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Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
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
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <178f8ea7-cebd-0e81-3dc7-10a058d22c07@redhat.com>
Date:   Tue, 2 Mar 2021 18:53:50 +0800
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On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>> the kernel that it's a legacy guest. Ugh :(
>>>> Well, although I think adding an ioctl is doable, may I know what the use
>>>> case there will be for kernel to leverage such info directly? Is there a
>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>> differentiation (legacy v.s. modern) needed?
>>> BTW a good API could be
>>>
>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>
>>> we did it per vring but maybe that was a mistake ...
>>
>> Actually, I wonder whether it's good time to just not support legacy driver
>> for vDPA. Consider:
>>
>> 1) It's definition is no-normative
>> 2) A lot of budren of codes
>>
>> So qemu can still present the legacy device since the config space or other
>> stuffs that is presented by vhost-vDPA is not expected to be accessed by
>> guest directly. Qemu can do the endian conversion when necessary in this
>> case?
>>
>> Thanks
>>
> Overall I would be fine with this approach but we need to avoid breaking
> working userspace, qemu releases with vdpa support are out there and
> seem to work for people. Any changes need to take that into account
> and document compatibility concerns.


Agree, let me check.


>   I note that any hardware
> implementation is already broken for legacy except on platforms with
> strong ordering which might be helpful in reducing the scope.


Yes.

Thanks


>
>