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Date:   Fri, 26 Feb 2021 16:19:16 +0800
From:   Jason Wang <>
To:     "Michael S. Tsirkin" <>
Cc:     Cornelia Huck <>,
        Si-Wei Liu <>,,,,,
Subject: Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow
 reset to zero

On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
> On Thu, Feb 25, 2021 at 12:36:07PM +0800, Jason Wang wrote:
>> On 2021/2/24 7:12 下午, Cornelia Huck wrote:
>>> On Wed, 24 Feb 2021 17:29:07 +0800
>>> Jason Wang <> wrote:
>>>> On 2021/2/23 6:58 下午, Cornelia Huck wrote:
>>>>> On Tue, 23 Feb 2021 18:31:07 +0800
>>>>> Jason Wang <> wrote:
>>>>>> 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
>>>>>>>>> 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?
>> Yes, see the avove codes:
>>          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);
>>          }
>> 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.


>>>>>> 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.
>> 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).
>> Thanks

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