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: <c7c8f49c-484f-f5b3-39e6-0d17f396cca7@intel.com>
Date:   Wed, 13 Jul 2022 11:03:20 +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 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.

I think it is still uniform, it there is _MQ, we return 
cfg.max_queue_pair, if no _MQ, return 1, still by netlink.

Thanks