arch/arm/kernel/ftrace.c

   1 /*
   2  * Dynamic function tracing support.
   3  *
   4  * Copyright (C) 2008 Abhishek Sagar <sagar.abhishek@gmail.com>
   5  *
   6  * For licencing details, see COPYING.
   7  *
   8  * Defines low-level handling of mcount calls when the kernel
   9  * is compiled with the -pg flag. When using dynamic ftrace, the
  10  * mcount call-sites get patched lazily with NOP till they are
  11  * enabled. All code mutation routines here take effect atomically.
  12  */
  13
  14 #include <linux/ftrace.h>
  15
  16 #include <asm/cacheflush.h>
  17 #include <asm/ftrace.h>
  18
  19 #define PC_OFFSET      8
  20 #define BL_OPCODE      0xeb000000
  21 #define BL_OFFSET_MASK 0x00ffffff
  22
  23 static unsigned long bl_insn;
  24 static const unsigned long NOP = 0xe1a00000; /* mov r0, r0 */
  25
  26 unsigned char *ftrace_nop_replace(void)
  27 {
  28         return (char *)&NOP;
  29 }
  30
  31 /* construct a branch (BL) instruction to addr */
  32 unsigned char *ftrace_call_replace(unsigned long pc, unsigned long addr)
  33 {
  34         long offset;
  35
  36         offset = (long)addr - (long)(pc + PC_OFFSET);
  37         if (unlikely(offset < -33554432 || offset > 33554428)) {
  38                 /* Can't generate branches that far (from ARM ARM). Ftrace
  39                  * doesn't generate branches outside of kernel text.
  40                  */
  41                 WARN_ON_ONCE(1);
  42                 return NULL;
  43         }
  44         offset = (offset >> 2) & BL_OFFSET_MASK;
  45         bl_insn = BL_OPCODE | offset;
  46         return (unsigned char *)&bl_insn;
  47 }
  48
  49 int ftrace_modify_code(unsigned long pc, unsigned char *old_code,
  50                        unsigned char *new_code)
  51 {
  52         unsigned long err = 0, replaced = 0, old, new;
  53
  54         old = *(unsigned long *)old_code;
  55         new = *(unsigned long *)new_code;
  56
  57         __asm__ __volatile__ (
  58                 "1:  ldr    %1, [%2]  \n"
  59                 "    cmp    %1, %4    \n"
  60                 "2:  streq  %3, [%2]  \n"
  61                 "    cmpne  %1, %3    \n"
  62                 "    movne  %0, #2    \n"
  63                 "3:\n"
  64
  65                 ".section .fixup, \"ax\"\n"
  66                 "4:  mov  %0, #1  \n"
  67                 "    b    3b      \n"
  68                 ".previous\n"
  69
  70                 ".section __ex_table, \"a\"\n"
  71                 "    .long 1b, 4b \n"
  72                 "    .long 2b, 4b \n"
  73                 ".previous\n"
  74
  75                 : "=r"(err), "=r"(replaced)
  76                 : "r"(pc), "r"(new), "r"(old), "0"(err), "1"(replaced)
  77                 : "memory");
  78
  79         if (!err && (replaced == old))
  80                 flush_icache_range(pc, pc + MCOUNT_INSN_SIZE);
  81
  82         return err;
  83 }
  84
  85 int ftrace_update_ftrace_func(ftrace_func_t func)
  86 {
  87         int ret;
  88         unsigned long pc, old;
  89         unsigned char *new;
  90
  91         pc = (unsigned long)&ftrace_call;
  92         memcpy(&old, &ftrace_call, MCOUNT_INSN_SIZE);
  93         new = ftrace_call_replace(pc, (unsigned long)func);
  94         ret = ftrace_modify_code(pc, (unsigned char *)&old, new);
  95         return ret;
  96 }
  97
  98 /* run from ftrace_init with irqs disabled */
  99 int __init ftrace_dyn_arch_init(void *data)
 100 {
 101         ftrace_mcount_set(data);
 102         return 0;
 103 }