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Re: [Qemu-devel] Unified device model

From: Jim C. Brown
Subject: Re: [Qemu-devel] Unified device model
Date: Sat, 8 Apr 2006 15:12:19 -0400
User-agent: Mutt/

On Sat, Apr 08, 2006 at 09:57:10PM +0200, Stanislav Shwartsman wrote:
> Hello All,
> It is not a secret that all open source emulators (QEMU, Bochs, Xen) use the
> same emulated devices and mostly copy-paste their emulation one from
> another.

While from my understanding Xen uses qemu's hardware emulation for it's VT
support, this is not really true otherwise.

The devices emulated by qemu and bochs are quite similar, but the code looks
completely different (appears to be a ground-up rewrite).

> I don't know who originally wrote the device models but now Bochs and QEMU
> maintain two similar implementations of the same devices.
> If one of the teams fixes the implementation or add functionality, another
> team mostly copy-paste the changes to their model.

I don't know how well Bochs and qemu keep in touch with each other. I've never
seen a Bochs developer announce themselves here, though.

> Xen project forked from QEMU and want to stay in touch with Bochs and QEMU
> device models and contribute the changes to make the model better.

Not true. Xen is completely independent. Unless you are refering to the
hardware emulation - which I believe is qemu's stuff.

> I am wondering about making unified device models architecture for open
> source simulators.
> The device models will be used in QEMU, Bochs, Xen and other open source
> simulators which would use the device models.

I would support this idea, if it was possible.

> I know about two professional teams working in simulation which would like
> to use these device models in their simulator and 
> could enrich the device library with new devices device interfaces, for
> example with AGP and 3D graphics.
> Bochs is already in middle of definition of new true pluginable devices
> architecture. 

This is welcome news.

> In near future Bochs devices will fully separatable from Bochs binary and
> when could be developed separately from Bochs.
> I call to QEMU developers join to this project and come with their
> requirements to plugin architecture.
> I don't know if QEMU supports device plugins now but I would like to see
> QEMU a part of this idea, 

I would as well.

> I would like to get single device shared library which could be loaded to
> Bochs and QEMU and work perfectly for both.
> This will eliminate the need to maintain two separate implementations of the
> same devices, 
> these implementations very fast will converge to single one, C or C++ based,
> Bochs or QEMU based, doesn't matter.
> I am listening for your opinions !

The primary reason given for not making a plugin API for qemu hardware emulation
is that qemu isn't stable enough - the code changes too often to support a 

Still, it might be easier to add support for plugins based on an external API,
rather than trying to keep a qemu plugin API consistent.

> Thanks,
> Stanislav

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