Michael S. Tsirkin
2021-Feb-23 10:01 UTC
[PATCH] vdpa/mlx5: set_features should allow reset to zero
On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote:> > On 2021/2/23 ??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 ??, 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. > >
Jason Wang
2021-Feb-23 10:17 UTC
[PATCH] vdpa/mlx5: set_features should allow reset to zero
On 2021/2/23 6:01 ??, Michael S. Tsirkin wrote:> On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote: >> On 2021/2/23 ??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 ??, 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.What if e.g the device advertise a large MTU. E.g 64K here? In that case, the driver can not live either. Clearing MTU won't help here. Thanks> > >>> } >>> >>> 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. >>>