linux-kernel - [PATCH v5 3/7] Change the symbols order when --ffunction-sections is enabled

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Message-ID: <20241023224409.201771-4-xur@google.com>
Date: Wed, 23 Oct 2024 15:44:02 -0700
From: Rong Xu <xur@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Arnd Bergmann <arnd@...db.de>, Bill Wendling <morbo@...gle.com>, Borislav Petkov <bp@...en8.de>, 
	Breno Leitao <leitao@...ian.org>, Brian Gerst <brgerst@...il.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, David Li <davidxl@...gle.com>, 
	Han Shen <shenhan@...gle.com>, Heiko Carstens <hca@...ux.ibm.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Juergen Gross <jgross@...e.com>, 
	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, "Mike Rapoport (IBM)" <rppt@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Nicolas Schier <nicolas@...sle.eu>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Rong Xu <xur@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Wei Yang <richard.weiyang@...il.com>, workflows@...r.kernel.org, 
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Maksim Panchenko <max4bolt@...il.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, Yabin Cui <yabinc@...gle.com>, 
	Krzysztof Pszeniczny <kpszeniczny@...gle.com>, Sriraman Tallam <tmsriram@...gle.com>, 
	Stephane Eranian <eranian@...gle.com>
Cc: x86@...nel.org, linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: [PATCH v5 3/7] Change the symbols order when --ffunction-sections is enabled

When the -ffunction-sections compiler option is enabled, each function
is placed in a separate section named .text.function_name rather than
putting all functions in a single .text section.

However, using -function-sections can cause problems with the
linker script. The comments included in include/asm-generic/vmlinux.lds.h
note these issues.:
  “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
   code elimination is enabled, so these sections should be converted
   to use ".." first.”

It is unclear whether there is a straightforward method for converting
a suffix to "..".

This patch modifies the order of subsections within the text output
section. Specifically, it repositions sections with certain fixed patterns
(for example .text.unlikely) before TEXT_MAIN, ensuring that they are
grouped and matched together. It also places .text.hot section at the
beginning of a page to help the TLB performance.

Note that the limitation arises because the linker script employs glob
patterns instead of regular expressions for string matching. While there
is a method to maintain the current order using complex patterns, this
significantly complicates the pattern and increases the likelihood of
errors.

Co-developed-by: Han Shen <shenhan@...gle.com>
Signed-off-by: Han Shen <shenhan@...gle.com>
Signed-off-by: Rong Xu <xur@...gle.com>
Suggested-by: Sriraman Tallam <tmsriram@...gle.com>
Suggested-by: Krzysztof Pszeniczny <kpszeniczny@...gle.com>
Tested-by: Yonghong Song <yonghong.song@...ux.dev>
---
 include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index eeadbaeccf88..fd901951549c 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -553,19 +553,24 @@
  * .text section. Map to function alignment to avoid address changes
  * during second ld run in second ld pass when generating System.map
  *
- * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
- * code elimination is enabled, so these sections should be converted
- * to use ".." first.
+ * TEXT_MAIN here will match symbols with a fixed pattern (for example,
+ * .text.hot or .text.unlikely) if dead code elimination or
+ * function-section is enabled. Match these symbols first before
+ * TEXT_MAIN to ensure they are grouped together.
+ *
+ * Also placing .text.hot section at the beginning of a page, this
+ * would help the TLB performance.
  */
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
+		*(.text.asan.* .text.tsan.*)				\
+		*(.text.unknown .text.unknown.*)			\
+		*(.text.unlikely .text.unlikely.*)			\
+		. = ALIGN(PAGE_SIZE);					\
 		*(.text.hot .text.hot.*)				\
 		*(TEXT_MAIN .text.fixup)				\
-		*(.text.unlikely .text.unlikely.*)			\
-		*(.text.unknown .text.unknown.*)			\
 		NOINSTR_TEXT						\
-		*(.ref.text)						\
-		*(.text.asan.* .text.tsan.*)
+		*(.ref.text)
 
 
 /* sched.text is aling to function alignment to secure we have same
-- 
2.47.0.105.g07ac214952-goog