linux-kernel - Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

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Message-ID: <20100316122903.GA8831@elte.hu>
Date:	Tue, 16 Mar 2010 13:29:03 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	"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
Subject: Re: [PATCH] Enhance perf to collect KVM guest os statistics from
 host side


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

> On 03/16/2010 01:25 PM, Ingo Molnar wrote:
> >
> >>I haven't followed vmchannel closely, but I think it is.  vmchannel is
> >>terminated in qemu on the host side, not in the host kernel.  So perf would
> >>need to connect to qemu.
> >Hm, that sounds rather messy if we want to use it to basically expose kernel
> >functionality in a guest/host unified way. Is the qemu process discoverable in
> >some secure way?
> 
> We know its pid.

How do i get a list of all 'guest instance PIDs', and what is the way to talk 
to Qemu?

> > Can we trust it?
> 
> No choice, it contains the guest address space.

I mean, i can trust a kernel service and i can trust /proc/kallsyms.

Can perf trust a random process claiming to be Qemu? What's the trust 
mechanism here?

> > Is there some proper tooling available to do it, or do we have to push it 
> > through 2-3 packages to get such a useful feature done?
> 
> libvirt manages qemu processes, but I don't think this should go through 
> libvirt.  qemu can do this directly by opening a unix domain socket in a 
> well-known place.

So Qemu has never run into such problems before?

( Sounds weird - i think Qemu configuration itself should be done via a 
  unix domain socket driven configuration protocol as well. )

> >( That is the general thought process how many cross-discipline useful
> >   desktop/server features hit the bit bucket before having had any chance of
> >   being vetted by users, and why Linux sucks so much when it comes to feature
> >   integration and application usability. )
> 
> You can't solve everything in the kernel, even with a well populated tools/.

Certainly not, but this is a technical problem in the kernel's domain, so it's 
a fair (and natural) expectation to be able to solve this within the kernel 
project.

	Ingo
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