Re: [PATCH v2 08/59] x86/build: Ensure proper function alignment

[Peter Zijlstra]

On Fri, Sep 02, 2022 at 11:08:54AM -0700, Linus Torvalds wrote:

> Let's just do this right.

Something like so then?

---
--- a/Makefile
+++ b/Makefile
@@ -940,8 +940,8 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_CFI)
 export CC_FLAGS_CFI
 endif
 
-ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B
-KBUILD_CFLAGS += -falign-functions=64
+ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0)
+KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
 endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1419,4 +1419,24 @@ source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"
 
+config FUNCTION_ALIGNMENT_8B
+	bool
+
+config FUNCTION_ALIGNMENT_16B
+	bool
+
+config FUNCTION_ALIGNMENT_32B
+	bool
+
+config FUNCTION_ALIGNMENT_64B
+	bool
+
+config FUNCTION_ALIGNMENT
+	int
+	default 64 if FUNCTION_ALIGNMENT_64B
+	default 32 if FUNCTION_ALIGNMENT_32B
+	default 16 if FUNCTION_ALIGNMENT_16B
+	default 8 if FUNCTION_ALIGNMENT_8B
+	default 0
+
 endmenu
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig
@@ -63,6 +63,7 @@ config IA64
 	select NUMA if !FLATMEM
 	select PCI_MSI_ARCH_FALLBACKS if PCI_MSI
 	select ZONE_DMA32
+	select FUNCTION_ALIGNMENT_32B
 	default y
 	help
 	  The Itanium Processor Family is Intel's 64-bit successor to
--- a/arch/ia64/Makefile
+++ b/arch/ia64/Makefile
@@ -23,7 +23,7 @@ KBUILD_AFLAGS_KERNEL := -mconstant-gp
 EXTRA		:=
 
 cflags-y	:= -pipe $(EXTRA) -ffixed-r13 -mfixed-range=f12-f15,f32-f127 \
-		   -falign-functions=32 -frename-registers -fno-optimize-sibling-calls
+		   -frename-registers -fno-optimize-sibling-calls
 KBUILD_CFLAGS_KERNEL := -mconstant-gp
 
 GAS_STATUS	= $(shell $(srctree)/arch/ia64/scripts/check-gas "$(CC)" "$(OBJDUMP)")
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -283,6 +283,8 @@ config X86
 	select X86_FEATURE_NAMES		if PROC_FS
 	select PROC_PID_ARCH_STATUS		if PROC_FS
 	select HAVE_ARCH_NODE_DEV_GROUP		if X86_SGX
+	select FUNCTION_ALIGNMENT_16B		if X86_64 || X86_ALIGNMENT_16
+	select FUNCTION_ALIGNMENT_8B
 	imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI
 
 config INSTRUCTION_DECODER
@@ -2442,6 +2444,7 @@ config HAVE_CALL_THUNKS
 	depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL
 
 config CALL_THUNKS
+	select FUNCTION_ALIGNMENT_16B
 	def_bool n
 
 menuconfig SPECULATION_MITIGATIONS
@@ -2515,6 +2518,7 @@ config CALL_DEPTH_TRACKING
 
 config CALL_THUNKS_DEBUG
 	bool "Enable call thunks and call depth tracking debugging"
+	select FUNCTION_ALIGNMENT_32B
 	depends on CALL_DEPTH_TRACKING
 	default n
 	help
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -517,9 +517,3 @@ config CPU_SUP_VORTEX_32
 	  makes the kernel a tiny bit smaller.
 
 	  If unsure, say N.
-
-# Defined here so it is defined for UM too
-config FUNCTION_ALIGNMENT
-	int
-	default 16 if X86_64 || X86_ALIGNMENT_16
-	default 8
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -84,10 +84,6 @@ else
 KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
 endif
 
-ifneq ($(CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B),y)
-KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
-endif
-
 ifeq ($(CONFIG_X86_32),y)
         BITS := 32
         UTS_MACHINE := i386
--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -12,16 +12,7 @@
 #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
 #endif /* CONFIG_X86_32 */
 
-#if CONFIG_FUNCTION_ALIGNMENT == 8
-# define __ALIGN		.p2align 3, 0x90;
-#elif CONFIG_FUNCTION_ALIGNMENT == 16
-# define __ALIGN		.p2align 4, 0x90;
-#elif CONFIG_FUNCTION_ALIGNMENT == 32
-# define __ALIGN		.p2align 5, 0x90
-#else
-# error Unsupported function alignment
-#endif
-
+#define __ALIGN			.balign CONFIG_FUNCTION_ALIGNMENT, 0x90
 #define __ALIGN_STR		__stringify(__ALIGN)
 
 #ifdef CONFIG_CFI_CLANG
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -69,8 +69,8 @@
 #endif
 
 #ifndef __ALIGN
-#define __ALIGN		.align 4,0x90
-#define __ALIGN_STR	".align 4,0x90"
+#define __ALIGN			.balign CONFIG_FUNCTION_ALIGNMENT
+#define __ALIGN_STR		__stringify(__ALIGN)
 #endif
 
 #ifdef __ASSEMBLY__
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -459,6 +459,7 @@ config SECTION_MISMATCH_WARN_ONLY
 config DEBUG_FORCE_FUNCTION_ALIGN_64B
 	bool "Force all function address 64B aligned"
 	depends on EXPERT && (X86_64 || ARM64 || PPC32 || PPC64 || ARC)
+	select FUNCTION_ALIGNMENT_64B
 	help
 	  There are cases that a commit from one domain changes the function
 	  address alignment of other domains, and cause magic performance