Date:   Thu, 28 Jul 2022 09:21:51 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Parav Pandit <parav@...dia.com>,
        "Zhu, Lingshan" <lingshan.zhu@...el.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 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.

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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