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Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
To:     Si-Wei Liu <si-wei.liu@oracle.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     elic@nvidia.com, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <7e6291a4-30b1-6b59-a2bf-713e7b56826d@redhat.com>
Date:   Wed, 24 Feb 2021 11:20:01 +0800
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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


>
> -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@oracle.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)
>