如何编写从汇编代码调用的 C 函数

Question

我需要编写一个 C 函数，该函数将从 linux 内核中的汇编代码中调用。

应该考虑哪些特殊问题？

我有一些想法，但谁能提供更多细节：

(1) 调用约定

确保汇编中的调用者和c中的被调用者握手良好。但是我应该使用哪种调用约定？如何声明c函数并在汇编代码中声明？

(2) 保护寄存器

在调用之前，一些寄存器应该保存在程序集中。我大致记得它们是eax，ecx和edx。但是我不必保存它们，除非我需要在调用 C 函数之前引用旧值，对吧？

还有什么问题？

Answer 1

当您的任务标记为 linux-kernel 时，请查看以下内核标头：arch/x86/include/asm/calling.h。

   3 x86 function call convention, 64-bit:
   4 -------------------------------------
   5  arguments           |  callee-saved      | extra caller-saved | return
   6 [callee-clobbered]   |                    | [callee-clobbered] |
   7 ---------------------------------------------------------------------------
   8 rdi rsi rdx rcx r8-9 | rbx rbp [*] r12-15 | r10-11             | rax, rdx [**]
   9
  10 ( rsp is obviously invariant across normal function calls. (gcc can 'merge'
  11   functions when it sees tail-call optimization possibilities) rflags is
  12   clobbered. Leftover arguments are passed over the stack frame.)
  13
  14 [*]  In the frame-pointers case rbp is fixed to the stack frame.
  15
  16 [**] for struct return values wider than 64 bits the return convention is a
  17      bit more complex: up to 128 bits width we return small structures
  18      straight in rax, rdx. For structures larger than that (3 words or
  19      larger) the caller puts a pointer to an on-stack return struct
  20      [allocated in the caller's stack frame] into the first argument - i.e.
  21      into rdi. All other arguments shift up by one in this case.
  22      Fortunately this case is rare in the kernel.
  23
  24 For 32-bit we have the following conventions - kernel is built with
  25 -mregparm=3 and -freg-struct-return:
  26
  27 x86 function calling convention, 32-bit:
  28 ----------------------------------------
  29  arguments         | callee-saved        | extra caller-saved | return
  30 [callee-clobbered] |                     | [callee-clobbered] |
  31 -------------------------------------------------------------------------
  32 eax edx ecx        | ebx edi esi ebp [*] | <none>             | eax, edx [**]
  33
  34 ( here too esp is obviously invariant across normal function calls. eflags
  35   is clobbered. Leftover arguments are passed over the stack frame. )
  36
  37 [*]  In the frame-pointers case ebp is fixed to the stack frame.
  38
  39 [**] We build with -freg-struct-return, which on 32-bit means similar
  40      semantics as on 64-bit: edx can be used for a second return value
  41      (i.e. covering integer and structure sizes up to 64 bits) - after that
  42      it gets more complex and more expensive: 3-word or larger struct returns
  43      get done in the caller's frame and the pointer to the return struct goes
  44      into regparm0, i.e. eax - the other arguments shift up and the
  45      function's register parameters degenerate to regparm=2 in essence.