netdev - Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F

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Message-ID: <dbfac5e0-70a2-7d6a-c857-8d66126b268e@intel.com>
Date:   Thu, 28 Jul 2022 14:02:39 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        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 1:53 PM, Jason Wang wrote:
> On Thu, Jul 28, 2022 at 11:47 AM Zhu, Lingshan <lingshan.zhu@...el.com> wrote:
>>
>>
>> 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.
> Or we can just read MTU from the device.
>
> But It looks to me Michael wants it conditional.
if _F_MTU is offered, we can read it from the device config space. If 
_F_MTU not
offered, I think it is conditional, however there can be a min default 
value,
1500 for Ethernet.

Thanks,
Zhu Lingshan
>
> Thanks
>
>> 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
>>>>>>>>