linux-kernel - Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

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Message-ID: <4B56D027.3010808@redhat.com>
Date:	Wed, 20 Jan 2010 11:43:03 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jim Keniston <jkenisto@...ibm.com>
CC:	Pekka Enberg <penberg@...helsinki.fi>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, ananth@...ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	utrace-devel <utrace-devel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

On 01/19/2010 07:47 PM, Jim Keniston wrote:
>
>> This is still with a kernel entry, yes?
>>      
> Yes, this involves setting a breakpoint and trapping into the kernel
> when it's hit.  The 6-7x figure is with the current 2-trap approach
> (breakpoint, single-step).  Boosting could presumably make that more
> like 12-14x.
>    

A trap is IIRC ~1000 cycles, we can reduce this to ~50 (totally 
negligible from the executed code's point of view).

>> Do you have plans for a variant
>> that's completely in userspace?
>>      
> I don't know of any such plans, but I'd be interested to read more of
> your thoughts here.  As I understand it, you've suggested replacing the
> probed instruction with a jump into an instrumentation vma (the XOL
> area, or something similar).  Masami has demonstrated -- through his
> djprobes enhancement to kprobes -- that this can be done for many x86
> instructions.
>
> What does the code in the jumped-to vma do?

1. Write a trace entry into shared memory, trap into the kernel on overflow.
2. Trap if a condition is satisfied (fast watchpoint implementation).

> Is the instrumentation code
> that corresponds to the uprobe handlers encoded in an ad hoc .so?
>    

Looks like a good idea, but it doesn't matter much to me.

-- 
error compiling committee.c: too many arguments to function

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