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Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
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
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <ce6b0380-bc4c-bcb8-db82-2605e819702c@redhat.com>
Date:   Wed, 24 Feb 2021 14:53:08 +0800
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On 2021/2/24 2:46 下午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 02:04:36PM +0800, Jason Wang wrote:
>> On 2021/2/24 1:04 下午, Michael S. Tsirkin wrote:
>>> On Tue, Feb 23, 2021 at 11:35:57AM -0800, 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.
>>>>
>>>> -Siwie
>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>> the kernel that it's a legacy guest. Ugh :(
>>
>> I'm not sure I get this, how can we know if there's a legacy driver before
>> set_features()?
> qemu knows for sure. It does not communicate this information to the
> kernel right now unfortunately.


I may miss something, but I still don't get how the new ioctl is 
supposed to work.

Thanks


>
>> And I wonder what will hapeen if we just revert the set_features(0)?
>>
>> 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@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)