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Subject: Re: [RFC] vhost: introduce mdev based hardware vhost backend
To:     Tiwei Bie <tiwei.bie@intel.com>
Cc:     mst@redhat.com, alex.williamson@redhat.com, ddutile@redhat.com,
        alexander.h.duyck@intel.com, virtio-dev@lists.oasis-open.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        dan.daly@intel.com, cunming.liang@intel.com,
        zhihong.wang@intel.com, jianfeng.tan@intel.com,
        xiao.w.wang@intel.com
References: <20180402152330.4158-1-tiwei.bie@intel.com>
 <622f4bd7-1249-5545-dc5a-5a92b64f5c26@redhat.com>
 <20180410045723.rftsb7l4l3ip2ioi@debian>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <30a63fff-7599-640a-361f-a27e5783012a@redhat.com>
Date:   Tue, 10 Apr 2018 15:25:45 +0800
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On 2018年04月10日 12:57, Tiwei Bie wrote:
> On Tue, Apr 10, 2018 at 10:52:52AM +0800, Jason Wang wrote:
>> On 2018年04月02日 23:23, Tiwei Bie wrote:
>>> This patch introduces a mdev (mediated device) based hardware
>>> vhost backend. This backend is an abstraction of the various
>>> hardware vhost accelerators (potentially any device that uses
>>> virtio ring can be used as a vhost accelerator). Some generic
>>> mdev parent ops are provided for accelerator drivers to support
>>> generating mdev instances.
>>>
>>> What's this
>>> ===========
>>>
>>> The idea is that we can setup a virtio ring compatible device
>>> with the messages available at the vhost-backend. Originally,
>>> these messages are used to implement a software vhost backend,
>>> but now we will use these messages to setup a virtio ring
>>> compatible hardware device. Then the hardware device will be
>>> able to work with the guest virtio driver in the VM just like
>>> what the software backend does. That is to say, we can implement
>>> a hardware based vhost backend in QEMU, and any virtio ring
>>> compatible devices potentially can be used with this backend.
>>> (We also call it vDPA -- vhost Data Path Acceleration).
>>>
>>> One problem is that, different virtio ring compatible devices
>>> may have different device interfaces. That is to say, we will
>>> need different drivers in QEMU. It could be troublesome. And
>>> that's what this patch trying to fix. The idea behind this
>>> patch is very simple: mdev is a standard way to emulate device
>>> in kernel.
>> So you just move the abstraction layer from qemu to kernel, and you still
>> need different drivers in kernel for different device interfaces of
>> accelerators. This looks even more complex than leaving it in qemu. As you
>> said, another idea is to implement userspace vhost backend for accelerators
>> which seems easier and could co-work with other parts of qemu without
>> inventing new type of messages.
> I'm not quite sure. Do you think it's acceptable to
> add various vendor specific hardware drivers in QEMU?
>

I don't object but we need to figure out the advantages of doing it in 
qemu too.

Thanks