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 18:11:50 +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:

> >  - Easy default reference to guest instances, and a way for tools to
> >    reference them symbolically as well in the multi-guest case. Preferably
> >    something trustable and kernel-provided - not some indirect information
> >    like a PID file created by libvirt-manager or so.
> 
> A guest is not a KVM concept. [...]

Well, in a sense a guest is a KVM concept too: it's in essence represented via 
the 'vcpu state attached to a struct mm' abstraction that is attached to the 
/dev/kvm file descriptor attached to a Linux process.

Multiple vcpus can be started by the same process to represent SMP, but the 
whole guest notion is present: a Linux MM that carries KVM state.

In that sense when we type 'perf kvm list' we'd like to get a list of all 
currently present guests that the developer has permission to profile: i.e. 
we'd like a list of all [debuggable] Linux tasks that have a KVM instance 
attached to them.

A convenient way to do that would be to use the Qemu process's ->comm[] name, 
and to have a KVM ioctl that gets us a list of all vcpus that the querying 
task has ptrace permission to. [the standard permission check we do for 
instrumentation]

No need for communication with Qemu for that - just an ioctl, and an 
always-guaranteed result that works fine on a whole-system and on a per user 
basis as well.

Thanks,

	Ingo
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