linux-kernel - [PATCH] x86/ftrace: Don't bother preserving/restoring R10/R11

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Message-ID: <20241009153901.3878233-2-ardb+git@google.com>
Date: Wed,  9 Oct 2024 17:39:01 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-trace-kernel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>
Subject: [PATCH] x86/ftrace: Don't bother preserving/restoring R10/R11

From: Ard Biesheuvel <ardb@...nel.org>

The x86_64 calling convention specifies that R10 and R11 are
caller-save, and are not used to pass function arguments. Given that any
compiler optimizations that may deviate from this rule (e.g., -fipa-ra)
are disabled when profiling is enabled, R10 and R11 are dead when the
ftrace hook is called, and so there is no point in preserving and
restoring them.

Case in point: the code inadvertently never restored R11 to begin with
(added via [0] in 2012)

[0] 08f6fba50311 ("ftrace/x86: Add separate function to save regs")

Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/ftrace_64.S | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index 214f30e9f0c0..9e13f9b203d1 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -216,8 +216,6 @@ SYM_INNER_LABEL(ftrace_regs_caller_op_ptr, SYM_L_GLOBAL)
 	movq %r14, R14(%rsp)
 	movq %r13, R13(%rsp)
 	movq %r12, R12(%rsp)
-	movq %r11, R11(%rsp)
-	movq %r10, R10(%rsp)
 	movq %rbx, RBX(%rsp)
 	/* Copy saved flags */
 	movq MCOUNT_REG_SIZE(%rsp), %rcx
@@ -256,7 +254,6 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
 	movq R14(%rsp), %r14
 	movq R13(%rsp), %r13
 	movq R12(%rsp), %r12
-	movq R10(%rsp), %r10
 	movq RBX(%rsp), %rbx
 
 	movq ORIG_RAX(%rsp), %rax
-- 
2.47.0.rc0.187.ge670bccf7e-goog