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From: Ard Biesheuvel <ardb@kernel.org>
To: linux@armlinux.org.uk
Cc: linux-arm-kernel@lists.infradead.org,
	Ard Biesheuvel <ardb@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Cristian Marussi <cristian.marussi@arm.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Linus Walleij <linus.walleij@linaro.org>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: [PATCH v2 02/12] ARM: ftrace: use ADD not POP to counter PUSH at entry
Date: Mon, 31 Jan 2022 18:03:37 +0100	[thread overview]
Message-ID: <20220131170347.381551-3-ardb@kernel.org> (raw)
In-Reply-To: <20220131170347.381551-1-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>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
---
 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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  parent reply	other threads:[~2022-01-31 17:17 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-31 17:03 [PATCH v2 00/12] ARM: ftrace fixes and cleanups Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 01/12] ARM: ftrace: ensure that ADR take Thumb bit into account Ard Biesheuvel
2022-01-31 17:03 ` Ard Biesheuvel [this message]
2022-01-31 17:03 ` [PATCH v2 03/12] ARM: ftrace: use trampolines to keep .init.text in branching range Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 04/12] ARM: ftrace: avoid redundant loads or clobbering IP Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 05/12] ARM: ftrace: avoid unnecessary literal loads Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 06/12] ARM: ftrace: enable HAVE_FUNCTION_GRAPH_FP_TEST Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 07/12] ARM: unwind: track location of LR value in stack frame Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 08/12] ARM: ftrace: enable the graph tracer with the EABI unwinder Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 09/12] ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds Ard Biesheuvel
2022-01-31 18:31   ` Nick Desaulniers
2022-02-01  7:42     ` Ard Biesheuvel
2022-02-01 13:18   ` Masami Hiramatsu
2022-02-01 14:05     ` Ard Biesheuvel
2022-02-02  6:10       ` Masami Hiramatsu
2022-02-02  8:00         ` Ard Biesheuvel
2022-01-31 17:03 ` [PATCH v2 10/12] drivers/firmware/scmi: disable ftrace for Clang " Ard Biesheuvel
2022-01-31 18:37   ` Nick Desaulniers
2022-02-01  8:12     ` Ard Biesheuvel
2022-01-31 22:04   ` Sudeep Holla
2022-01-31 17:03 ` [PATCH v2 11/12] ARM: cacheflush: avoid clobbering the frame pointer in Thumb2 mode Ard Biesheuvel
2022-01-31 18:40   ` Nick Desaulniers
2022-01-31 17:03 ` [PATCH v2 12/12] Revert "ARM: 9144/1: forbid ftrace with clang and thumb2_kernel" Ard Biesheuvel
2022-01-31 18:42   ` Nick Desaulniers
2022-01-31 17:21 ` [PATCH v2 00/12] ARM: ftrace fixes and cleanups Steven Rostedt

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