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Re: Support for ftrace in MIPS

To: Frédéric Weisbecker <fweisbec@gmail.com>
Subject: Re: Support for ftrace in MIPS
From: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 23 Oct 2008 10:07:46 +0100
Cc: linux-mips@linux-mips.org
In-reply-to: <c62985530810201800r3ddfbb99s1a0809f2eba45cdf@mail.gmail.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <c62985530810201800r3ddfbb99s1a0809f2eba45cdf@mail.gmail.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.18 (2008-05-17)
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 */

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