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Date:   Tue, 23 Feb 2021 10:03:57 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>,
        "Michael S. Tsirkin" <mst@...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 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


>
>>
>>
>>>> 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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