x86/asm/entry/64: Improve the THREAD_INFO() macro explanation

Explain the background, and add a real example.
Acked-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/20150324184311.GA14760@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/thread_info.h

@@ -206,10 +206,29 @@ static inline unsigned long current_stack_pointer(void)
 	_ASM_SUB $(THREAD_SIZE),reg ;
 
 /*
- * ASM operand which evaluates to thread_info address
- * if it is known that "reg" is exactly "off" bytes below stack top.
- * Example (fetch thread_info->fieldname):
- *  mov TI_fieldname+THREAD_INFO(reg, off),%eax
+ * ASM operand which evaluates to a 'thread_info' address of
+ * the current task, if it is known that "reg" is exactly "off"
+ * bytes below the top of the stack currently.
+ *
+ * ( The kernel stack's size is known at build time, it is usually
+ *   2 or 4 pages, and the bottom  of the kernel stack contains
+ *   the thread_info structure. So to access the thread_info very
+ *   quickly from assembly code we can calculate down from the
+ *   top of the kernel stack to the bottom, using constant,
+ *   build-time calculations only. )
+ *
+ * For example, to fetch the current thread_info->flags value into %eax
+ * on x86-64 defconfig kernels, in syscall entry code where RSP is
+ * currently at exactly SIZEOF_PTREGS bytes away from the top of the
+ * stack:
+ *
+ *      mov TI_flags+THREAD_INFO(%rsp, SIZEOF_PTREGS), %eax
+ *
+ * will translate to:
+ *
+ *      8b 84 24 b8 c0 ff ff      mov    -0x3f48(%rsp), %eax
+ *
+ * which is below the current RSP by almost 16K.
  */
 #define THREAD_INFO(reg, off) ((off)-THREAD_SIZE)(reg)