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

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Date:	Mon, 22 Mar 2010 20:27:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Avi Kivity <avi@...hat.com>, Pekka Enberg <penberg@...helsinki.fi>,
	"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>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project


* Anthony Liguori <anthony@...emonkey.ws> wrote:

> On 03/22/2010 12:34 PM, Ingo Molnar wrote:
> >This is really just the much-discredited microkernel approach for keeping
> >global enumeration data that should be kept by the kernel ...
> >
> >Lets look at the ${HOME}/.qemu/qmp/ enumeration method suggested by Anthony.
> >There's numerous ways that this can break:
> >
> >  - Those special files can get corrupted, mis-setup, get out of sync, or can
> >    be hard to discover.
> >
> >  - The ${HOME}/.qemu/qmp/ solution suggested by Anthony has a very obvious
> >    design flaw: it is per user. When i'm root i'd like to query _all_ current
> >    guest images, not just the ones started by root. A system might not even
> >    have a notion of '${HOME}'.
> >
> >  - Apps might start KVM vcpu instances without adhering to the
> >    ${HOME}/.qemu/qmp/ access method.
> >
> >  - There is no guarantee for the Qemu process to reply to a request - while
> >    the kernel can always guarantee an enumeration result. I dont want 'perf
> >    kvm' to hang or misbehave just because Qemu has hung.
> 
> If your position basically boils down to, we can't trust userspace
> and we can always trust the kernel, I want to eliminate any
> userspace path, then I can't really help you out.

Why would you want to 'help me out'? I can tell a good solution from a bad one 
just fine.

You should instead read the long list of disadvantages above, invert them and 
list then as advantages for the kernel-based vcpu enumeration solution, apply 
common sense and go admit to yourself that indeed in this situation a kernel 
provided enumeration of vcpu contexts is the most robust solution.

It's really as simple as that :-)

Thanks,

	Ingo
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