linux-kernel - Re: [RFC v4 0/3] vhost: introduce mdev based hardware backend

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Date:   Thu, 19 Sep 2019 21:08:11 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Tiwei Bie <tiwei.bie@...el.com>, alex.williamson@...hat.com,
        maxime.coquelin@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, dan.daly@...el.com,
        cunming.liang@...el.com, zhihong.wang@...el.com,
        lingshan.zhu@...el.com
Subject: Re: [RFC v4 0/3] vhost: introduce mdev based hardware backend


On 2019/9/18 下午10:32, Michael S. Tsirkin wrote:
>>>> So I have some questions:
>>>>
>>>> 1) Compared to method 2, what's the advantage of creating a new vhost char
>>>> device? I guess it's for keep the API compatibility?
>>> One benefit is that we can avoid doing vhost ioctls on
>>> VFIO device fd.
>> Yes, but any benefit from doing this?
> It does seem a bit more modular, but it's certainly not a big deal.


Ok, if we go this way, it could be as simple as provide some callback to 
vhost, then vhost can just forward the ioctl through parent_ops.


>
>>>> 2) For method 2, is there any easy way for user/admin to distinguish e.g
>>>> ordinary vfio-mdev for vhost from ordinary vfio-mdev?
>>> I think device-api could be a choice.
>> Ok.
>>
>>
>>>> I saw you introduce
>>>> ops matching helper but it's not friendly to management.
>>> The ops matching helper is just to check whether a given
>>> vfio-device is based on a mdev device.
>>>
>>>> 3) A drawback of 1) and 2) is that it must follow vfio_device_ops that
>>>> assumes the parameter comes from userspace, it prevents support kernel
>>>> virtio drivers.
>>>>
>>>> 4) So comes the idea of method 3, since it register a new vhost-mdev driver,
>>>> we can use device specific ops instead of VFIO ones, then we can have a
>>>> common API between vDPA parent and vhost-mdev/virtio-mdev drivers.
>>> As the above draft shows, this requires introducing a new
>>> VFIO device driver. I think Alex's opinion matters here.


Just to clarify, a new type of mdev driver but provides dummy 
vfio_device_ops for VFIO to make container DMA ioctl work.

Thanks


>> Yes, it is.
>>
>> Thanks
>>
>>