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

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Date:	Fri, 3 Apr 2009 14:26:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
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


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

> Ingo Molnar wrote:
>>> 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.
>>>     
>>
>> Well, this has to be driven from the KVM side as the kprobes use 
>> will only be for decoding so if it's modified from the kprobes 
>> side the KVM-only functionality might regress.
>>
>> So ... we can do the library decoder for kprobes purposes, and 
>> someone versed in the KVM emulator can then combine the two.
>
> Problem is, anyone versed in the kvm emulator will want to run as 
> far away from this work as possible.

Are you suggesting that the KVM emulator should never have been 
merged in the first place? ;-)

Anyway, we'll make sure the kprobes/library decoder is as clean as 
possible - so it ought to be hackable and extensible without the 
risk of permanent brain damage. Mmiotrace and kmemcheck has decoding 
smarts too, and i think the sw-breakpoint injection code of KGDB 
could use it as well - so there's broader utility in all this.

	Ingo
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