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, 13 Jul 2022 11:45:46 +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 7/13/2022 11:06 AM, Parav Pandit wrote:
>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>> Sent: Tuesday, July 12, 2022 11:03 PM
>>
>>
>> On 7/13/2022 12:48 AM, Parav Pandit wrote:
>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>>>> Sent: Sunday, July 10, 2022 10:30 PM
>>>>> Showing max_vq_pairs of 1 even when _MQ is not negotiated,
>>>>> incorrectly
>>>> says that max_vq_pairs is exposed to the guest, but it is not offered.
>>>>> So, please fix the iproute2 to not print max_vq_pairs when it is not
>>>> returned by the kernel.
>>>> iproute2 can report whether there is MQ feature in the device /
>>>> driver feature bits.
>>>> I think iproute2 only queries the number of max queues here.
>>>>
>>>> max_vq_pairs shows how many queue pairs there, this attribute's
>>>> existence does not depend on MQ, if no MQ, there are still one queue
>>>> pair, so just show one.
>>> This netlink attribute's existence is depending on the _MQ feature bit
>> existence.
>> why? If no MQ, then no queues?
>>> We can break that and report the value, but if we break that there are
>> many other config space bits who doesn’t have good default like
>> max_vq_pairs.
>> max_vq_paris may not have a default value, but we know if there is no MQ,
>> a virtio-net still have one queue pair to be functional.
>>> There is ambiguity for user space what to do with it and so in the kernel
>> space..
>>> Instead of dealing with them differently in kernel, at present we attach
>> each netlink attribute to a respective feature bit wherever applicable.
>>> And code in kernel and user space is uniform to handle them.
>> I get your point, but you see, by "max_vq_pairs", the user space tool is
>> asking how many queue pairs there, it is not asking whether the device have
>> MQ.
>> Even no _MQ, we still need to tell the users that there are one queue pair, or
>> it is not a functional virtio-net, we should detect this error earlier in the
>> device initialization.
> It is not an error. :)
I meant if no queues, it should be non-functional, which is an error.
>
> 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.
>
>> I think it is still uniform, it there is _MQ, we return cfg.max_queue_pair, if no
>> _MQ, return 1, still by netlink.
> Better to do that in user space because we cannot do same for other config fields.
same as above
>
>> Thanks