +		/*
>>>>>> +		 * Workaround when Virtio device status protocol
>>>>>> +		 * feature is not supported, wait for SET_VRING_CALL
>>>>>> +		 * request. This is not ideal as some frontends like
>>>>>> +		 * Virtio-user may not send this request, so vDPA device
>>>>>> +		 * may never be configured. Virtio device status support
>>>>>> +		 * on frontend side is strongly advised.
>>>>>> +		 */
>>>>>> +		if (!(dev->protocol_features &
>>>>>> +				(1ULL <<
>>>>>> VHOST_USER_PROTOCOL_F_STATUS)) &&
>>>>>> +				(request !=
>>>>>> VHOST_USER_SET_VRING_CALL))
>>>>>> +			goto out;
>>>>>> +
>>>>>
>>>>> When status protocol feature is not supported, in the current code,
>>>>> the
>>>> vDPA configuration triggering depends in:
>>>>> 1. Device is ready - all the queues are configured (datapath
>>>>> addresses,
>>>> callfd and kickfd) .
>>>>> 2. last command is callfd.
>>>>>
>>>>>
>>>>> The code doesn't take into account that some queues may stay disabled.
>>>>> Maybe the correct timing is:
>>>>> 1. Device is ready - all the enabled queues are configured and MEM
>>>>> table is
>>>> configured.
>>>>
>>>> I think current virtio_is_ready() already assumes the mem table is
>>>> configured, otherwise we would not have vq->desc, vq->used and
>>>> vq->avail being set as it needs to be translated using the mem table.
>>>>
>>> Yes, but if you don't expect to check them for disabled queues you need to
>> check mem table to be sure it was set.
>>
>> Even disabled queues should be allocated/configured by the guest driver.
> Is it by spec?

Sorry, that was a misunderstanding from my side.
The number of queues set by the driver using MQ_VQ_PAIRS_SET control
message have to be allocated and configured by the driver:
http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-1940001

> We saw that windows virtio guest driver doesn't configure disabled queues.
> Is it bug in windows guest?
> You probably can take a look here:
> https://github.com/virtio-win/kvm-guest-drivers-windows
> 

Indeed it limits the number of queue pairs to the number of CPUs.
This is done here:
https://github.com/virtio-win/kvm-guest-drivers-windows/blob/edda3f50a17015aab1450ca09e3263c7409e4001/NetKVM/Common/ParaNdis_Common.cpp#L956

Linux does the same by the way:
https://elixir.bootlin.com/linux/latest/source/drivers/net/virtio_net.c#L3092

We rarely face this issue because by default, the management
layers usually set the number of queue pairs to the number of vCPUs if
multiqueue is enabled. But the problem is real.

In my opinion, the problem is more on Vhost-user spec side and/or Vhost-
user backend.

The DPDK backend allocates queue pairs for every time it receives a
Vhost-user message setting a new queue (callfd, kickfd, enable,... see
vhost_user_check_and_alloc_queue_pair()). And then virtio_is_ready()
waits for all the allocated queue pairs to be initialized.

The problem is that QEMU sends some if these messages even for queues
that aren't (or won't be) initialized, as you can see in below log where
I reproduced the issue with Windows 10:
https://pastebin.com/YYCfW9Y3

I don't see how the backend could know the guest driver is done with
currently received information from Qemu as it seems to him some queues
are partially initialized (callfd is set).

With VHOST_USER_SET_STATUS, we will be able to handle this properly, as
the backend can be sure the guest won't initialize more queues as soon
as DRIVER_OK Virtio status bit is set. In my v2, I can add one patch to
handle this case properly, by "destorying" queues metadata as soon as
DRIVER_OK is received.

Note that it was the exact reason why I first tried to add support for
VHOST_USER_SET_STATUS more than two years ago...:
https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg04560.html

What do you think?

Regards,
Maxime