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

[Jason Wang]

On 2021/2/23 6:04 下午, Cornelia Huck wrote:
> On Tue, 23 Feb 2021 17:46:20 +0800
> Jason Wang <jasowang@xxxxxxxxxx> 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?
> I think both failing or not accepting the feature can be argued to make
> sense: "the device presented us with a mtu size that does not make
> sense" would point to failing, "we cannot work with the mtu size that
> the device presented us" would point to not negotiating the feature.
>
> > >           }
> > >
> > >           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.
> Before FEATURES_OK is set by the driver, I guess it means "the device
> has offered the feature";


For me this part is ok since it clarify that it's the driver that set 
the bit.



> during normal usage, it means "the feature
> has been negotiated".

/?

It looks to me the feature negotiation is done only after device set 
FEATURES_OK, or FEATURES_OK could be read from device status?


>   (This is a bit fuzzy for legacy mode.)


The problem is the MTU description for example:

"The following driver-read-only field, mtu only exists if 
VIRTIO_NET_F_MTU is set."

It looks to me need to use "if VIRTIO_NET_F_MTU is set by device". 
Otherwise readers (at least for me), may think the MTU is only valid if 
driver set the bit.


> Should we add a wording clarification to the spec?


I think so.

Thanks

>