x86: document 64-bit and 32-bit function call convention ABI

/*

 * Some macros to handle stack frames in assembly.

 x86 function call convention, 64-bit:

 -------------------------------------

  arguments           |  callee-saved      | extra caller-saved | return

 [callee-clobbered]   |                    | [callee-clobbered] |

 ---------------------------------------------------------------------------

 rdi rsi rdx rcx r8-9 | rbx rbp [*] r12-15 | r10-11             | rax, rdx [**]

 ( rsp is obviously invariant across normal function calls. (gcc can 'merge'

   functions when it sees tail-call optimization possibilities) rflags is

   clobbered. Leftover arguments are passed over the stack frame.)

 [*]  In the frame-pointers case rbp is fixed to the stack frame.

 [**] for struct return values wider than 64 bits the return convention is a

      bit more complex: up to 128 bits width we return small structures

      straight in rax, rdx. For structures larger than that (3 words or

      larger) the caller puts a pointer to an on-stack return struct

      [allocated in the caller's stack frame] into the first argument - i.e.

      into rdi. All other arguments shift up by one in this case.

      Fortunately this case is rare in the kernel.

For 32-bit we have the following conventions - kernel is built with

-mregparm=3 and -freg-struct-return:

 x86 function calling convention, 32-bit:

 ----------------------------------------

  arguments         | callee-saved        | extra caller-saved | return

 [callee-clobbered] |                     | [callee-clobbered] |

 -------------------------------------------------------------------------

 eax edx ecx        | ebx edi esi ebp [*] | <none>             | eax, edx [**]

 ( here too esp is obviously invariant across normal function calls. eflags

   is clobbered. Leftover arguments are passed over the stack frame. )

 [*]  In the frame-pointers case ebp is fixed to the stack frame.

 [**] We build with -freg-struct-return, which on 32-bit means similar

      semantics as on 64-bit: edx can be used for a second return value

      (i.e. covering integer and structure sizes up to 64 bits) - after that

      it gets more complex and more expensive: 3-word or larger struct returns

      get done in the caller's frame and the pointer to the return struct goes

      into regparm0, i.e. eax - the other arguments shift up and the

      function's register parameters degenerate to regparm=2 in essence.

*/

/*

 * 64-bit system call stack frame layout defines and helpers,

 * for assembly code:

 */

#define R15		  0

#define RBP		 32

#define RBX		 40

/* arguments: interrupts/non tracing syscalls only save upto here*/

/* arguments: interrupts/non tracing syscalls only save up to here: */

#define R11		 48

#define R10		 56

#define R9		 64

#define ORIG_RAX	120       /* + error_code */

/* end of arguments */

/* cpu exception frame or undefined in case of fast syscall. */

/* cpu exception frame or undefined in case of fast syscall: */

#define RIP		128

#define CS		136

#define EFLAGS		144