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SubjectRe: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

On 2021/2/23 6:04 下午, Cornelia Huck wrote:
> On Tue, 23 Feb 2021 17:46:20 +0800
> Jason Wang <> 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
>>> 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

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.



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