netdev - RE: [PATCH V4 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F

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Date:   Tue, 9 Aug 2022 19:48:57 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Zhu Lingshan <lingshan.zhu@...el.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "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 V4 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0

> From: Michael S. Tsirkin <mst@...hat.com>
> Sent: Tuesday, August 9, 2022 3:36 PM

> > > After applying this commit, when MQ = 0, iproute2 output:
> > > $vdpa dev config show vdpa0
> > > vdpa0: mac 00:e8:ca:11:be:05 link up link_announce false
> > > max_vq_pairs 1 mtu 1500
> > >
> > No. We do not want to diverge returning values of config space for fields
> which are not present as discussed in previous versions.
> > Please drop this patch.
> > Nack on this patch.
> 
> Wrt this patch as far as I'm concerned you guys are bikeshedding.
> 
> Parav generally don't send nacks please they are not helpful.

Ok. I explained the technical reasoning that we don't diverge in fields. All are linked to the respective feature bits uniformly.
This I explained repeatedly in almost v1 to v3.
And reporting 1 is mis-leading, because it says _MQ is not negotiated, how come this device tells its config space has max_qp = 1.

But if you want to proceed to diverge kernel on feature bits go ahead. It requires inspection feature but feature.
I just don't see how this can be well maintained.

Commit log doesn't even describe the weird use case that says "as user space, I do not want to read device feature bits and just want to read config space to decide...".
Odd..