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: <e98fc062-021b-848b-5cf4-15bd63a11c5c@intel.com>
Date:   Wed, 27 Jul 2022 10:53:02 +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 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.
> 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.
>