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

On 2021/3/1 5:25 äå, Michael S. Tsirkin wrote:
On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
On 2021/2/26 2:53 äå, Michael S. Tsirkin wrote:
On Thu, Feb 25, 2021 at 12:36:07PM +0800, Jason Wang wrote:
On 2021/2/24 7:12 äå, Cornelia Huck wrote:
On Wed, 24 Feb 2021 17:29:07 +0800
Jason Wang <jasowang@redhat.com> wrote:

On 2021/2/23 6:58 äå, Cornelia Huck wrote:
On Tue, 23 Feb 2021 18:31:07 +0800
Jason Wang <jasowang@redhat.com> wrote:
On 2021/2/23 6:04 äå, Cornelia Huck wrote:
On Tue, 23 Feb 2021 17:46:20 +0800
Jason Wang <jasowang@redhat.com> wrote:
On 2021/2/23 äå5:25, 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 äå, 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",
                      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,
                      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 deviceâs virtio configuration space, and population of virtqueues.
8. Set the DRIVER_OK status bit. At this point the device is âliveâ.

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?
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

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?
Yes, see the avove codes:

  ÂÂÂÂÂÂÂ if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ int mtu = virtio_cread16(vdev,

So it's probably too late to fix the driver.

Confused. What is wrong with the above? It never reads the
field unless the feature has been offered by device.

So the spec said:


The following driver-read-only field, max_virtqueue_pairs only exists if


If I read this correctly, there will be no max_virtqueue_pairs field if the
VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
what spec said.

I think that's a misunderstanding. This text was never intended to
imply that field offsets change beased on feature bits.
We had this pain with legacy and we never wanted to go back there.

This merely implies that without VIRTIO_NET_F_MQ the field
should not be accessed. Exists in the sense "is accessible to driver".

Let's just clarify that in the spec, job done.

Ok, agree. That will make things more eaiser.


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
See Michael's reply, the spec allows read the config before setting
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
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
So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
will look like:

struct virtio_net_config {
    ÂÂÂÂÂÂÂ u8 mac[6];
    ÂÂÂÂÂÂÂ le16 status;
    ÂÂÂÂÂÂÂ le16 mtu;

I agree.
So consider it's probably too late to fix the driver which assumes some
field are always persent, it looks to me need fix the spec do declare the
fields are always existing instead.

     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"?
So the questions is should we use this for all over the spec or it will be
only used in this speicifc part (device configuration).


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