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" <[email-masked]>
Date: Tue, 26 Jul 2022 15:48:50 -0400

On Tue, Jul 26, 2022 at 03:54:06PM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <[email-masked]>
> > Sent: Wednesday, July 13, 2022 1:27 AM
> > 
> > On Fri, Jul 01, 2022 at 10:07:59PM +0000, Parav Pandit wrote:
> > >
> > >
> > > > From: Zhu Lingshan <[email-masked]>
> > > > 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.

>From spec POV there's just driver and device, user would be part of
driver 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.

Consistency is good for sure. What are these other fields though?
Can you give examples so I understand please?

-- 
MST