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

	_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)