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Date:   Mon, 22 Feb 2021 17:12:12 -0800
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     elic@...dia.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 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

>
>
>>> 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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