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 02:17:41 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.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


> 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.

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.