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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Si-Wei Liu <si-wei.liu@oracle.com>,
	elic@nvidia.com, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
Date: Tue, 23 Feb 2021 05:01:31 -0500
Message-ID: <20210223045600-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com>

On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote:
> 
> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
> > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
> > > 
> > > On 2/21/2021 8:14 PM, Jason Wang wrote:
> > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > for legacy") made an exception for legacy guests to reset
> > > > > features to 0, when config space is accessed before features
> > > > > are set. We should relieve the verify_min_features() check
> > > > > and allow features reset to 0 for this case.
> > > > > 
> > > > > It's worth noting that not just legacy guests could access
> > > > > config space before features are set. For instance, when
> > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > will try to access and validate the MTU present in the config
> > > > > space before virtio features are set.
> > > > 
> > > > This looks like a spec violation:
> > > > 
> > > > "
> > > > 
> > > > The following driver-read-only field, mtu only exists if
> > > > VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> > > > driver to use.
> > > > "
> > > > 
> > > > Do we really want to workaround this?
> > > Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> > > 
> > > I think the point is, since there's legacy guest we'd have to support, this
> > > host side workaround is unavoidable. Although I agree the violating driver
> > > should be fixed (yes, it's in today's upstream kernel which exists for a
> > > while now).
> > Oh  you are right:
> > 
> > 
> > static int virtnet_validate(struct virtio_device *vdev)
> > {
> >          if (!vdev->config->get) {
> >                  dev_err(&vdev->dev, "%s failure: config access disabled\n",
> >                          __func__);
> >                  return -EINVAL;
> >          }
> > 
> >          if (!virtnet_validate_features(vdev))
> >                  return -EINVAL;
> > 
> >          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> >                  int mtu = virtio_cread16(vdev,
> >                                           offsetof(struct virtio_net_config,
> >                                                    mtu));
> >                  if (mtu < MIN_MTU)
> >                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
> 
> 
> I wonder why not simply fail here?

Back in 2016 it went like this:

	On Thu, Jun 02, 2016 at 05:10:59PM -0400, Aaron Conole wrote:
	> +     if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
	> +             dev->mtu = virtio_cread16(vdev,
	> +                                       offsetof(struct virtio_net_config,
	> +                                                mtu));
	> +     }
	> +
	>       if (vi->any_header_sg)
	>               dev->needed_headroom = vi->hdr_len;
	> 

	One comment though: I think we should validate the mtu.
	If it's invalid, clear VIRTIO_NET_F_MTU and ignore.


Too late at this point :)

I guess it's a way to tell device "I can not live with this MTU",
device can fail FEATURES_OK if it wants to. MIN_MTU
is an internal linux thing and at the time I felt it's better to
try to make progress.


> 
> >          }
> > 
> >          return 0;
> > }
> > 
> > And the spec says:
> > 
> > 
> > The driver MUST follow this sequence to initialize a device:
> > 1. Reset the device.
> > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> > 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> > device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> > fields to check that it can support the device before accepting it.
> > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> > 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> > support our subset of features and the device is unusable.
> > 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> > reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > 
> > 
> > Item 4 on the list explicitly allows reading config space before
> > FEATURES_OK.
> > 
> > I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
> 
> 
> So this probably need some clarification. "is set" is used many times in the
> spec that has different implications.
> 
> Thanks
> 
> 
> > 
> > Generally it is worth going over feature dependent config fields
> > and checking whether they should be present when device feature is set
> > or when feature bit has been negotiated, and making this clear.
> > 


  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 11:54 Si-Wei Liu
2021-02-21 14:44 ` Eli Cohen
2021-02-21 21:52   ` Michael S. Tsirkin
2021-02-22  6:05     ` Eli Cohen
2021-02-23  9:26       ` Michael S. Tsirkin
2021-02-23  9:48         ` Jason Wang
2021-02-23  9:55           ` Michael S. Tsirkin
2021-02-22  4:14 ` Jason Wang
2021-02-22  7:34   ` Michael S. Tsirkin
2021-02-23  1:12     ` Si-Wei Liu
2021-02-23  2:03       ` Jason Wang
2021-02-23 13:26         ` Michael S. Tsirkin
2021-02-23 19:35           ` Si-Wei Liu
2021-02-24  3:20             ` Jason Wang
2021-02-24  5:17               ` Michael S. Tsirkin
2021-02-24  6:02                 ` Jason Wang
2021-02-24  6:45                 ` Eli Cohen
2021-02-24  6:47                   ` Michael S. Tsirkin
2021-02-24  6:55                     ` Jason Wang
2021-02-24  7:12                       ` Michael S. Tsirkin
2021-02-24 12:40                         ` Eli Cohen
2021-02-24  7:17                       ` Eli Cohen
2021-02-24  5:04             ` Michael S. Tsirkin
2021-02-24  6:04               ` Jason Wang
2021-02-24  6:46                 ` Michael S. Tsirkin
2021-02-24  6:53                   ` Jason Wang
2021-02-24  7:17                     ` Michael S. Tsirkin
     [not found]                       ` <babc654d-8dcd-d8a2-c3b6-d20cc4fc554c@redhat.com>
2021-02-24  8:43                         ` Michael S. Tsirkin
2021-02-24  9:30                           ` Jason Wang
2021-02-28 21:30                             ` Michael S. Tsirkin
2021-03-01  3:53                               ` Jason Wang
2021-02-24 18:24               ` Si-Wei Liu
2021-02-26  0:56                 ` Si-Wei Liu
2021-02-28 21:27                   ` Michael S. Tsirkin
2021-03-01 18:08                     ` Si-Wei Liu
2021-02-28 21:28                 ` Michael S. Tsirkin
2021-02-28 21:34                 ` Michael S. Tsirkin
2021-03-01  3:56                   ` Jason Wang
2021-03-02  9:47                     ` Michael S. Tsirkin
2021-03-02 10:53                       ` Jason Wang
2021-02-22 17:09   ` Si-Wei Liu
2021-02-23  2:03     ` Jason Wang
2021-02-23  9:25     ` Michael S. Tsirkin
2021-02-23  9:46       ` Jason Wang
2021-02-23 10:01         ` Michael S. Tsirkin [this message]
2021-02-23 10:17           ` Jason Wang
2021-02-24  9:40             ` Jason Wang
2021-02-23 10:04         ` [virtio-dev] " Cornelia Huck
2021-02-23 10:31           ` Jason Wang
2021-02-23 10:58             ` Cornelia Huck
2021-02-24  9:29               ` Jason Wang
2021-02-24 11:12                 ` Cornelia Huck
2021-02-25  4:36                   ` Jason Wang
2021-02-25 13:26                     ` Cornelia Huck
2021-02-25 18:53                     ` Michael S. Tsirkin
2021-02-26  8:19                       ` Jason Wang
2021-02-28 21:25                         ` Michael S. Tsirkin
2021-03-01  3:51                           ` Jason Wang
2021-03-02 12:08                             ` Cornelia Huck
     [not found]                               ` <5f6972fe-7246-b622-958d-9cab8dd98e21@redhat.com>
2021-03-03  8:29                                 ` Cornelia Huck
2021-03-04  8:24                                   ` Jason Wang
2021-03-04 13:50                                     ` Cornelia Huck
2021-03-05  3:01                                       ` Jason Wang
2021-02-23 12:26 ` Michael S. Tsirkin

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