netdev - RE: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F

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Date:   Tue, 26 Jul 2022 15:54:06 +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 V3 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: Wednesday, July 13, 2022 1:27 AM
> 
> On Fri, Jul 01, 2022 at 10:07:59PM +0000, Parav Pandit wrote:
> >
> >
> > > From: Zhu Lingshan <lingshan.zhu@...el.com>
> > > Sent: Friday, July 1, 2022 9:28 AM
> > > If VIRTIO_NET_F_MQ == 0, the virtio device should have one queue
> > > pair, so when userspace querying queue pair numbers, it should
> > > return mq=1 than zero.
> > >
> > > Function vdpa_dev_net_config_fill() fills the attributions of the
> > > vDPA devices, so that it should call vdpa_dev_net_mq_config_fill()
> > > so the parameter in vdpa_dev_net_mq_config_fill() should be
> > > feature_device than feature_driver for the vDPA devices themselves
> > >
> > > Before this change, 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 0 mtu 1500
> > >
> > The fix belongs to user space.
> > When a feature bit _MQ is not negotiated, vdpa kernel space will not add
> attribute VDPA_ATTR_DEV_NET_CFG_MAX_VQP.
> > When such attribute is not returned by kernel, max_vq_pairs should not be
> shown by the iproute2.
> >
> > We have many config space fields that depend on the feature bits and
> some of them do not have any defaults.
> > To keep consistency of existence of config space fields among all, we don't
> want to show default like below.
> >
> > Please fix the iproute2 to not print max_vq_pairs when it is not returned by
> the kernel.
> 
> Parav I read the discussion and don't get your argument. From driver's POV
> _MQ with 1 VQ pair and !_MQ are exactly functionally equivalent.
But we are talking from user POV here.

> 
> It's true that iproute probably needs to be fixed too, to handle old kernels.
> But iproute is not the only userspace, why not make it's life easier by fixing
> the kernel?
Because it cannot be fixed for other config space fields which are control by feature bits those do not have any defaults.
So better to treat all in same way from user POV.