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Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
To:     Si-Wei Liu <si-wei.liu@oracle.com>, mst@redhat.com, elic@nvidia.com
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org
References: <1613735698-3328-1-git-send-email-si-wei.liu@oracle.com>
 <605e7d2d-4f27-9688-17a8-d57191752ee7@redhat.com>
 <ee31e93b-5fbb-1999-0e82-983d3e49ad1e@oracle.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <716ad747-501b-4e95-9785-87161da5940f@redhat.com>
Date:   Tue, 23 Feb 2021 10:03:08 +0800
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On 2021/2/23 1:09 上午, Si-Wei Liu wrote:
>
>
> On 2/21/2021 8:14 PM, 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?
>
> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?


Yes, but the problem is we can't detect whether or not it's an legacy 
guest (e.g feature is not set).


>
> I think the point is, since there's legacy guest we'd have to support, 
> this host side workaround is unavoidable.


Since from vhost-vDPA point of view the driver is Qemu, it means we need 
make qemu vhost-vDPA driver spec complaint.

E.g how about:

1) revert 452639a64ad8 and fix Qemu? In Qemu, during vhost-vDPA 
initialization, do a minial config sync by neogitating minimal features 
(e.g just VIRTIO_F_ACCESS_PLATFORM). When FEATURE_OK is not set from 
guest, we can only allow it to access the config space that is emulated 
by Qemu?

Then


> Although I agree the violating driver should be fixed (yes, it's in 
> today's upstream kernel which exists for a while now).


2) Fix the virtio driver bug.

Or a quick workaround is to set (VIRTIO_F_ACCESS_PLATFORM instead of 0) 
in this case.

Thanks


>
> -Siwei
>
>>
>> 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)
>>
>