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Date:	Mon, 26 Oct 2009 12:06:15 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Wu Zhangjin <wuzhangjin@...il.com>
Cc:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Nicholas Mc Guire <der.herr@...r.at>,
	David Daney <ddaney@...iumnetworks.com>,
	Adam Nemet <anemet@...iumnetworks.com>,
	Patrik Kluba <kpajko79@...il.com>,
	Wu Zhangjin <wuzj@...ote.com>
Subject: Re: [PATCH -v6 07/13] tracing: add dynamic function tracer support
 for MIPS

On Mon, 2009-10-26 at 23:13 +0800, Wu Zhangjin wrote:
> With dynamic function tracer, by default, _mcount is defined as an
> "empty" function, it just return without any more action, just like the
> static function tracer does. When enabling it in user-space, it will
> jump to a real tracing function(ftrace_caller), and do some real job for
> us.
> 
> Differ from the static function tracer, dynamic function tracer provides
> two functions ftrace_make_call()/ftrace_make_nop() to enable/disable the
> tracing of some indicated kernel functions(set_ftrace_filter).
> 
> In the -v4 version, the implementation of this support is basically the same as
> X86 version does: _mcount is implemented as an empty function and ftrace_caller
> is implemented as a real tracing function respectively.
> 
> But in this version, to support module tracing with the help of
> -mlong-calls. the stuff becomes a little more complex, let's have a look
> at the new calling to _mcount with -mlong-calls(result of "objdump -hdr
> vmlinux").
> 

Note in cases where the call to mcount is more that one instruction, it
may be better to add a jump over everything.

> 	c:	3c030000 	lui	v1,0x0

				b	1f


> 			c: R_MIPS_HI16	_mcount
> 			c: R_MIPS_NONE	*ABS*
> 			c: R_MIPS_NONE	*ABS*
> 	10:	64630000 	daddiu	v1,v1,0
> 			10: R_MIPS_LO16	_mcount
> 			10: R_MIPS_NONE	*ABS*
> 			10: R_MIPS_NONE	*ABS*
> 	14:	03e0082d 	move	at,ra
> 	18:	0060f809 	jalr	v1

1:

Yes, it will still execute in the delay slot daddiu. Not sure if this is
better than two adds and a nop. I'll have to play with it.

> 
> and the old version without -mlong-calls looks like this:
> 
> 	108:   03e0082d        move    at,ra
> 	10c:   0c000000        jal     0 <fpcsr_pending>
>                         10c: R_MIPS_26  _mcount
>                         10c: R_MIPS_NONE        *ABS*
>                         10c: R_MIPS_NONE        *ABS*
> 	110:   00020021        nop
> 
> In the old version, there is only one "_mcount" string for every kernel
> function, so, we just need to match this one in mcount_regex of
> scripts/recordmcount.pl, but in the new version, we need to choose one
> of the two to match. Herein, I choose the first one with "R_MIPS_HI16
> _mcount".
> 
> and In the old verion, without module tracing support, we just need to replace
> "jal _mcount" by "jal ftrace_caller" to do real tracing, and filter the tracing
> of some kernel functions via replacing it by a nop instruction.
> 
> but as we have described before, the instruction "jal ftrace_caller" only left
> 32bit length for the address of ftrace_caller, it will fail when calling from
> the module space. so, herein, we must replace something else.
> 
> the basic idea is loading the address of ftrace_caller to v1 via changing these
> two instructions:
> 
> 	lui	v1,0x0
> 	addiu	v1,v1,0
> 
> If we want to enable the tracing, we need to replace the above instructions to:
> 
> 	lui	v1, HI_16BIT_ftrace_caller
> 	addiu	v1, v1, LOW_16BIT_ftrace_caller
> 
> If we want to stop the tracing of the indicated kernel functions, we
> just need to replace the "jalr v1" to a nop instruction. but we need to
> replace two instructions and encode the above two instructions
> oursevles.
> 
> Is there a simpler solution, Yes! Here it is, in this version, we put _mcount
> and ftrace_caller together, which means the address of _mcount and
> ftrace_caller is the same:
> 
> _mcount:
> ftrace_caller:
> 	j	ftrace_stub
> 	nop
> 
> 	...(do real tracing here)...
> 
> ftrace_stub:
> 	jr	ra
> 	move	ra, at
> 
> By default, the kernel functions call _mcount, and then jump to ftrace_stub and
> return. and when we want to do real tracing, we just need to remove that "j
> ftrace_stub", and it will run through the two "nop" instructions and then do
> the real tracing job.

The problem with this is that all .init functions always call _mcount
and we never nop it. Since the initial nop is done at boot up with only
one CPU running, it is safe to modify all locations with the jump to
something other than _mcount and not to worry about it later.


> 
> what about filtering job? we just need to switching the "jalr v1" and "nop"
> instruction.
> 
> In linux-mips64, there will be some local symbols, whose name are
> prefixed by $L, which need to be filtered. thanks goes to Steven for
> writing the mips64-specific function_regex.
> 
> In a conclusion, with RISC, things becomes easier with such a "stupid"
> trick, RISC is something like K.I.S.S, and also, there are lots of
> "simple" tricks in the whole ftrace support, thanks goes to Steven and
> the other folks for providing such a wonderful tracing framework!
> 
> Signed-off-by: Wu Zhangjin <wuzj@...ote.com>
> ---
>  arch/mips/Kconfig              |    2 +
>  arch/mips/include/asm/ftrace.h |   12 ++++++
>  arch/mips/kernel/Makefile      |    3 +-
>  arch/mips/kernel/ftrace.c      |   76 ++++++++++++++++++++++++++++++++++++++++
>  arch/mips/kernel/mcount.S      |   29 +++++++++++++++
>  scripts/recordmcount.pl        |   38 ++++++++++++++++++++
>  6 files changed, 159 insertions(+), 1 deletions(-)
>  create mode 100644 arch/mips/kernel/ftrace.c
> 
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index a9bd992..147fbbc 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -6,6 +6,8 @@ config MIPS
>  	select HAVE_ARCH_KGDB
>  	select HAVE_FUNCTION_TRACER
>  	select HAVE_FUNCTION_TRACE_MCOUNT_TEST
> +	select HAVE_DYNAMIC_FTRACE
> +	select HAVE_FTRACE_MCOUNT_RECORD
>  	# Horrible source of confusion.  Die, die, die ...
>  	select EMBEDDED
>  	select RTC_LIB if !LEMOTE_FULOONG2E
> diff --git a/arch/mips/include/asm/ftrace.h b/arch/mips/include/asm/ftrace.h
> index 5f8ebcf..d5771e8 100644
> --- a/arch/mips/include/asm/ftrace.h
> +++ b/arch/mips/include/asm/ftrace.h
> @@ -19,6 +19,18 @@
>  extern void _mcount(void);
>  #define mcount _mcount
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +static inline unsigned long ftrace_call_adjust(unsigned long addr)
> +{
> +	/* 12 is the offset of "jalr v1" from the first loading instruction
> +	 * "lui v1, HI_16BIT", please get more information from
> +	 * scripts/recordmcount.pl */
> +	return addr + 12;
> +}
> +
> +struct dyn_arch_ftrace {
> +};
> +#endif /*  CONFIG_DYNAMIC_FTRACE */
>  #endif /* __ASSEMBLY__ */
>  #endif /* CONFIG_FUNCTION_TRACER */
>  #endif /* _ASM_MIPS_FTRACE_H */
> diff --git a/arch/mips/kernel/Makefile b/arch/mips/kernel/Makefile
> index 38e704a..3cda3ab 100644
> --- a/arch/mips/kernel/Makefile
> +++ b/arch/mips/kernel/Makefile
> @@ -10,6 +10,7 @@ obj-y		+= cpu-probe.o branch.o entry.o genex.o irq.o process.o \
>  
>  ifdef CONFIG_FUNCTION_TRACER
>  CFLAGS_REMOVE_trace_clock.o = -pg
> +CFLAGS_REMOVE_ftrace.o = -pg
>  CFLAGS_REMOVE_early_printk.o = -pg
>  endif
>  
> @@ -30,7 +31,7 @@ obj-$(CONFIG_STACKTRACE)	+= stacktrace.o
>  obj-$(CONFIG_MODULES)		+= mips_ksyms.o module.o
>  
>  obj-$(CONFIG_FTRACE)		+= trace_clock.o
> -obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o
> +obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o ftrace.o
>  
>  obj-$(CONFIG_CPU_LOONGSON2)	+= r4k_fpu.o r4k_switch.o
>  obj-$(CONFIG_CPU_MIPS32)	+= r4k_fpu.o r4k_switch.o
> diff --git a/arch/mips/kernel/ftrace.c b/arch/mips/kernel/ftrace.c
> new file mode 100644
> index 0000000..0be30cf
> --- /dev/null
> +++ b/arch/mips/kernel/ftrace.c
> @@ -0,0 +1,76 @@
> +/*
> + * Code for replacing ftrace calls with jumps.
> + *
> + * Copyright (C) 2007-2008 Steven Rostedt <srostedt@...hat.com>
> + * Copyright (C) 2009 DSLab, Lanzhou University, China
> + * Author: Wu Zhangjin <wuzj@...ote.com>
> + *
> + * Thanks goes to Steven Rostedt for writing the original x86 version.
> + */
> +
> +#include <linux/uaccess.h>
> +#include <linux/init.h>
> +#include <linux/ftrace.h>
> +
> +#include <asm/cacheflush.h>
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +
> +#define JAL 0x0c000000		/* jump & link: ip --> ra, jump to target */
> +#define ADDR_MASK 0x03ffffff	/*  op_code|addr : 31...26|25 ....0 */
> +
> +#ifdef CONFIG_CPU_LOONGSON2F
> +static unsigned int ftrace_nop = 0x00020021;
> +#else
> +static unsigned int ftrace_nop = 0x00000000;
> +#endif
> +
> +static int ftrace_modify_code(unsigned long ip, unsigned int new_code)
> +{
> +	*(unsigned int *)ip = new_code;
> +
> +	flush_icache_range(ip, ip + 8);
> +
> +	return 0;
> +}
> +
> +int ftrace_make_nop(struct module *mod,
> +		    struct dyn_ftrace *rec, unsigned long addr)
> +{
> +	return ftrace_modify_code(rec->ip, ftrace_nop);
> +}
> +
> +static int modified;	/* initialized as 0 by default */
> +#define JALR_V1 0x0060f809
> +
> +int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> +{
> +	/* We just need to remove the "j ftrace_stub" at the fist time! */
> +	if (modified == 0) {
> +		modified = 1;
> +		ftrace_modify_code(addr, ftrace_nop);
> +	}
> +	return ftrace_modify_code(rec->ip, JALR_V1);
> +}
> +
> +#define FTRACE_CALL_IP ((unsigned long)(&ftrace_call))
> +#define jump_insn_encode(op_code, addr) \
> +	((unsigned int)((op_code) | (((addr) >> 2) & ADDR_MASK)))
> +
> +int ftrace_update_ftrace_func(ftrace_func_t func)
> +{
> +	unsigned int new;
> +
> +	new = jump_insn_encode(JAL, (unsigned long)func);
> +
> +	return ftrace_modify_code(FTRACE_CALL_IP, new);
> +}
> +
> +int __init ftrace_dyn_arch_init(void *data)
> +{
> +	/* The return code is retured via data */
> +	*(unsigned long *)data = 0;
> +
> +	return 0;
> +}
> +#endif				/* CONFIG_DYNAMIC_FTRACE */
> diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S
> index 5dfaca8..389be7b 100644
> --- a/arch/mips/kernel/mcount.S
> +++ b/arch/mips/kernel/mcount.S
> @@ -63,6 +63,33 @@
>  	move ra, AT
>  	.endm
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +
> +NESTED(ftrace_caller, PT_SIZE, ra)
> +	.globl _mcount
> +_mcount:
> +	j	ftrace_stub
> +	nop
> +	lw	t0, function_trace_stop
> +	bnez	t0, ftrace_stub
> +	nop
> +
> +	MCOUNT_SAVE_REGS
> +
> +	MCOUNT_SET_ARGS
> +	.globl ftrace_call
> +ftrace_call:
> +	nop	/* a placeholder for the call to a real tracing function */
> +	nop	/* Do not touch me, I'm in the dealy slot of "jal func" */

Indent the second nop ;-)

> +
> +	MCOUNT_RESTORE_REGS
> +	.globl ftrace_stub
> +ftrace_stub:
> +	RETURN_BACK
> +	END(ftrace_caller)
> +
> +#else	/* ! CONFIG_DYNAMIC_FTRACE */
> +
>  NESTED(_mcount, PT_SIZE, ra)
>  	lw	t0, function_trace_stop
>  	bnez	t0, ftrace_stub
> @@ -89,5 +116,7 @@ ftrace_stub:
>  	RETURN_BACK
>  	END(_mcount)
>  
> +#endif	/* ! CONFIG_DYNAMIC_FTRACE */
> +
>  	.set at
>  	.set reorder
> diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
> index 465fe5e..d83c0c1 100755
> --- a/scripts/recordmcount.pl
> +++ b/scripts/recordmcount.pl
> @@ -246,6 +246,44 @@ if ($arch eq "x86_64") {
>      $ld .= " -m elf64_sparc";
>      $cc .= " -m64";
>      $objcopy .= " -O elf64-sparc";
> +
> +} elsif ($arch eq "mips") {
> +    # To enable module support, we need to enable the -mlong-calls option
> +    # of gcc for MIPS, after using this option, we can not get the real
> +    # offset of the calling to _mcount, but the offset of the first loading
> +    # instruction or the second one, here we get the first one, and then,
> +    # we plus it with 12 in arch/mips/kernel/asm/ftrace.h.
> +    #
> +    #       c:	3c030000 	lui	v1,0x0
> +    #			c: R_MIPS_HI16	_mcount
> +    #			c: R_MIPS_NONE	*ABS*
> +    #			c: R_MIPS_NONE	*ABS*
> +    #      10:	64630000 	daddiu	v1,v1,0
> +    #			10: R_MIPS_LO16	_mcount
> +    #			10: R_MIPS_NONE	*ABS*
> +    #			10: R_MIPS_NONE	*ABS*
> +    #      14:	03e0082d 	move	at,ra
> +    #      18:	0060f809 	jalr	v1
> +    $mcount_regex = "^\\s*([0-9a-fA-F]+): R_MIPS_HI16\\s+_mcount\$";
> +    $objdump .= " -Melf-trad".$endian."mips ";
> +
> +    if ($endian eq "big") {
> +	    $endian = " -EB ";
> +	    $ld .= " -melf".$bits."btsmip";
> +    } else {
> +	    $endian = " -EL ";
> +	    $ld .= " -melf".$bits."ltsmip";
> +    }
> +
> +    $cc .= " -mno-abicalls -fno-pic -mabi=" . $bits . $endian;
> +    $ld .= $endian;
> +
> +    if ($bits == 64) {
> +	    $function_regex =
> +		"^([0-9a-fA-F]+)\\s+<(.|[^\$]L.*?|\$[^L].*?|[^\$][^L].*?)>:";
> +	    $type = ".dword";
> +    }
> +
>  } else {
>      die "Arch $arch is not supported with CONFIG_FTRACE_MCOUNT_RECORD";
>  }

So if I convert mips to do what ppc does, we can remove the long jump
thing, right?

-- Steve


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