On Wed, 24 Feb 2021 17:29:07 +0800
Jason Wang <jasow...@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 <jasow...@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 <jasow...@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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