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Message-ID: <ef775724-b5fb-ca70-ed2f-f23d8fbf4cd8@redhat.com>
Date: Wed, 24 Feb 2021 14:55:13 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, Eli Cohen <elic@...dia.com>
Cc: Si-Wei Liu <si-wei.liu@...cle.com>, linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
On 2021/2/24 2:47 下午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 08:45:20AM +0200, Eli Cohen wrote:
>> On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
>>> On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
>>>> On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
>>>>>
>>>>> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>>>>>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>>>>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, 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?
>>>>>>>>>>
>>>>>>>>>> Thanks
>>>>>>>>> And also:
>>>>>>>>>
>>>>>>>>> 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”.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> so accessing config space before FEATURES_OK is a spec
>>>>>>>>> violation, right?
>>>>>>>> It is, but it's not relevant to what this commit tries to address. I
>>>>>>>> thought the legacy guest still needs to be supported.
>>>>>>>>
>>>>>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>>>>>> issue where this discrepancy is introduced to
>>>>>>>> virtnet_validate() (since
>>>>>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>>>>>
>>>>>>>> -Siwei
>>>>>>> I think it's a bug to read config space in validate, we should
>>>>>>> move it to
>>>>>>> virtnet_probe().
>>>>>>>
>>>>>>> Thanks
>>>>>> I take it back, reading but not writing seems to be explicitly
>>>>>> allowed by spec.
>>>>>> So our way to detect a legacy guest is bogus, need to think what is
>>>>>> the best way to handle this.
>>>>> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
>>>>> legacy guest? Supposedly only config space write access needs to be
>>>>> guarded before setting FEATURES_OK.
>>>>
>>>> I agree. My understanding is that all vDPA must be modern device (since
>>>> VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
>>>>
>>>> Thanks
>>> Well mlx5 has some code to handle legacy guests ...
>>> Eli, could you comment? Is that support unused right now?
>>>
>> If you mean support for version 1.0, well the knob is there but it's not
>> set in the firmware I use. Note sure if we will support this.
> Hmm you mean it's legacy only right now?
> Well at some point you will want advanced goodies like RSS
> and all that is gated on 1.0 ;)
So if my understanding is correct the device/firmware is legacy but
require VIRTIO_F_ACCESS_PLATFORM semanic? Looks like a spec violation?
Thanks
>
>>>>> -Siwie
>>>>>
>>>>>>>>>>> Rejecting reset to 0
>>>>>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>>>>>> for the very first config space access, rendering issues like
>>>>>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>>>>>> out-of-range MTU.
>>>>>>>>>>>
>>>>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>>>>>> supported mlx5 devices")
>>>>>>>>>>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
>>>>>>>>>>> ---
>>>>>>>>>>> drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>>>>> 1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> index 7c1f789..540dd67 100644
>>>>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>>>>> return mvdev->mlx_features;
>>>>>>>>>>> }
>>>>>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>>>>>> u64 features)
>>>>>>>>>>> -{
>>>>>>>>>>> - if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>>>>>> - return -EOPNOTSUPP;
>>>>>>>>>>> -
>>>>>>>>>>> - return 0;
>>>>>>>>>>> -}
>>>>>>>>>>> -
>>>>>>>>>>> static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>>>>> {
>>>>>>>>>>> int err;
>>>>>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>>>>>> features)
>>>>>>>>>>> {
>>>>>>>>>>> struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>>>>> struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>>>>>> - int err;
>>>>>>>>>>> print_features(mvdev, features, true);
>>>>>>>>>>> - err = verify_min_features(mvdev, features);
>>>>>>>>>>> - if (err)
>>>>>>>>>>> - return err;
>>>>>>>>>>> -
>>>>>>>>>>> ndev->mvdev.actual_features = features &
>>>>>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>>>>> ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>>>>> ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>>>>>> - return err;
>>>>>>>>>>> + return 0;
>>>>>>>>>>> }
>>>>>>>>>>> static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>>>>>> *vdev, struct vdpa_callback *cb)
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