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    SubjectRe: [PATCH] vdpa/mlx5: set_features should allow reset to zero
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    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()?

    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)

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