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

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Date:	Wed, 20 Jan 2010 14:31:40 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Jim Keniston <jkenisto@...ibm.com>, Avi Kivity <avi@...hat.com>,
	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>,
	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)

Frederic Weisbecker wrote:
> On Tue, Jan 19, 2010 at 09:47:45AM -0800, Jim Keniston wrote:
>>> 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?  Is the instrumentation code
>> that corresponds to the uprobe handlers encoded in an ad hoc .so?
> 
> 
> Once the instrumentation is requested by a process that is not the
> instrumented one, this looks impossible to set a uprobe without a
> minimal voluntary collaboration from the instrumented process
> (events sent through IPC or whatever). So that looks too limited,
> this is not anymore a true dynamic uprobe.

Agreed. Since uprobe's handler must be running in kernel,
we need to jump into kernel space anyway. "Booster" (just skips
a single-stepping(trap) exception) may be useful for
improving uprobe performance.

And also as Andi said, using jump instead of int3 in userspace
has 2GB address space limitation. It's not a problem for kernel
inside, but a big problem in userspace.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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