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From: Jason Wang <jasowang@redhat.com>
To: Cornelia Huck <cohuck@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Si-Wei Liu <si-wei.liu@oracle.com>,
	elic@nvidia.com, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org
Subject: Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
Date: Tue, 23 Feb 2021 18:31:07 +0800
Message-ID: <bbb0a09e-17e1-a397-1b64-6ce9afe18e44@redhat.com> (raw)
In-Reply-To: <20210223110430.2f098bc0.cohuck@redhat.com>


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?


>   (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 
VIRTIO_NET_F_MTU is set."

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.

Thanks

>


  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 11:54 Si-Wei Liu
2021-02-21 14:44 ` Eli Cohen
2021-02-21 21:52   ` Michael S. Tsirkin
2021-02-22  6:05     ` Eli Cohen
2021-02-23  9:26       ` Michael S. Tsirkin
2021-02-23  9:48         ` Jason Wang
2021-02-23  9:55           ` Michael S. Tsirkin
2021-02-22  4:14 ` Jason Wang
2021-02-22  7:34   ` Michael S. Tsirkin
2021-02-23  1:12     ` Si-Wei Liu
2021-02-23  2:03       ` Jason Wang
2021-02-23 13:26         ` Michael S. Tsirkin
2021-02-23 19:35           ` Si-Wei Liu
2021-02-24  3:20             ` Jason Wang
2021-02-24  5:17               ` Michael S. Tsirkin
2021-02-24  6:02                 ` Jason Wang
2021-02-24  6:45                 ` Eli Cohen
2021-02-24  6:47                   ` Michael S. Tsirkin
2021-02-24  6:55                     ` Jason Wang
2021-02-24  7:12                       ` Michael S. Tsirkin
2021-02-24 12:40                         ` Eli Cohen
2021-02-24  7:17                       ` Eli Cohen
2021-02-24  5:04             ` Michael S. Tsirkin
2021-02-24  6:04               ` Jason Wang
2021-02-24  6:46                 ` Michael S. Tsirkin
2021-02-24  6:53                   ` Jason Wang
2021-02-24  7:17                     ` Michael S. Tsirkin
     [not found]                       ` <babc654d-8dcd-d8a2-c3b6-d20cc4fc554c@redhat.com>
2021-02-24  8:43                         ` Michael S. Tsirkin
2021-02-24  9:30                           ` Jason Wang
2021-02-28 21:30                             ` Michael S. Tsirkin
2021-03-01  3:53                               ` Jason Wang
2021-02-24 18:24               ` Si-Wei Liu
2021-02-26  0:56                 ` Si-Wei Liu
2021-02-28 21:27                   ` Michael S. Tsirkin
2021-03-01 18:08                     ` Si-Wei Liu
2021-02-28 21:28                 ` Michael S. Tsirkin
2021-02-28 21:34                 ` Michael S. Tsirkin
2021-03-01  3:56                   ` Jason Wang
2021-03-02  9:47                     ` Michael S. Tsirkin
2021-03-02 10:53                       ` Jason Wang
2021-02-22 17:09   ` Si-Wei Liu
2021-02-23  2:03     ` Jason Wang
2021-02-23  9:25     ` Michael S. Tsirkin
2021-02-23  9:46       ` Jason Wang
2021-02-23 10:01         ` Michael S. Tsirkin
2021-02-23 10:17           ` Jason Wang
2021-02-24  9:40             ` Jason Wang
2021-02-23 10:04         ` [virtio-dev] " Cornelia Huck
2021-02-23 10:31           ` Jason Wang [this message]
2021-02-23 10:58             ` Cornelia Huck
2021-02-24  9:29               ` Jason Wang
2021-02-24 11:12                 ` Cornelia Huck
2021-02-25  4:36                   ` Jason Wang
2021-02-25 13:26                     ` Cornelia Huck
2021-02-25 18:53                     ` Michael S. Tsirkin
2021-02-26  8:19                       ` Jason Wang
2021-02-28 21:25                         ` Michael S. Tsirkin
2021-03-01  3:51                           ` Jason Wang
2021-03-02 12:08                             ` Cornelia Huck
     [not found]                               ` <5f6972fe-7246-b622-958d-9cab8dd98e21@redhat.com>
2021-03-03  8:29                                 ` Cornelia Huck
2021-03-04  8:24                                   ` Jason Wang
2021-03-04 13:50                                     ` Cornelia Huck
2021-03-05  3:01                                       ` Jason Wang
2021-02-23 12:26 ` Michael S. Tsirkin

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