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Date: Tue, 25 Jan 2011 10:38:15 -0800
From: Josh Stone <jistone@redhat.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Srikar Dronamraju <srikar@linux.vnet.ibm.com>, Ingo Molnar <mingo@elte.hu>,
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        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 10/20] 10: uprobes: task specific
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On 01/25/2011 05:56 AM, Peter Zijlstra wrote:
> Ah, I think I found it while reading patch 13, you need the pre/post_xol
> callbacks, can't you simply synthesize their effect into the replacement
> sequence?
> 
>   push %rax
>   mov $vaddr, %rax
>   $INSN
>   pop %rax
>   jmp $next_insn
> 
> like replacements would obviate the need for the pre/post callbacks and
> allow you to run straight through.

For this particular example, you'd better be sure that $INSN doesn't
need %rsp intact.

Control flow in general also makes this challenging.  If $INSN is a
call, then any inline fixups won't get a chance until after return.  If
$INSN is a jump, then its target must be modified so that both taken and
not-taken paths land in respective fixup locations.  I'm sure there are
more cases that I'm not thinking of.

> It would also remove the whole single-step need since they're proper
> boosted probes.

Kprobes has boosting, but it doesn't apply to all opcodes.  I would
guess that the same could be done for uprobes, where certain opcodes get
a fixup sequence like you suggest, but the pre/post_xol mechanism is
still needed in general.

Josh