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

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Date:   Wed, 27 Jul 2022 12:24:56 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Parav Pandit <parav@...dia.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "mst@...hat.com" <mst@...hat.com>
Cc:     "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 7/27/2022 11:47 AM, 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.
The user space tools asks for the number of vq pairs, not whether the 
device has _MQ.
_MQ and _RSS are not the same kind of concepts, as we have discussed above.
You have pointed out the logic: If there is _MQ, kernel answers 
max_vq_paris, if no _MQ, num_vq_paris=1.

So as MST pointed out, implementing this in kernel space can make our 
life easier, once for all.
>
>>> For feature X, kernel reports default and for feature Y, kernel skip
>> reporting it, because there is no default. <- This is what we are trying to
>> avoid here.
>> Kernel reports one queue pair because there is actually one.