On Mon, Jun 17, 2024 at 10:38:55AM +0800, Andy Chiu wrote:

On Fri, Jun 14, 2024 at 3:09 AM Nathan Chancellor [off-list ref] wrote:

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Hi Andy,

On Thu, Jun 13, 2024 at 03:11:09PM +0800, Andy Chiu wrote:

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We are changing ftrace code patching in order to remove dependency from
stop_machine() and enable kernel preemption. This requires us to align
functions entry at a 4-B align address.

However, -falign-functions on older versions of GCC alone was not strong
enoungh to align all functions. In fact, cold functions are not aligned
after turning on optimizations. We consider this is a bug in GCC and
turn off guess-branch-probility as a workaround to align all functions.

GCC bug id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345

The option -fmin-function-alignment is able to align all functions
properly on newer versions of gcc. So, we add a cc-option to test if
the toolchain supports it.

Suggested-by: Evgenii Shatokhin <redacted>
Signed-off-by: Andy Chiu <redacted>
---
 arch/riscv/Kconfig  | 1 +
 arch/riscv/Makefile | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index b94176e25be1..80b8d48e1e46 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig

@@ -203,6 +203,7 @@ config CLANG_SUPPORTS_DYNAMIC_FTRACE
 config GCC_SUPPORTS_DYNAMIC_FTRACE
      def_bool CC_IS_GCC
      depends on $(cc-option,-fpatchable-function-entry=8)
+     depends on $(cc-option,-fmin-function-alignment=4) || !RISCV_ISA_C

Please use CC_HAS_MIN_FUNCTION_ALIGNMENT (from arch/Kconfig), which
already checks for support for this option.

Thanks for the suggestion!

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 config HAVE_SHADOW_CALL_STACK
      def_bool $(cc-option,-fsanitize=shadow-call-stack)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 06de9d365088..74628ad8dcf8 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile

@@ -14,8 +14,13 @@ endif
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
      LDFLAGS_vmlinux += --no-relax
      KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
+ifeq ($(CONFIG_CC_IS_CLANG),y)

Same here, please invert this and use

  ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT

like the main Makefile does.

Hope this makes sense to you. I am going to add the following in riscv Kconig:

select FUNCTION_ALIGNMENT_4B if DYNAMIC_FTRACE && !RISCV_ISA_C

So we will not need any of these

Yes, that definitely makes sense, I forgot that this has been abstracted
away via the use of those alignment Kconfig symbols that can be selected
when needed. Much better than my suggestions, thanks!

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+     cflags_ftrace_align := -falign-functions=4
+else
+     cflags_ftrace_align := -fmin-function-alignment=4
+endif
 ifeq ($(CONFIG_RISCV_ISA_C),y)
-     CC_FLAGS_FTRACE := -fpatchable-function-entry=4
+     CC_FLAGS_FTRACE := -fpatchable-function-entry=4 $(cflags_ftrace_align)
 else
      CC_FLAGS_FTRACE := -fpatchable-function-entry=2
 endif

--
2.43.0

Thanks,
Andy