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Date: Thu, 16 Apr 2020 21:15:17 +0200
From: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Subject: Re: RPMSG over VirtIO under KVM
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To: Mathieu Poirier <mathieu.poirier@linaro.org>
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Hi Mathieu,

On Thu, Apr 16, 2020 at 11:50:48AM -0600, Mathieu Poirier wrote:
> Good day Guennadi,
> 
> On Thu, 16 Apr 2020 at 08:06, Guennadi Liakhovetski
> <guennadi.liakhovetski@linux.intel.com> wrote:
> >
> > Hi,
> >
> > It has been proposed to port the VirtIO SOF driver [1], used to
> > implement audio support under Linux, running in a KVM guest, to use
> > RPMSG to communicate with the SOF vhost driver, running on the Linux
> > host. On one hand I see an rpmsg-virtio driver, which should make such
> > a port possible, on the other hand I don't see a single VirtIO driver
> > in the kernel, using RPMSG for Linux virtualisation.
> 
> Above you wrote "rpmsg-virtio" driver, which I take to mean the code
> found in file virtio_rpmsg_bus.c [1].

Exactly.

> The code in [1] centers around
> the communication between an application processor and some form of
> remote processor (micro controller or dsp).  The "virtio" part of the
> name refers to the underlying infrastructure put in place to
> communicate with the remote processor, all coming from the virtio
> space.  Here instead of using the virtio mechanic to communicate
> between a host and a guest, it is used to communicate with a remote
> processor.
> 
> I came to the same conclusion a while back - as of today no virtio
> drivers are using RPMSG to communicate between host and guest.  I
> suppose nobody needed it or implemented their own schemes.
> 
> [1].  https://elixir.bootlin.com/linux/v5.7-rc1/source/drivers/rpmsg/virtio_rpmsg_bus.c
> 
> >
> > Hence my questions: is this a good idea? Is there anything in the
> > kernel VirtIO RPMSG implementation, that would make this impossible?
> 
> I don't see why it wouldn't be a good idea, nor what would technically
> prevent such a thing from happening.  Two things work in your favour:
> 1) the RPMSG foundation has been tailored to be used over different
> kinds of hardware and 2) an existing implementation is already using
> virtioqueues.
> 
> I suggest to start looking at function rpmsg_register_device(), used
> by different RPMSG drivers - the magic is really in the RPMSG
> operations (struct rpmsg_device_ops) that are used to abstract the HW
> implementation.

Exactly, just what I was thinking too. And I also think it can well be 
possible to reuse the code in virtio_rpmsg_bus.c, possibly with some 
limited extensions and modifications. I'll get it rolling then.

Thanks
Guennadi

> Regards,
> Mathieu
> 
> > [1] https://thesofproject.github.io/latest/developer_guides/virtualization/virtualization.html