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Subject: Re: [RFC v2 17/37] lkl tools: host lib: virtio devices
From: Johannes Berg <johannes@sipsolutions.net>
Date: Tue, 26 Nov 2019 09:52:34 +0100
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To: Richard Weinberger <richard@nod.at>
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On Tue, 2019-11-26 at 09:50 +0100, Richard Weinberger wrote:

> > This isn't really a driver, this is virtio *device-side* code. Our
> > virtio_uml is *guest-side* code, and only speaks vhost-user.
> 
> Sorry, bad wording from my side. I meant with "driver" a kernel component.

Yeah, though arguably this isn't a kernel component, this is a
hypervisor component... We just link them both into the same binary due
to the way UML/LKL work.
 
> > I'm not sure how MMIO devices could possibly work though, does LKL
> > intercept MMIO somehow?
> 
> My point is that UML and LKL should try to do use the same concept/code
> regarding virtio. At the end of day both use virtual devices which use
> facilities from the host.
> If this is really not possible it needs a good explanation.

I think it isn't possible, unless you use vhost-user over a unix domain
socket internally to talk between the kernel (virtio_uml) and hypervisor
(device) components.

In virtio_uml, the device implementation is assumed to be a separate
process with a vhost-user connection. Here in LKL, the virtio device is
part of the "hypervisor", i.e. in the same process.

johannes