[PATCH v6 8/8] x86-64: select HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS

[Rasmus Villemoes]

This allows reducing the size of struct _ddebug from 56 to 40
bytes. There's one such struct for each pr_debug(), netdev_debug()
etc. in a CONFIG_DYNAMIC_DEBUG kernel. An Ubuntu 4.15 kernel has about
2550 entries in the __verbose section of vmlinux, amounting to ~40K
saved. (Modules also become smaller, but it's harder to quantify how
much that yields at runtime.)

For comparison, the __bug_table section of that Ubuntu kernel is 75576
bytes, i.e. 6298 12-byte bug_entrys, so GENERIC_BUG_RELATIVE_POINTERS
saves ~50K.

Due to the build-time sanity checks in asm-generic/dynamic_debug.h, we
need to add another #undef to vclock_gettime.c.

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 arch/x86/Kconfig                            | 1 +
 arch/x86/entry/vdso/vdso32/vclock_gettime.c | 1 +
 arch/x86/include/asm/Kbuild                 | 1 +
 3 files changed, 3 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2bbbd4d1ba31..ed44ea2b9556 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -30,6 +30,7 @@ config X86_64
 	select NEED_DMA_MAP_STATE
 	select SWIOTLB
 	select ARCH_HAS_SYSCALL_WRAPPER
+	select HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS
 
 config FORCE_DYNAMIC_FTRACE
 	def_bool y
diff --git a/arch/x86/entry/vdso/vdso32/vclock_gettime.c b/arch/x86/entry/vdso/vdso32/vclock_gettime.c
index 9242b28418d5..9acec4426206 100644
--- a/arch/x86/entry/vdso/vdso32/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vdso32/vclock_gettime.c
@@ -17,6 +17,7 @@
 #undef CONFIG_ILLEGAL_POINTER_VALUE
 #undef CONFIG_SPARSEMEM_VMEMMAP
 #undef CONFIG_NR_CPUS
+#undef CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS
 
 #define CONFIG_X86_32 1
 #define CONFIG_PGTABLE_LEVELS 2
diff --git a/arch/x86/include/asm/Kbuild b/arch/x86/include/asm/Kbuild
index 8b52bc5ddf69..3b630b4d37d7 100644
--- a/arch/x86/include/asm/Kbuild
+++ b/arch/x86/include/asm/Kbuild
@@ -8,6 +8,7 @@ generated-y += unistd_64_x32.h
 generated-y += xen-hypercalls.h
 
 generic-y += dma-contiguous.h
+generic-y += dynamic_debug.h
 generic-y += early_ioremap.h
 generic-y += export.h
 generic-y += mcs_spinlock.h
-- 
2.20.1