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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.
#ifdef CONFIG_X86_64
* 64-bit system call stack frame layout defines and helpers,
* for assembly code:
/* The layout forms the "struct pt_regs" on the stack: */
* C ABI says these regs are callee-preserved. They aren't saved on kernel entry
* unless syscall needs a complete, fully filled "struct pt_regs".
#define R15 0*8
#define R14 1*8
#define R13 2*8
#define R12 3*8
#define RBP 4*8
#define RBX 5*8
/* These regs are callee-clobbered. Always saved on kernel entry. */
#define R11 6*8
#define R10 7*8
#define R9 8*8
#define R8 9*8
#define RAX 10*8
#define RCX 11*8
#define RDX 12*8
#define RSI 13*8
#define RDI 14*8
* On syscall entry, this is syscall#. On CPU exception, this is error code.
* On hw interrupt, it's IRQ number:
#define ORIG_RAX 15*8
/* Return frame for iretq */
#define RIP 16*8
#define CS 17*8
#define EFLAGS 18*8
#define RSP 19*8
#define SS 20*8
#define SIZEOF_PTREGS 21*8
.macro ALLOC_PT_GPREGS_ON_STACK addskip=0
addq $-(15*8+\addskip), %rsp
.macro SAVE_C_REGS_HELPER offset=0 rax=1 rcx=1 r8910=1 r11=1
.if \r11
movq %r11, 6*8+\offset(%rsp)
.if \r8910
movq %r10, 7*8+\offset(%rsp)
movq %r9, 8*8+\offset(%rsp)
movq %r8, 9*8+\offset(%rsp)
.if \rax
movq %rax, 10*8+\offset(%rsp)
.if \rcx
movq %rcx, 11*8+\offset(%rsp)
movq %rdx, 12*8+\offset(%rsp)
movq %rsi, 13*8+\offset(%rsp)
movq %rdi, 14*8+\offset(%rsp)
.macro SAVE_C_REGS offset=0
SAVE_C_REGS_HELPER \offset, 1, 1, 1, 1
.macro SAVE_C_REGS_EXCEPT_RAX_RCX offset=0
SAVE_C_REGS_HELPER \offset, 0, 0, 1, 1
.macro SAVE_C_REGS_EXCEPT_R891011
SAVE_C_REGS_HELPER 0, 1, 1, 0, 0
SAVE_C_REGS_HELPER 0, 1, 0, 0, 0
SAVE_C_REGS_HELPER 0, 0, 0, 1, 0
.macro SAVE_EXTRA_REGS offset=0
movq %r15, 0*8+\offset(%rsp)
movq %r14, 1*8+\offset(%rsp)
movq %r13, 2*8+\offset(%rsp)
movq %r12, 3*8+\offset(%rsp)
movq %rbp, 4*8+\offset(%rsp)
movq %rbx, 5*8+\offset(%rsp)
.macro SAVE_EXTRA_REGS_RBP offset=0
movq %rbp, 4*8+\offset(%rsp)
.macro RESTORE_EXTRA_REGS offset=0
movq 0*8+\offset(%rsp), %r15
movq 1*8+\offset(%rsp), %r14
movq 2*8+\offset(%rsp), %r13
movq 3*8+\offset(%rsp), %r12
movq 4*8+\offset(%rsp), %rbp
movq 5*8+\offset(%rsp), %rbx
xorl %r15d, %r15d
xorl %r14d, %r14d
xorl %r13d, %r13d
xorl %r12d, %r12d
xorl %ebp, %ebp
xorl %ebx, %ebx
.macro RESTORE_C_REGS_HELPER rstor_rax=1, rstor_rcx=1, rstor_r11=1, rstor_r8910=1, rstor_rdx=1
.if \rstor_r11
movq 6*8(%rsp), %r11
.if \rstor_r8910
movq 7*8(%rsp), %r10
movq 8*8(%rsp), %r9
movq 9*8(%rsp), %r8
.if \rstor_rax
movq 10*8(%rsp), %rax
.if \rstor_rcx
movq 11*8(%rsp), %rcx
.if \rstor_rdx
movq 12*8(%rsp), %rdx
movq 13*8(%rsp), %rsi
movq 14*8(%rsp), %rdi
subq $-(15*8+\addskip), %rsp
.macro icebp
.byte 0xf1
#else /* CONFIG_X86_64 */
* For 32bit only simplified versions of SAVE_ALL/RESTORE_ALL. These
* are different from the entry_32.S versions in not changing the segment
* registers. So only suitable for in kernel use, not when transitioning
* from or to user space. The resulting stack frame is not a standard
* pt_regs frame. The main use case is calling C code from assembler
* when all the registers need to be preserved.
.macro SAVE_ALL
pushl %eax
pushl %ebp
pushl %edi
pushl %esi
pushl %edx
pushl %ecx
pushl %ebx
popl %ebx
popl %ecx
popl %edx
popl %esi
popl %edi
popl %ebp
popl %eax
#endif /* CONFIG_X86_64 */