linux-kernel - Re: [RFC] Unify KVM kernel-space and user-space code into a single project

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Message-ID: <20100318140933.GE25642@elte.hu>
Date:	Thu, 18 Mar 2010 15:09:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Sheng Yang <sheng@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, ziteng.huang@...el.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Fr?d?ric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project

* Avi Kivity <avi@...hat.com> wrote:

> > That is not what i said. I said they are closely related, and where 
> > technologies are closely related, project proximity turns into project 
> > unification at a certain stage.
> 
> I really don't see how.  So what if both qemu and kvm implement an i8254?  
> They can't share any code since the internal APIs are so different. [...]

I wouldnt jump to assumptions there. perf shares some facilities with the 
kernel on the source code level - they can be built both in the kernel and in 
user-space.

But my main thought wasnt even to actually share the implementation - but to 
actually synchronize when a piece of device emulation moves into the kernel. 
It is arguably bad for performance in most cases when Qemu handles a given 
device - so all the common devices should be kernel accelerated.

The version and testing matrix would be simplified significantly as well: as 
kernel and qemu goes hand in hand, they are always on the same version.

> [...] Even worse for the x86 emulator as qemu and kvm are fundamentally 
> different.

So is it your argument that the difference and the duplication in x86 
instruction emulation is a good thing? You said it some time ago that
the kvm x86 emulator was very messy and you wish it was cleaner.

While qemu's is indeed rather different (it's partly a translator/JIT), i'm 
sure the decoder logic could be shared - and qemu has a slow-path 
full-emulation fallback in any case, which is similar to what in-kernel 
emulator does (IIRC ...).

That might have changed meanwhile.

	Ingo
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