linux-kernel - Re: [PATCH -tip 0/6 V4] tracing: kprobe-based event tracer

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Date:	Fri, 03 Apr 2009 14:50:35 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Masami Hiramatsu <mhiramat@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Jim Keniston <jkenisto@...ibm.com>, kvm@...r.kernel.org,
	systemtap-ml <systemtap@...rces.redhat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip 0/6 V4] tracing: kprobe-based event tracer

Ingo Molnar wrote:
> ok, the structure and concept looks quite good now, really nice!
>
> I'm wondering about something i suggested many moons ago: to look 
> into the KVM decoder+emulator (arch/x86/kvm/x86_emulate.c).
>
> I remember there were some issues with that (one problem being that 
> the KVM decoder is a special-purpose thing covering specific range 
> of execution environments - not a near-full integer-ops decoder like 
> the one we are aiming for here) - are there any other fundamental 
> problems beyond 'it has to be done' ?
>
> Conceptually we want just a single piece of decoder logic in 
> arch/x86/. If the KVM folks are cool with it we could factor out the 
> KVM one into arch/x86/lib/. But ... if there are compelling reasons 
> to leave the KVM one alone in its limited environment we can do that 
> too.
>   

kvm has three requirements not needed by kprobes:
- it wants to execute instructions, not just decode them, including 
generating faults where appropriate
- it is performance critical
- it needs to support 16-bit, 32-bit, and 64-bit instructions simultaneously

If an arch/x86/ decoder/emulator gives me these I'll gladly switch to 
it.  x86_emulate.c is high on my list of most disliked code.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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