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Message-ID: <20100322202937.GA18126@elte.hu>
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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