[PATCH v7 3/7] Adjust symbol ordering in text output section

[Rong Xu <xur@google.com>]

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 changes current order:
  .text.hot, .text, .text_unlikely, .text.unknown, .text.asan
to the new order:
  .text.asan, .text.unknown, .text_unlikely, .text.hot, .text

Here is the rationale behind the new layout:

The majority of the code resides in three sections: .text.hot, .text,
and .text.unlikely, with .text.unknown containing a negligible amount.
.text.asan is only generated in ASAN builds.

The primary goal is to group code segments based on their execution
frequency (hotness).

First, we want to place .text.hot adjacent to .text. Since we cannot put
.text.hot after .text (Due to constraints with -ffunction-sections,
placing .text.hot after .text is problematic), we need to put
.text.hot before .text.

Then it comes to .text.unlikely, we cannot put it after .text (same
-ffunction-sections issue) . Therefore, we position .text.unlikely
before .text.hot.

.text.unknown and .tex.asan follow the same logic.

This revised ordering effectively reverses the original arrangement (for
.text.unlikely, .text.unknown, and .tex.asan), maintaining a similar level
of affinity between sections.

It also places .text.hot section at the beginning of a page to better
utilize the TLB entry.

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.

This patch also changes vmlinux.lds.S for the sparc64 architecture to
accommodate specific symbol placement requirements.

Co-developed-by: Han Shen <shenhan@google.com>
Signed-off-by: Han Shen <shenhan@google.com>
Signed-off-by: Rong Xu <xur@google.com>
Suggested-by: Sriraman Tallam <tmsriram@google.com>
Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
---
 arch/sparc/kernel/vmlinux.lds.S   |  5 +++++
 include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/sparc/kernel/vmlinux.lds.S b/arch/sparc/kernel/vmlinux.lds.S
index d317a843f7ea9..f1b86eb303404 100644
--- a/arch/sparc/kernel/vmlinux.lds.S
+++ b/arch/sparc/kernel/vmlinux.lds.S
@@ -48,6 +48,11 @@ SECTIONS
 	{
 		_text = .;
 		HEAD_TEXT
+	        ALIGN_FUNCTION();
+#ifdef CONFIG_SPARC64
+	        /* Match text section symbols in head_64.S first */
+	        *head_64.o(.text)
+#endif
 		TEXT_TEXT
 		SCHED_TEXT
 		LOCK_TEXT
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index eeadbaeccf88b..fd901951549c0 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
-- 
2.47.0.163.g1226f6d8fa-goog