Date:   Thu, 28 Jul 2022 13:53:51 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Parav Pandit <parav@...dia.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

On Thu, Jul 28, 2022 at 11:47 AM Zhu, Lingshan <lingshan.zhu@...el.com> wrote:
>
>
>
> On 7/28/2022 9:21 AM, Jason Wang wrote:
> > On Wed, Jul 27, 2022 at 11:45 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >> On Wed, Jul 27, 2022 at 05:50:59PM +0800, Jason Wang wrote:
> >>> On Wed, Jul 27, 2022 at 5:03 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >>>> On Wed, Jul 27, 2022 at 02:54:13PM +0800, Jason Wang wrote:
> >>>>> On Wed, Jul 27, 2022 at 2:01 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >>>>>> On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote:
> >>>>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >>>>>>>> Sent: Tuesday, July 26, 2022 10:53 PM
> >>>>>>>>
> >>>>>>>> On 7/27/2022 10:17 AM, Parav Pandit wrote:
> >>>>>>>>>> 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.
> >>>>>>>> MQ and other features like RSS are different. If there is no _RSS_XX, there
> >>>>>>>> are no attributes like max_rss_key_size, and there is not a default value.
> >>>>>>>> But for MQ, we know it has to be 1 wihtout _MQ.
> >>>>>>> "we" = user space.
> >>>>>>> To keep the consistency among all the config space fields.
> >>>>>> Actually I looked and the code some more and I'm puzzled:
> >>>>>>
> >>>>>>
> >>>>>>          struct virtio_net_config config = {};
> >>>>>>          u64 features;
> >>>>>>          u16 val_u16;
> >>>>>>
> >>>>>>          vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
> >>>>>>
> >>>>>>          if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac),
> >>>>>>                      config.mac))
> >>>>>>                  return -EMSGSIZE;
> >>>>>>
> >>>>>>
> >>>>>> Mac returned even without VIRTIO_NET_F_MAC
> >>>>>>
> >>>>>>
> >>>>>>          val_u16 = le16_to_cpu(config.status);
> >>>>>>          if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
> >>>>>>                  return -EMSGSIZE;
> >>>>>>
> >>>>>>
> >>>>>> status returned even without VIRTIO_NET_F_STATUS
> >>>>>>
> >>>>>>          val_u16 = le16_to_cpu(config.mtu);
> >>>>>>          if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
> >>>>>>                  return -EMSGSIZE;
> >>>>>>
> >>>>>>
> >>>>>> MTU returned even without VIRTIO_NET_F_MTU
> >>>>>>
> >>>>>>
> >>>>>> What's going on here?
> >>>>> Probably too late to fix, but this should be fine as long as all
> >>>>> parents support STATUS/MTU/MAC.
> >>>> Why is this too late to fix.
> >>> If we make this conditional on the features. This may break the
> >>> userspace that always expects VDPA_ATTR_DEV_NET_CFG_MTU?
> >>>
> >>> Thanks
> >> Well only on devices without MTU. I'm saying said userspace
> >> was reading trash on such devices anyway.
> > It depends on the parent actually. For example, mlx5 query the lower
> > mtu unconditionally:
> >
> >          err = query_mtu(mdev, &mtu);
> >          if (err)
> >                  goto err_alloc;
> >
> >          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, mtu);
> >
> > Supporting MTU features seems to be a must for real hardware.
> > Otherwise the driver may not work correctly.
> >
> >> We don't generally maintain bug for bug compatiblity on a whim,
> >> only if userspace is actually known to break if we fix a bug.
> >   So I think it should be fine to make this conditional then we should
> > have a consistent handling of other fields like MQ.
> For some fields that have a default value, like MQ =1, we can return the
> default value.
> For other fields without a default value, like MAC, we return nothing.
>
> Does this sounds good? So, for MTU, if without _F_MTU, I think we can
> return 1500 by default.

Or we can just read MTU from the device.

But It looks to me Michael wants it conditional.

Thanks

>
> Thanks,
> Zhu Lingshan
> >
> > Thanks
> >
> >>
> >>>>> I wonder if we can add a check in the core and fail the device
> >>>>> registration in this case.
> >>>>>
> >>>>> Thanks
> >>>>>
> >>>>>>
> >>>>>> --
> >>>>>> MST
> >>>>>>
>

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