Re: [PATCH 2/3] mlx5_vdpa: fix feature negotiation across device reset

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 2/7/2021 9:35 PM, Eli Cohen wrote:
> On Sat, Feb 06, 2021 at 04:29:23AM -0800, Si-Wei Liu wrote:
>> The mlx_features denotes the capability for which
>> set of virtio features is supported by device. In
>> principle, this field needs not be cleared during
>> virtio device reset, as this capability is static
>> and does not change across reset.
>>
>> In fact, the current code may have the assumption
>> that mlx_features can be reloaded from firmware
>> via the .get_features ops after device is reset
>> (via the .set_status ops), which is unfortunately
>> not true. The userspace VMM might save a copy
>> of backend capable features and won't call into
>> kernel again to get it on reset. This causes all
>> virtio features getting disabled on newly created
>> virtqs after device reset, while guest would hold
>> mismatched view of available features. For e.g.,
>> the guest may still assume tx checksum offload
>> is available after reset and feature negotiation,
>> causing frames with bogus (incomplete) checksum
>> transmitted on the wire.
>>
>> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
>> ---
>>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> index b8416c4..aa6f8cd 100644
>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> @@ -1788,7 +1788,6 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev, u8 status)
>>   		clear_virtqueues(ndev);
>>   		mlx5_vdpa_destroy_mr(&ndev->mvdev);
>>   		ndev->mvdev.status = 0;
>> -		ndev->mvdev.mlx_features = 0;
>>   		++mvdev->generation;
>>   		return;
>>   	}
> Since we assume that device capabilities don't change, I think I would
> get the features through a call done in mlx5v_probe after the netdev
> object is created and change mlx5_vdpa_get_features() to just return
> ndev->mvdev.mlx_features.
Yep, it makes sense. Will post a revised patch. If vdpa tool allows 
reconfiguration post probing, the code has to be reconciled then.

>
> Did you actually see this issue in action? If you did, can you share
> with us how you trigerred this?
Issue is indeed seen in action. The mismatched tx-checksum offload as 
described in the commit message was one of such examples. You would need 
a guest reboot though (triggering device reset via the .set_status ops 
and zero'ed mlx_features was loaded to deduce new actual_features for 
creating the h/w virtq object) for repro.

-Siwei
>
>> -- 
>> 1.8.3.1
>>