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    SubjectRe: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
    On Wed, 24 Feb 2021 17:29:07 +0800
    Jason Wang <jasowang@redhat.com> wrote:

    > 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.

    I think so, yes.

    >
    > 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.

    That would be a bug, but it seems to me that the virtio-net driver
    reads max_virtqueue_pairs conditionally and handles absence of the
    feature correctly?

    >
    >
    > >
    > >> 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.

    Yes, the period prior to finishing negotiation is obviously special.

    >
    >
    > >
    > > 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;
    > };
    >

    I agree.

    >
    > > 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.

    Maybe "a shorthand expression"?

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