Commit ad1c2f39 authored by Ard Biesheuvel's avatar Ard Biesheuvel
Browse files

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: default avatarArd Biesheuvel <ardb@kernel.org>
Reviewed-by: default avatarNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Reviewed-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent dd88b03f

arch/arm/kernel/entry-ftrace.S

+1 −1
Original line number Diff line number Diff line
@@ -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).

 */

arch/arm/kernel/ftrace.c

+13 −2
Original line number Diff line number Diff line
@@ -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