diff options
| author | Ard Biesheuvel <ardb@kernel.org> | 2021-12-08 08:42:03 +0100 |
|---|---|---|
| committer | Ard Biesheuvel <ardb@kernel.org> | 2022-02-09 09:12:32 +0100 |
| commit | ad1c2f39fda0acb51b5a93604c5e1a703b849a7d (patch) | |
| tree | 47f173a82e141c35d4afee2953ccc29497bec46f /arch/arm/kernel | |
| parent | dd88b03ff0c84f4bcbe1419b93a4bed429fed3be (diff) | |
ARM: ftrace: use ADD not POP to counter PUSH at entry
The compiler emitted hook used for ftrace consists of a PUSH {LR} to
preserve the link register, followed by a branch-and-link (BL) to
__gnu_mount_nc. Dynamic ftrace patches away the latter to turn the
combined sequence into a NOP, using a POP {LR} instruction.
This is not necessary, since the link register does not get clobbered in
this case, and simply adding #4 to the stack pointer is sufficient, and
avoids a memory access that may take a few cycles to resolve depending
on the micro-architecture.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Diffstat (limited to 'arch/arm/kernel')
| -rw-r--r-- | arch/arm/kernel/entry-ftrace.S | 2 | ||||
| -rw-r--r-- | arch/arm/kernel/ftrace.c | 15 |
2 files changed, 14 insertions, 3 deletions
diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S index f4886fb6e9ba..dca12a09322a 100644 --- a/arch/arm/kernel/entry-ftrace.S +++ b/arch/arm/kernel/entry-ftrace.S @@ -27,7 +27,7 @@ * allows it to be clobbered in subroutines and doesn't use it to hold * parameters.) * - * When using dynamic ftrace, we patch out the mcount call by a "pop {lr}" + * When using dynamic ftrace, we patch out the mcount call by a "add sp, #4" * instead of the __gnu_mcount_nc call (see arch/arm/kernel/ftrace.c). */ diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c index a006585e1c09..811cadf7eefc 100644 --- a/arch/arm/kernel/ftrace.c +++ b/arch/arm/kernel/ftrace.c @@ -24,10 +24,21 @@ #include <asm/set_memory.h> #include <asm/patch.h> +/* + * The compiler emitted profiling hook consists of + * + * PUSH {LR} + * BL __gnu_mcount_nc + * + * To turn this combined sequence into a NOP, we need to restore the value of + * SP before the PUSH. Let's use an ADD rather than a POP into LR, as LR is not + * modified anyway, and reloading LR from memory is highly likely to be less + * efficient. + */ #ifdef CONFIG_THUMB2_KERNEL -#define NOP 0xf85deb04 /* pop.w {lr} */ +#define NOP 0xf10d0d04 /* add.w sp, sp, #4 */ #else -#define NOP 0xe8bd4000 /* pop {lr} */ +#define NOP 0xe28dd004 /* add sp, sp, #4 */ #endif #ifdef CONFIG_DYNAMIC_FTRACE |