[PATCH] x86-64: Speed up dwarf2 unwinder

This changes the dwarf2 unwinder to do a binary search for CIEs
instead of a linear work. The linker is unfortunately not
able to build a proper lookup table at link time, instead it creates
one at runtime as soon as the bootmem allocator is usable (so you'll continue
using the linear lookup for the first [hopefully] few calls).
The code should be ready to utilize a build-time created table once
a fixed linker becomes available.
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>

Makefile

@@ -499,6 +499,7 @@ endif
 
 ifdef CONFIG_UNWIND_INFO
 CFLAGS		+= -fasynchronous-unwind-tables
+LDFLAGS_vmlinux	+= --eh-frame-hdr
 endif
 
 ifdef CONFIG_DEBUG_INFO

include/asm-generic/vmlinux.lds.h

@@ -125,6 +125,10 @@
 		*(__param)						\
 		VMLINUX_SYMBOL(__stop___param) = .;			\
 	}								\
+									\
+	/* Unwind data binary search table */				\
+	EH_FRAME_HDR							\
+									\
 	__end_rodata = .;						\
 	. = ALIGN(4096);
 
@@ -157,6 +161,18 @@
 		*(.kprobes.text)					\
 		VMLINUX_SYMBOL(__kprobes_text_end) = .;
 
+#ifdef CONFIG_STACK_UNWIND
+		/* Unwind data binary search table */
+#define EH_FRAME_HDR							\
+        	.eh_frame_hdr : AT(ADDR(.eh_frame_hdr) - LOAD_OFFSET) {	\
+			VMLINUX_SYMBOL(__start_unwind_hdr) = .;		\
+			*(.eh_frame_hdr)				\
+			VMLINUX_SYMBOL(__end_unwind_hdr) = .;		\
+		}
+#else
+#define EH_FRAME_HDR
+#endif
+
 		/* DWARF debug sections.
 		Symbols in the DWARF debugging sections are relative to
 		the beginning of the section so we begin them at 0.  */

include/linux/unwind.h

@@ -26,6 +26,7 @@ struct module;
  * Initialize unwind support.
  */
 extern void unwind_init(void);
+extern void unwind_setup(void);
 
 #ifdef CONFIG_MODULES
 
@@ -73,6 +74,7 @@ extern int unwind_to_user(struct unwind_frame_info *);
 struct unwind_frame_info {};
 
 static inline void unwind_init(void) {}
+static inline void unwind_setup(void) {}
 
 #ifdef CONFIG_MODULES
 

init/main.c

@@ -503,6 +503,7 @@ asmlinkage void __init start_kernel(void)
 	printk(KERN_NOTICE);
 	printk(linux_banner);
 	setup_arch(&command_line);
+	unwind_setup();
 	setup_per_cpu_areas();
 	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */