Messages in this thread |  | | Subject | Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero | From | Jason Wang <> | Date | Tue, 23 Feb 2021 17:46:20 +0800 |
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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. >
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