Date:   Thu, 28 Jul 2022 11:46:53 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Parav Pandit <parav@...dia.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xieyongji@...edance.com" <xieyongji@...edance.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>
Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0



On 7/28/2022 9:21 AM, Jason Wang wrote:
> On Wed, Jul 27, 2022 at 11:45 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>> On Wed, Jul 27, 2022 at 05:50:59PM +0800, Jason Wang wrote:
>>> On Wed, Jul 27, 2022 at 5:03 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>>>> On Wed, Jul 27, 2022 at 02:54:13PM +0800, Jason Wang wrote:
>>>>> On Wed, Jul 27, 2022 at 2:01 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>>>>>> On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote:
>>>>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>>>>>>>> Sent: Tuesday, July 26, 2022 10:53 PM
>>>>>>>>
>>>>>>>> On 7/27/2022 10:17 AM, Parav Pandit wrote:
>>>>>>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>>>>>>>>>> Sent: Tuesday, July 26, 2022 10:15 PM
>>>>>>>>>>
>>>>>>>>>> On 7/26/2022 11:56 PM, Parav Pandit wrote:
>>>>>>>>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>>>>>>>>>>>> Sent: Tuesday, July 12, 2022 11:46 PM
>>>>>>>>>>>>> When the user space which invokes netlink commands, detects that
>>>>>>>>>> _MQ
>>>>>>>>>>>> is not supported, hence it takes max_queue_pair = 1 by itself.
>>>>>>>>>>>> I think the kernel module have all necessary information and it is
>>>>>>>>>>>> the only one which have precise information of a device, so it
>>>>>>>>>>>> should answer precisely than let the user space guess. The kernel
>>>>>>>>>>>> module should be reliable than stay silent, leave the question to
>>>>>>>>>>>> the user space
>>>>>>>>>> tool.
>>>>>>>>>>> Kernel is reliable. It doesn’t expose a config space field if the
>>>>>>>>>>> field doesn’t
>>>>>>>>>> exist regardless of field should have default or no default.
>>>>>>>>>> so when you know it is one queue pair, you should answer one, not try
>>>>>>>>>> to guess.
>>>>>>>>>>> User space should not guess either. User space gets to see if _MQ
>>>>>>>>>> present/not present. If _MQ present than get reliable data from kernel.
>>>>>>>>>>> If _MQ not present, it means this device has one VQ pair.
>>>>>>>>>> it is still a guess, right? And all user space tools implemented this
>>>>>>>>>> feature need to guess
>>>>>>>>> No. it is not a guess.
>>>>>>>>> It is explicitly checking the _MQ feature and deriving the value.
>>>>>>>>> The code you proposed will be present in the user space.
>>>>>>>>> It will be uniform for _MQ and 10 other features that are present now and
>>>>>>>> in the future.
>>>>>>>> MQ and other features like RSS are different. If there is no _RSS_XX, there
>>>>>>>> are no attributes like max_rss_key_size, and there is not a default value.
>>>>>>>> But for MQ, we know it has to be 1 wihtout _MQ.
>>>>>>> "we" = user space.
>>>>>>> To keep the consistency among all the config space fields.
>>>>>> Actually I looked and the code some more and I'm puzzled:
>>>>>>
>>>>>>
>>>>>>          struct virtio_net_config config = {};
>>>>>>          u64 features;
>>>>>>          u16 val_u16;
>>>>>>
>>>>>>          vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
>>>>>>
>>>>>>          if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac),
>>>>>>                      config.mac))
>>>>>>                  return -EMSGSIZE;
>>>>>>
>>>>>>
>>>>>> Mac returned even without VIRTIO_NET_F_MAC
>>>>>>
>>>>>>
>>>>>>          val_u16 = le16_to_cpu(config.status);
>>>>>>          if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
>>>>>>                  return -EMSGSIZE;
>>>>>>
>>>>>>
>>>>>> status returned even without VIRTIO_NET_F_STATUS
>>>>>>
>>>>>>          val_u16 = le16_to_cpu(config.mtu);
>>>>>>          if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
>>>>>>                  return -EMSGSIZE;
>>>>>>
>>>>>>
>>>>>> MTU returned even without VIRTIO_NET_F_MTU
>>>>>>
>>>>>>
>>>>>> What's going on here?
>>>>> Probably too late to fix, but this should be fine as long as all
>>>>> parents support STATUS/MTU/MAC.
>>>> Why is this too late to fix.
>>> If we make this conditional on the features. This may break the
>>> userspace that always expects VDPA_ATTR_DEV_NET_CFG_MTU?
>>>
>>> Thanks
>> Well only on devices without MTU. I'm saying said userspace
>> was reading trash on such devices anyway.
> It depends on the parent actually. For example, mlx5 query the lower
> mtu unconditionally:
>
>          err = query_mtu(mdev, &mtu);
>          if (err)
>                  goto err_alloc;
>
>          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, mtu);
>
> Supporting MTU features seems to be a must for real hardware.
> Otherwise the driver may not work correctly.
>
>> We don't generally maintain bug for bug compatiblity on a whim,
>> only if userspace is actually known to break if we fix a bug.
>   So I think it should be fine to make this conditional then we should
> have a consistent handling of other fields like MQ.
For some fields that have a default value, like MQ =1, we can return the 
default value.
For other fields without a default value, like MAC, we return nothing.

Does this sounds good? So, for MTU, if without _F_MTU, I think we can 
return 1500 by default.

Thanks,
Zhu Lingshan
>
> Thanks
>
>>
>>>>> I wonder if we can add a check in the core and fail the device
>>>>> registration in this case.
>>>>>
>>>>> Thanks
>>>>>
>>>>>>
>>>>>> --
>>>>>> MST
>>>>>>

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