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 21:29:37 +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:

> > I think you didnt understand my point. I am talking about 'perf kvm top' 
> > hanging if Qemu hangs.
> 
> Use non-blocking I/O, report that guest as dead.  No point in profiling it, 
> it isn't making any progress.

Erm, at what point do i decide that a guest is 'dead' versus 'just lagged due 
to lots of IO' ?

Also, do you realize that you increase complexity (the use of non-blocking 
IO), just to protect against something that wouldnt happen if the right 
solution was used in the first place?

> > With a proper in-kernel enumeration the kernel would always guarantee the 
> > functionality, even if the vcpu does not make progress (i.e. it's "hung").
> >
> > With this implemented in Qemu we lose that kind of robustness guarantee.
> 
> If qemu has a bug in the resource enumeration code, you can't profile one 
> guest.  If the kernel has a bug in the resource enumeration code, the system 
> either panics or needs to be rebooted later.

This is really simple code, not rocket science. If there's a bug in it we'll 
fix it. On the other hand a 500KLOC+ piece of Qemu code has lots of places to 
hang, so that is a large cross section.

	Ingo
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