Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

[Jason Wang]

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?


>          }
>
>          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.