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 20:20:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	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


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

> > Lets look at the ${HOME}/.qemu/qmp/ enumeration method suggested by 
> > Anthony. There's numerous ways that this can break:
> 
> I don't like it either.  We have libvirt for enumerating guests.

Which has pretty much the same problems to the ${HOME}/.qemu/qmp/ solution, 
obviously.

> >  - 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.
> 
> - it doesn't work with nfs.

So out of a list of 4 disadvantages your reply is that you agree with 3?

> >  - 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 qemu doesn't reply, your guest is dead anyway.

Erm, but i'm talking about a dead tool here. There's a world of a difference 
between 'kvm top' not showing new entries (because the guest is dead), and 
'perf kvm top' hanging due to Qemu hanging.

So it's essentially 4 our of 4. Yet your reply isnt "Ingo you are right" but 
"hey, too bad" ?

> > Really, for such reasons user-space is pretty poor at doing system-wide 
> > enumeration and resource management. Microkernels lost for a reason.
> 
> Take a look at your desktop, userspace is doing all of that everywhere, from 
> enumerating users and groups, to deciding how your disks are named.  The 
> kernel only provides the bare facilities.

We dont do that for robust system instrumentation, for heaven's sake!

By your argument it would be perfectly fine to implement /proc purely via 
user-space, correct?

> > You are committing several grave design mistakes here.
> 
> I am committing on the shoulders of giants.

Really, this is getting outright ridiculous. You agree with me that Anothony 
suggested a technically inferior solution, yet you even seem to be proud of it 
and are joking about it?

And _you_ are complaining about lkml-style hard-talk discussions?

Thanks,

	Ingo
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