Messages in this thread | | | Subject | Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero | From | Jason Wang <> | Date | Wed, 24 Feb 2021 17:29:07 +0800 |
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On 2021/2/23 6:58 下午, Cornelia Huck wrote: > On Tue, 23 Feb 2021 18:31:07 +0800 > Jason Wang <jasowang@redhat.com> wrote: > >> On 2021/2/23 6:04 下午, Cornelia Huck wrote: >>> On Tue, 23 Feb 2021 17:46:20 +0800 >>> Jason Wang <jasowang@redhat.com> 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? > I'd consider feature negotiation done when the driver reads FEATURES_OK > back from the status.
I agree.
> >> >>> (This is a bit fuzzy for legacy mode.) > ...because legacy does not have FEATURES_OK. > >> >> 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". > "offered by the device"? I don't think it should 'disappear' from the > config space if the driver won't use it. (Same for other config space > fields that are tied to feature bits.)
But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks to according to the spec there will be no mtu field.
And a more interesting case is VIRTIO_NET_F_MQ is not offered but VIRTIO_NET_F_MTU offered. To me, it means we don't have max_virtqueue_pairs but it's not how the driver is wrote today.
> >> Otherwise readers (at least for me), may think the MTU is only valid >> if driver set the bit. > I think it would still be 'valid' in the sense that it exists and has > some value in there filled in by the device, but a driver reading it > without negotiating the feature would be buggy. (Like in the kernel > code above; the kernel not liking the value does not make the field > invalid.)
See Michael's reply, the spec allows read the config before setting features.
> > Maybe a statement covering everything would be: > > "The following driver-read-only field mtu only exists if the device > offers VIRTIO_NET_F_MTU and may be read by the driver during feature > negotiation and after VIRTIO_NET_F_MTU has been successfully > negotiated." > >> >>> Should we add a wording clarification to the spec? >> >> I think so. > Some clarification would be needed for each field that depends on a > feature; that would be quite verbose. Maybe we can get away with a > clarifying statement? > > "Some config space fields may depend on a certain feature. In that > case, the field exits if the device has offered the corresponding > feature,
So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config will look like:
struct virtio_net_config { u8 mac[6]; le16 status; le16 mtu; };
> and may be read by the driver during feature negotiation, and > accessed by the driver after the feature has been successfully > negotiated. A shorthand for this is a statement that a field only > exists if a certain feature bit is set."
I'm not sure using "shorthand" is good for the spec, at least we can limit the its scope only to the configuration space part.
Thanks
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