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Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
To:     "Michael S. Tsirkin" <mst@redhat.com>, Eli Cohen <elic@nvidia.com>
Cc:     Si-Wei Liu <si-wei.liu@oracle.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: <65494f6b-9613-1c0e-4a36-e4af2965235e@redhat.com>
Date:   Tue, 23 Feb 2021 17:48:10 +0800
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On 2021/2/23 下午5:26, Michael S. Tsirkin wrote:
> On Mon, Feb 22, 2021 at 08:05:26AM +0200, Eli Cohen wrote:
>> On Sun, Feb 21, 2021 at 04:52:05PM -0500, Michael S. Tsirkin wrote:
>>> On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
>>>> On Fri, Feb 19, 2021 at 06:54:58AM -0500, 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. 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;
>>>>> -}
>>>>> -
>>>> But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
>>>> support such cases.
>>> Did you mean "catch such cases" rather than "support"?
>>>
>> Actually I meant this driver/device does not support such cases.
> Well the removed code merely failed without VIRTIO_F_ACCESS_PLATFORM
> it didn't actually try to support anything ...


I think it's used to catch the driver that doesn't support ACCESS_PLATFORM?

Thanks


>