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From: Jason Wang <jasowang@redhat.com>
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
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
Date: Wed, 24 Feb 2021 14:53:08 +0800
Message-ID: <ce6b0380-bc4c-bcb8-db82-2605e819702c@redhat.com> (raw)
In-Reply-To: <20210224014232-mutt-send-email-mst@kernel.org>


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)


  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 11:54 Si-Wei Liu
2021-02-21 14:44 ` Eli Cohen
2021-02-21 21:52   ` Michael S. Tsirkin
2021-02-22  6:05     ` Eli Cohen
2021-02-23  9:26       ` Michael S. Tsirkin
2021-02-23  9:48         ` Jason Wang
2021-02-23  9:55           ` Michael S. Tsirkin
2021-02-22  4:14 ` Jason Wang
2021-02-22  7:34   ` Michael S. Tsirkin
2021-02-23  1:12     ` Si-Wei Liu
2021-02-23  2:03       ` Jason Wang
2021-02-23 13:26         ` Michael S. Tsirkin
2021-02-23 19:35           ` Si-Wei Liu
2021-02-24  3:20             ` Jason Wang
2021-02-24  5:17               ` Michael S. Tsirkin
2021-02-24  6:02                 ` Jason Wang
2021-02-24  6:45                 ` Eli Cohen
2021-02-24  6:47                   ` Michael S. Tsirkin
2021-02-24  6:55                     ` Jason Wang
2021-02-24  7:12                       ` Michael S. Tsirkin
2021-02-24 12:40                         ` Eli Cohen
2021-02-24  7:17                       ` Eli Cohen
2021-02-24  5:04             ` Michael S. Tsirkin
2021-02-24  6:04               ` Jason Wang
2021-02-24  6:46                 ` Michael S. Tsirkin
2021-02-24  6:53                   ` Jason Wang [this message]
2021-02-24  7:17                     ` Michael S. Tsirkin
     [not found]                       ` <babc654d-8dcd-d8a2-c3b6-d20cc4fc554c@redhat.com>
2021-02-24  8:43                         ` Michael S. Tsirkin
2021-02-24  9:30                           ` Jason Wang
2021-02-28 21:30                             ` Michael S. Tsirkin
2021-03-01  3:53                               ` Jason Wang
2021-02-24 18:24               ` Si-Wei Liu
2021-02-26  0:56                 ` Si-Wei Liu
2021-02-28 21:27                   ` Michael S. Tsirkin
2021-03-01 18:08                     ` Si-Wei Liu
2021-02-28 21:28                 ` Michael S. Tsirkin
2021-02-28 21:34                 ` Michael S. Tsirkin
2021-03-01  3:56                   ` Jason Wang
2021-03-02  9:47                     ` Michael S. Tsirkin
2021-03-02 10:53                       ` Jason Wang
2021-02-22 17:09   ` Si-Wei Liu
2021-02-23  2:03     ` Jason Wang
2021-02-23  9:25     ` Michael S. Tsirkin
2021-02-23  9:46       ` Jason Wang
2021-02-23 10:01         ` Michael S. Tsirkin
2021-02-23 10:17           ` Jason Wang
2021-02-24  9:40             ` Jason Wang
2021-02-23 10:04         ` [virtio-dev] " Cornelia Huck
2021-02-23 10:31           ` Jason Wang
2021-02-23 10:58             ` Cornelia Huck
2021-02-24  9:29               ` Jason Wang
2021-02-24 11:12                 ` Cornelia Huck
2021-02-25  4:36                   ` Jason Wang
2021-02-25 13:26                     ` Cornelia Huck
2021-02-25 18:53                     ` Michael S. Tsirkin
2021-02-26  8:19                       ` Jason Wang
2021-02-28 21:25                         ` Michael S. Tsirkin
2021-03-01  3:51                           ` Jason Wang
2021-03-02 12:08                             ` Cornelia Huck
     [not found]                               ` <5f6972fe-7246-b622-958d-9cab8dd98e21@redhat.com>
2021-03-03  8:29                                 ` Cornelia Huck
2021-03-04  8:24                                   ` Jason Wang
2021-03-04 13:50                                     ` Cornelia Huck
2021-03-05  3:01                                       ` Jason Wang
2021-02-23 12:26 ` Michael S. Tsirkin

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