From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: 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: [PATCH] vdpa/mlx5: set_features should allow reset to zero
Date: Wed, 24 Feb 2021 17:40:24 +0800 [thread overview]
Message-ID: <0e79975a-9130-f437-bcea-928968fa523e@redhat.com> (raw)
In-Reply-To: <c188353e-9aca-a94c-e8f5-4bad5942481c@redhat.com>
On 2021/2/23 6:17 下午, Jason Wang wrote:
>
> On 2021/2/23 6:01 下午, Michael S. Tsirkin wrote:
>> On Tue, Feb 23, 2021 at 05:46:20PM +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?
>> Back in 2016 it went like this:
>>
>> On Thu, Jun 02, 2016 at 05:10:59PM -0400, Aaron Conole wrote:
>> > + if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>> > + dev->mtu = virtio_cread16(vdev,
>> > + offsetof(struct
>> virtio_net_config,
>> > + mtu));
>> > + }
>> > +
>> > if (vi->any_header_sg)
>> > dev->needed_headroom = vi->hdr_len;
>> >
>>
>> One comment though: I think we should validate the mtu.
>> If it's invalid, clear VIRTIO_NET_F_MTU and ignore.
>>
>>
>> Too late at this point :)
>>
>> I guess it's a way to tell device "I can not live with this MTU",
>> device can fail FEATURES_OK if it wants to. MIN_MTU
>> is an internal linux thing and at the time I felt it's better to
>> try to make progress.
>
>
> What if e.g the device advertise a large MTU. E.g 64K here?
Ok, consider we use add_recvbuf_small() when neither GSO nor mrg_rxbuf.
This means we should fail the probing if MTU is greater than 1500 in
this case.
Thanks
> In that case, the driver can not live either. Clearing MTU won't help
> here.
>
> Thanks
>
>
>>
>>
>>>> }
>>>>
>>>> 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.
>>>
>>> Thanks
>>>
>>>
>>>> Generally it is worth going over feature dependent config fields
>>>> and checking whether they should be present when device feature is set
>>>> or when feature bit has been negotiated, and making this clear.
>>>>
>
next prev parent reply other threads:[~2021-02-24 9:45 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-19 11:54 [PATCH] vdpa/mlx5: set_features should allow reset to zero 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
2021-12-11 1:44 ` vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero) Si-Wei Liu
2021-12-12 9:26 ` Michael S. Tsirkin
2021-12-13 3:02 ` Jason Wang
2021-12-13 8:06 ` Michael S. Tsirkin
2021-12-13 8:57 ` Jason Wang
2021-12-13 10:42 ` Michael S. Tsirkin
2021-12-14 1:13 ` Si-Wei Liu
2021-12-14 1:59 ` Si-Wei Liu
2021-12-14 3:01 ` Jason Wang
2021-12-14 5:06 ` Michael S. Tsirkin
2021-12-15 1:05 ` Si-Wei Liu
2021-12-15 2:06 ` Jason Wang
2021-12-15 20:52 ` Si-Wei Liu
2021-12-15 21:33 ` Michael S. Tsirkin
2021-12-16 2:01 ` Si-Wei Liu
2021-12-16 2:53 ` Jason Wang
2021-12-16 22:32 ` Si-Wei Liu
2021-12-17 1:57 ` Jason Wang
2021-12-17 2:00 ` Michael S. Tsirkin
2021-12-17 2:15 ` Jason Wang
2021-12-16 6:35 ` Michael S. Tsirkin
2021-12-16 3:43 ` Jason Wang
2021-12-17 1:08 ` Si-Wei Liu
2021-12-17 2:01 ` Jason Wang
2021-02-22 17:09 ` [PATCH] vdpa/mlx5: set_features should allow reset to zero 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 [this message]
2021-02-23 10:04 ` [virtio-dev] " Cornelia Huck
2021-02-23 10:31 ` Jason Wang
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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