Date: Fri, 20 Nov 2009 17:24:18 +0000
From: Ralf Baechle <ralf@linux-mips.org>
To: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: rostedt@goodmis.org, Nicholas Mc Guire <der.herr@hofr.at>,
       zhangfx@lemote.com, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>,
       Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org,
       linux-mips@linux-mips.org
Subject: Re: [PATCH v9 06/10] tracing: add function graph tracer support
	for MIPS
Message-ID: <20091120172418.GF6869@linux-mips.org>
References: <cover.1258719323.git.wuzhangjin@gmail.com> <2276758e661b2b2362432851003df1d7c99d6cc0.1258719323.git.wuzhangjin@gmail.com>
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On Fri, Nov 20, 2009 at 08:34:34PM +0800, Wu Zhangjin wrote:

> The implementation of function graph tracer for MIPS is a little
> different from X86.
> 
> in MIPS, gcc(with -pg) only transfer the caller's return address(at) and
> the _mcount's return address(ra) to us.
> 
> For the kernel part without -mlong-calls:
> 
> move at, ra
> jal _mcount
> 
> For the module part with -mlong-calls:
> 
> lui v1, hi16bit_of_mcount
> addiu v1, v1, low16bit_of_mcount
> move at, ra
> jal _mcount
> 
> Without -mlong-calls,
> 
> if the function is a leaf, it will not save the return address(ra):
> 
> ffffffff80101298 <au1k_wait>:
> ffffffff80101298:       67bdfff0        daddiu  sp,sp,-16
> ffffffff8010129c:       ffbe0008        sd      s8,8(sp)
> ffffffff801012a0:       03a0f02d        move    s8,sp
> ffffffff801012a4:       03e0082d        move    at,ra
> ffffffff801012a8:       0c042930        jal     ffffffff8010a4c0 <_mcount>
> ffffffff801012ac:       00020021        nop
> 
> so, we can hijack it directly in _mcount, but if the function is non-leaf, the
> return address is saved in the stack.
> 
> ffffffff80133030 <copy_process>:
> ffffffff80133030:       67bdff50        daddiu  sp,sp,-176
> ffffffff80133034:       ffbe00a0        sd      s8,160(sp)
> ffffffff80133038:       03a0f02d        move    s8,sp
> ffffffff8013303c:       ffbf00a8        sd      ra,168(sp)
> ffffffff80133040:       ffb70098        sd      s7,152(sp)
> ffffffff80133044:       ffb60090        sd      s6,144(sp)
> ffffffff80133048:       ffb50088        sd      s5,136(sp)
> ffffffff8013304c:       ffb40080        sd      s4,128(sp)
> ffffffff80133050:       ffb30078        sd      s3,120(sp)
> ffffffff80133054:       ffb20070        sd      s2,112(sp)
> ffffffff80133058:       ffb10068        sd      s1,104(sp)
> ffffffff8013305c:       ffb00060        sd      s0,96(sp)
> ffffffff80133060:       03e0082d        move    at,ra
> ffffffff80133064:       0c042930        jal     ffffffff8010a4c0 <_mcount>
> ffffffff80133068:       00020021        nop
> 
> but we can not get the exact stack address(which saved ra) directly in
> _mcount, we need to search the content of at register in the stack space
> or search the "s{d,w} ra, offset(sp)" instruction in the text. 'Cause we
> can not prove there is only a match in the stack space, so, we search
> the text instead.
> 
> as we can see, if the first instruction above "move at, ra" is not a
> store instruction, there should be a leaf function, so we hijack the at
> register directly via putting &return_to_handler into it, otherwise, we
> search the "s{d,w} ra, offset(sp)" instruction to get the stack offset,
> and then the stack address. we use the above copy_process() as an
> example, we at last find "ffbf00a8", 0xa8 is the stack offset, we plus
> it with s8(fp), that is the stack address, we hijack the content via
> writing the &return_to_handler in.
> 
> If with -mlong-calls, since there are two more instructions above "move
> at, ra", so, we can move the pointer to the position above "lui v1,
> hi16bit_of_mcount".
> 
> Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>

Queued for 2.6.33 but due to patch 3/3 I won't propagate this series
immediately to linux-next.

Thanks!

  Ralf
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