Cornelia Huck
2021-Feb-23 10:04 UTC
[virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
On Tue, 23 Feb 2021 17:46:20 +0800 Jason Wang <jasowang at redhat.com> 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?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"; during normal usage, it means "the feature has been negotiated". (This is a bit fuzzy for legacy mode.) Should we add a wording clarification to the spec?
Jason Wang
2021-Feb-23 10:31 UTC
[virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
On 2021/2/23 6:04 ??, Cornelia Huck wrote:> On Tue, 23 Feb 2021 17:46:20 +0800 > Jason Wang <jasowang at redhat.com> 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? > 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>