From: Ard Biesheuvel <ardb@kernel•org>
To: linux@armlinux•org.uk, linux-arm-kernel@lists•infradead.org
Cc: Ard Biesheuvel <ardb@kernel•org>, Arnd Bergmann <arnd@arndb•de>,
Linus Walleij <linus.walleij@linaro•org>,
Nick Desaulniers <ndesaulniers@google•com>
Subject: [PATCH 2/8] ARM: ftrace: use ADD not POP to counter PUSH at entry
Date: Tue, 25 Jan 2022 16:36:50 +0100 [thread overview]
Message-ID: <20220125153656.1802079-3-ardb@kernel.org> (raw)
In-Reply-To: <20220125153656.1802079-1-ardb@kernel.org>
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>
---
arch/arm/kernel/entry-ftrace.S | 2 +-
arch/arm/kernel/ftrace.c | 4 ++--
2 files changed, 3 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..db72d3a6522d 100644
--- a/arch/arm/kernel/ftrace.c
+++ b/arch/arm/kernel/ftrace.c
@@ -25,9 +25,9 @@
#include <asm/patch.h>
#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
--
2.30.2
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next prev parent reply other threads:[~2022-01-25 15:49 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-25 15:36 [PATCH 0/8] ARM: ftrace fixes and cleanups Ard Biesheuvel
2022-01-25 15:36 ` [PATCH 1/8] ARM: ftrace: ensure that ADR take Thumb bit into account Ard Biesheuvel
2022-01-25 19:14 ` Nick Desaulniers
2022-02-02 23:53 ` Linus Walleij
2022-01-25 15:36 ` Ard Biesheuvel [this message]
2022-01-25 19:23 ` [PATCH 2/8] ARM: ftrace: use ADD not POP to counter PUSH at entry Nick Desaulniers
2022-02-02 23:59 ` Linus Walleij
2022-01-25 15:36 ` [PATCH 3/8] ARM: ftrace: use trampolines to keep .init.text in branching range Ard Biesheuvel
2022-01-25 20:20 ` Nick Desaulniers
2022-02-03 0:12 ` Linus Walleij
2022-01-25 15:36 ` [PATCH 4/8] ARM: ftrace: avoid redundant loads or clobbering IP Ard Biesheuvel
2022-01-25 15:36 ` [PATCH 5/8] ARM: ftrace: avoid unnecessary literal loads Ard Biesheuvel
2022-01-25 20:27 ` Nick Desaulniers
2022-01-25 15:36 ` [PATCH 6/8] ARM: ftrace: enable HAVE_FUNCTION_GRAPH_FP_TEST Ard Biesheuvel
2022-01-25 15:36 ` [PATCH 7/8] ARM: unwind: track location of LR value in stack frame Ard Biesheuvel
2022-01-25 15:36 ` [PATCH 8/8] ARM: ftrace: enable the graph tracer with the EABI unwinder Ard Biesheuvel
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