Re: Support for ftrace in MIPS

[Ralf Baechle <ralf@linux-mips.org>]

Frédéric,

> I saw that there is no implementation of ftrace in mips currently and
> I would like to know if someone is currently working on it. If not I
> would be glad
> to work on patches for that.

I do have an ftrace implementation in the queue.  Static ftrace works as
far as I've tested but due to a restriction of the generic ftrace code the
MIPS code for dynamic ftrace can't work on most SMP systems - basically
only the R10000 and single core 34K SMP kernel variants have a chance to
work.  For others cache flushing requires a call to smp_call_function but
that can't work because other CPUs are stopped at the time of the flush.
So that still needs some sorting.

Patch below.

  Ralf

From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 10 Sep 2008 18:04:04 +0200
Subject: [PATCH] [MIPS] Implement fcount support for MIPS.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
---
 arch/mips/Kconfig             |    5 ++
 arch/mips/Makefile            |    2 +
 arch/mips/kernel/Makefile     |    2 +
 arch/mips/kernel/ftrace.c     |  118 +++++++++++++++++++++++++++++++++++++
 arch/mips/kernel/mips_ksyms.c |    6 ++
 arch/mips/lib/Makefile        |    2 +
 arch/mips/lib/mcount.S        |  128 +++++++++++++++++++++++++++++++++++++++++
 include/asm-mips/ftrace.h     |   14 +++++
 8 files changed, 277 insertions(+), 0 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 5f149b0..43202db 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1,6 +1,7 @@
 config MIPS
 	bool
 	default y
+	select HAVE_FTRACE
 	select HAVE_IDE
 	select HAVE_OPROFILE
 	select HAVE_ARCH_KGDB
@@ -1503,6 +1504,10 @@ config MIPS_APSP_KSPD
 	  "exit" syscall notifying other kernel modules the SP program is
 	  exiting.  You probably want to say yes here.
 
+config SYS_SUPPORTS_DYNAMIC_FTRACE
+	def_bool y
+	select HAVE_DYNAMIC_FTRACE if !SMP || CPU_R10000
+
 config MIPS_CMP
 	bool "MIPS CMP framework support"
 	depends on SYS_SUPPORTS_MIPS_CMP
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index 7f39fd8..ae9099b 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -14,7 +14,9 @@
 
 KBUILD_DEFCONFIG := ip22_defconfig
 
+ifndef CONFIG_FTRACE
 cflags-y := -ffunction-sections
+endif
 
 #
 # Select the object file format to substitute into the linker script.
diff --git a/arch/mips/kernel/Makefile b/arch/mips/kernel/Makefile
index d9da711..ea730e0 100644
--- a/arch/mips/kernel/Makefile
+++ b/arch/mips/kernel/Makefile
@@ -43,6 +43,8 @@ obj-$(CONFIG_CPU_TX39XX)	+= r2300_fpu.o r2300_switch.o
 obj-$(CONFIG_CPU_TX49XX)	+= r4k_fpu.o r4k_switch.o
 obj-$(CONFIG_CPU_VR41XX)	+= r4k_fpu.o r4k_switch.o
 
+obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
+
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_SMP_UP)		+= smp-up.o
 
diff --git a/arch/mips/kernel/ftrace.c b/arch/mips/kernel/ftrace.c
new file mode 100644
index 0000000..e24ae47
--- /dev/null
+++ b/arch/mips/kernel/ftrace.c
@@ -0,0 +1,118 @@
+#include <linux/ftrace.h>
+#include <linux/init.h>
+#include <linux/uaccess.h>
+
+#include <asm/cacheflush.h>
+#include <asm/sgidefs.h>
+#include <asm/inst.h>
+#include <asm/ftrace.h>
+
+/*
+ * On O32 the gcc generated code sequence to invoke _mcount looks like
+ *
+ *	move	$1, $31			# save current return address
+ *	subu	$sp, $sp, 8		# _mcount pops 2 words from  stack
+ *	jal	_mcount
+ *	 j	$31
+ *
+ * If the _mcount call is nop'ed out we still need to pop the stack frame.
+ * So we use an addiu $sp, $sp, 8 instead.  NABI doesn't allocate frames in
+ * such a nonsenical way so a normal nop does the magic.
+ */
+static const union mips_instruction ftrace_nop = {
+#if _MIPS_SIM == _MIPS_SIM_ABI32
+	.i_format.opcode = addiu_op,		/* addiu $sp, $sp, 8 */
+	.i_format.rs = 29,
+	.i_format.rt = 29,
+	.i_format.simmediate = 8
+#else
+	.word = 0				/* normal nop */
+#endif
+};
+
+notrace unsigned char *ftrace_nop_replace(void)
+{
+	return (unsigned char *) ftrace_nop.byte;
+}
+
+notrace unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
+{
+	static union mips_instruction inst;
+
+	if (unlikely(ip ^ addr) >> 26) {
+		WARN_ON_ONCE(1);
+		return NULL;
+	}
+
+	inst.j_format.opcode = jal_op;
+	inst.j_format.target = addr >> 2;
+
+	return (unsigned char *) &inst;
+}
+
+notrace int
+ftrace_modify_code(unsigned long ip, unsigned char *old_code,
+                   unsigned char *new_code)
+{
+	unsigned int old = *(unsigned int *) old_code;
+	unsigned int new = *(unsigned int *) new_code;
+	unsigned int replaced;
+	int err;
+
+	err = __get_user(replaced, (unsigned int *) ip);
+	if (unlikely(err))
+		return 1;
+
+	err = __put_user(new, (unsigned int *) ip);
+	if (unlikely(err))
+		return 1;
+
+	if (unlikely(replaced != old && replaced != new))
+		return 2;
+
+	/*
+	 * This is broken.  We need to flush the icache on all processors
+	 * but can't because ftrace_modify_code is executed with interrupts
+	 * disabled ...
+	 */
+	local_flush_icache_range(ip, ip + MCOUNT_INSN_SIZE);
+
+	return 0;
+}
+
+notrace int ftrace_update_ftrace_func(ftrace_func_t func)
+{
+	unsigned long ip = (unsigned long) &ftrace_call;
+	unsigned char old[MCOUNT_INSN_SIZE], *new;
+	int ret;
+
+	memcpy(old, &ftrace_call, MCOUNT_INSN_SIZE);
+	new = ftrace_call_replace(ip, (unsigned long)func);
+	ret = ftrace_modify_code(ip, old, new);
+
+	return ret;
+}
+
+notrace int ftrace_mcount_set(unsigned long *data)
+{
+	unsigned long ip = (unsigned long) &mcount_call;
+	unsigned long *addr = data;
+	unsigned char old[MCOUNT_INSN_SIZE], *new;
+
+	/*
+	 * Replace the mcount stub with a pointer to the
+	 * ip recorder function.
+	 */
+	memcpy(old, &mcount_call, MCOUNT_INSN_SIZE);
+	new = ftrace_call_replace(ip, *addr);
+	*addr = ftrace_modify_code(ip, old, new);
+
+	return 0;
+}
+
+int __init ftrace_dyn_arch_init(void *data)
+{
+	ftrace_mcount_set(data);
+
+	return 0;
+}
diff --git a/arch/mips/kernel/mips_ksyms.c b/arch/mips/kernel/mips_ksyms.c
index 225755d..18f5556 100644
--- a/arch/mips/kernel/mips_ksyms.c
+++ b/arch/mips/kernel/mips_ksyms.c
@@ -11,6 +11,7 @@
 #include <linux/interrupt.h>
 #include <linux/module.h>
 #include <asm/checksum.h>
+#include <asm/ftrace.h>
 #include <asm/pgtable.h>
 #include <asm/uaccess.h>
 
@@ -51,3 +52,8 @@ EXPORT_SYMBOL(csum_partial_copy_nocheck);
 EXPORT_SYMBOL(__csum_partial_copy_user);
 
 EXPORT_SYMBOL(invalid_pte_table);
+
+#ifdef CONFIG_FTRACE
+/* _mcount is defined in assembly */
+EXPORT_SYMBOL(_mcount);
+#endif
diff --git a/arch/mips/lib/Makefile b/arch/mips/lib/Makefile
index 8810dfb..c9ec0c3 100644
--- a/arch/mips/lib/Makefile
+++ b/arch/mips/lib/Makefile
@@ -27,5 +27,7 @@ obj-$(CONFIG_CPU_TX39XX)	+= r3k_dump_tlb.o
 obj-$(CONFIG_CPU_TX49XX)	+= dump_tlb.o
 obj-$(CONFIG_CPU_VR41XX)	+= dump_tlb.o
 
+obj-$(CONFIG_FTRACE)		+= mcount.o
+
 # libgcc-style stuff needed in the kernel
 obj-y += ashldi3.o ashrdi3.o cmpdi2.o lshrdi3.o ucmpdi2.o
diff --git a/arch/mips/lib/mcount.S b/arch/mips/lib/mcount.S
new file mode 100644
index 0000000..9817ce6
--- /dev/null
+++ b/arch/mips/lib/mcount.S
@@ -0,0 +1,128 @@
+#include <asm/regdef.h>
+#include <asm/ftrace.h>
+#include <asm/asm.h>
+
+/*
+ * The problem here is that _mcount() is not a normal function.  It does
+ * not obey the calling conventions of a normal function.  It takes its
+ * arguments in $at and $ra, and it is required to preserve $ra - and most
+ * other registers, in fact, if I remember correctly.  The arguments have
+ * not yet been saved.  You need to do this.  Another bizarreness is that
+ * the caller needs to pop an extra 8 bytes for the O32.
+ */
+
+#if _MIPS_SIM == _ABIO32
+#define POP_EXTRA 8
+#define ARG_SAVE 16
+#define N_ARG_REGS 4
+#else
+#define POP_EXTRA 0
+#define ARG_SAVE 0
+#define N_ARG_REGS 8
+#endif
+
+	.macro		save_mcount_frame
+	PTR_SUBU	sp, sp, ARG_SAVE + (N_ARG_REGS + 4) * LONGSIZE
+	LONG_S		AT, ARG_SAVE + 0 * LONGSIZE(sp)
+	LONG_S		ra, ARG_SAVE + 1 * LONGSIZE(sp)
+	LONG_S		a0, ARG_SAVE + 2 * LONGSIZE(sp)
+	LONG_S		a1, ARG_SAVE + 3 * LONGSIZE(sp)
+	LONG_S		a2, ARG_SAVE + 4 * LONGSIZE(sp)
+	LONG_S		a3, ARG_SAVE + 5 * LONGSIZE(sp)
+#if _MIPS_SIM == _ABI64
+	LONG_S		a4, ARG_SAVE + 6 * LONGSIZE(sp)
+	LONG_S		a5, ARG_SAVE + 7 * LONGSIZE(sp)
+	LONG_S		a6, ARG_SAVE + 8 * LONGSIZE(sp)
+	LONG_S		a7, ARG_SAVE + 9 * LONGSIZE(sp)
+#endif
+	LONG_S		v0, ARG_SAVE + (2 + N_ARG_REGS) * LONGSIZE(sp)
+	.endm
+
+	.macro		restore_mcount_frame
+	LONG_L		AT, ARG_SAVE + 0 * LONGSIZE(sp)
+	LONG_L		ra, ARG_SAVE + 1 * LONGSIZE(sp)
+	LONG_L		a0, ARG_SAVE + 2 * LONGSIZE(sp)
+	LONG_L		a1, ARG_SAVE + 3 * LONGSIZE(sp)
+	LONG_L		a2, ARG_SAVE + 4 * LONGSIZE(sp)
+	LONG_L		a3, ARG_SAVE + 5 * LONGSIZE(sp)
+#if _MIPS_SIM == _ABI64
+	LONG_L		a4, ARG_SAVE + 6 * LONGSIZE(sp)
+	LONG_L		a5, ARG_SAVE + 7 * LONGSIZE(sp)
+	LONG_L		a6, ARG_SAVE + 8 * LONGSIZE(sp)
+	LONG_L		a7, ARG_SAVE + 9 * LONGSIZE(sp)
+#endif
+	LONG_L		v0, ARG_SAVE + (2 + N_ARG_REGS) * LONGSIZE(sp)
+	PTR_ADDU	sp, sp, ARG_SAVE + (N_ARG_REGS + 4) * LONGSIZE + POP_EXTRA
+	.endm
+
+#ifdef CONFIG_DYNAMIC_FTRACE
+
+LEAF(_mcount)
+	.set		push
+	.set		noreorder
+	.set		noat
+	save_mcount_frame
+	PTR_SUBU	a0, ra, 2 * MCOUNT_INSN_SIZE
+
+FEXPORT(mcount_call)
+	jal		ftrace_stub
+	 nop
+
+	restore_mcount_frame
+	jr		ra
+	 move		ra, AT
+	.set		pop
+	END(_mcount)
+
+LEAF(ftrace_caller)
+	.set		push
+	.set		noreorder
+	.set		noat
+	save_mcount_frame
+	PTR_SUBU	a0, ra, 2 * MCOUNT_INSN_SIZE
+	move		a1, AT
+
+FEXPORT(ftrace_call)
+	jal		ftrace_stub
+	 nop
+
+	restore_mcount_frame
+	jr		ra
+	 move		ra, AT
+	.set		pop
+	END(ftrace_caller)
+
+#else
+
+LEAF(_mcount)
+	.set		push
+	.set		noreorder
+	.set		noat
+	PTR_L		t0, ftrace_trace_function
+	PTR_LA		t1, ftrace_stub
+	bne		t0, t1, trace
+	 nop
+
+#if POP_EXTRA
+	PTR_ADDIU	sp, sp, 8
+#endif
+	jr		ra
+	 move		 ra, AT
+
+trace:
+	save_mcount_frame
+	PTR_SUBU	a0, ra, 2 * MCOUNT_INSN_SIZE
+	move		a1, AT
+	jalr		t0
+	 nop
+	restore_mcount_frame
+	jr		ra
+	 move		ra, AT
+	.set		pop
+	END(_mcount)
+
+#endif /* !CONFIG_DYNAMIC_FTRACE */
+
+LEAF(ftrace_stub)
+	jr		ra
+	END(ftrace_stub)
diff --git a/include/asm-mips/ftrace.h b/include/asm-mips/ftrace.h
new file mode 100644
index 0000000..f0479ce
--- /dev/null
+++ b/include/asm-mips/ftrace.h
@@ -0,0 +1,14 @@
+#ifndef __ASM_FTRACE_H
+#define __ASM_FTRACE_H
+
+#ifdef CONFIG_FTRACE
+#define MCOUNT_ADDR             ((long)(_mcount))
+#define MCOUNT_INSN_SIZE        4 /* sizeof mcount call */
+
+#ifndef __ASSEMBLY__
+extern void _mcount(void);
+#endif
+
+#endif /* CONFIG_FTRACE */
+
+#endif /* __ASM_FTRACE_H */