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Subject: Re: [RFC] vhost: introduce mdev based hardware vhost backend

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.


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