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: <PH0PR12MB54811EB71F4D9C32DA2D6F02DC949@PH0PR12MB5481.namprd12.prod.outlook.com>
Date:   Tue, 26 Jul 2022 20:53:28 +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: Tuesday, July 26, 2022 3:49 PM
> 
> On Tue, Jul 26, 2022 at 03:54:06PM +0000, Parav Pandit wrote:
> >
> > > 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.
> 
> From spec POV there's just driver and device, user would be part of driver here.
User space application still need to inspect the _MQ bit to
> 
> > >
> > > 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?

speed only exists if VIRTIO_NET_F_SPEED_DUPLEX.
rss_max_key_size exists only if VIRTIO_NET_F_RSS exists.