Unverified Commit c41bf432 authored by Andy Chiu's avatar Andy Chiu Committed by Palmer Dabbelt
Browse files

riscv: ftrace: align patchable functions to 4 Byte boundary 

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: default avatarEvgenii Shatokhin <e.shatokhin@yadro.com>
Signed-off-by: default avatarAndy Chiu <andy.chiu@sifive.com>
Reviewed-by: default avatarBjörn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20250407180838.42877-3-andybnac@gmail.com


Signed-off-by: default avatarAlexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: default avatarPalmer Dabbelt <palmer@dabbelt.com>
parent 54ecbc8d

arch/riscv/Kconfig

+2 −0
Original line number Diff line number Diff line
@@ -150,6 +150,7 @@ config RISCV 
	select HAVE_DEBUG_KMEMLEAK

	select HAVE_DMA_CONTIGUOUS if MMU

	select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)

	select FUNCTION_ALIGNMENT_4B if HAVE_DYNAMIC_FTRACE && RISCV_ISA_C

	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS

	select HAVE_DYNAMIC_FTRACE_WITH_ARGS if HAVE_DYNAMIC_FTRACE

	select HAVE_FTRACE_GRAPH_FUNC
@@ -236,6 +237,7 @@ 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_HAS_MIN_FUNCTION_ALIGNMENT || !RISCV_ISA_C



config HAVE_SHADOW_CALL_STACK

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