内核黑科技之DEFINE宏

arch/arm/kernel/asm-offsets.c中会用到一个特殊的宏DEFINE，示例：

DEFINE(TI_FLAGS,        offsetof(struct thread_info, flags));

这句的意思就是将flags在struct thread_info中的偏移量赋值给TI_FLAGS，它会在vector_swi中使用。问题在于这个宏是怎么生效的呢？
首先找到DEFINE宏的定义，在include/linux/kbuild.h中：

#define DEFINE(sym, val) \
        asm volatile("\n->" #sym " %0 " #val : : "i" (val))

展开来是无法编译的，因为汇编指令中并没有->这个东西。
实际上，这个asm-offsets.c文件根本不是用来运行的，只是在编译的时候，用它生成一个asm-offsets.s文件，然后Kbuild会处理这个asm-offsets.s文件，生成asm-offsets.h文件。这个头文件最终被汇编文件引用，其中定义的变量最终得到应用。
如果要在自己的内核模块中使用这些变量，引用这个头文件即可。

#include 

引用设计者的说明。

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List:       linux-kernel 
Subject:    [RFC] Standard way of generating assembler offsets 
From:       Keith Owens  
Date:       2001-10-04 11:47:08 
[Download message RAW] 

Almost every architecture generates Assembler values to map the offsets 
of fields in C structures, about the only exception is i386 and that is 
because its offsets are hard coded into entry.S.  Every arch has done 
it differently, none of them have got it exactly right. 

As part of kbuild 2.5 I am standardizing on one method for generating 
Assembler offsets.  This change is required for kbuild 2.5 but it can 
be added to 2.4 without disturbing the current kbuild, I want to do 
this gradually now instead of a single massive change in kernel 2.5.  I 
will be issuing per architecture changes for generating Assembler 
offsets against 2.4. 

The kbuild 2.5 method for generating Assembler offsets satisfies these 
requirements: 

* No manual intervention required.  Many architectures rely on users 
  running make dep after changing config options that affect the 
  Assembler offsets.  If the user forgets to run make dep then the C 
  and Assembler code is out of sync - totally unacceptable.  This is 
  completely fixed in kbuild 2.5; I cannot do a complete fix in kbuild 
  2.4 but it is still better than the existing manual system. 

* Standard name for the related files.  There are 6+ different names 
  for the files used to generate Assembler offsets, kbuild 2.5 uses 
  asm-offsets.[csh] on all architectures. 

* Allows for multiple parallel compiles from the same source tree. 
  Writing the generated asm-offsets.h to include/asm is not an option, 
  it prevents concurrent compiles. 

* The method must work in native and cross compile mode and give 
  exactly the same results.  Some 2.4 code only works in native mode, 
  some architectures have different methods for native and cross 
  compile with different output formats.  Yeuch! 

* Standard scripts for generating the output.  Every arch does it 
  differently in 2.4, standards are good! 

* Correct dependency trees.  Because 2.4 make dep does not scan .S 
  files, there is little or no dependency information.  Even if the 
  offsets are regenerated, the affected Assembler code does not always 
  get rebuilt.  kbuild 2.5 handles dependencies for Assembler as well 
  as C; I cannot get kbuild 2.4 perfect but I can improve on the 
  existing (or non-existent) 2.4 dependencies. 

All architectures will define arch/$(ARCH)/asm-offsets.c.  This has a 
standard prologue for the macros that convert offsets to Assembler, 
followed by arch specific field references. 

arch/$(ARCH)/asm-offsets.s is generated from arch/$(ARCH)/asm-offsets.c 
using standard rules, although kbuild 2.4 needs some tweaking. 

arch/$(ARCH)/asm-offsets.h is generated from arch/$(ARCH)/asm-offsets.s 
by a semi-standard script.  Most of the script is common to all 
architectures but the precise format of the Assembler output is arch 
specific. 

The final result is included in *only* the Assembler programs that need 
it, as #include "asm-offsets.h" with -I arch/$(ARCH) in the relevant 
Makefiles.  Hard coding relative paths in source files is a pet hate, 
use #include "localname.h" and -I instead.  Including the generated 
file in C code is not allowed, it severly pollutes the dependency 
chain, to the extent that any config change can force a complete 
recompile, unacceptable. 


Example from i386: 

arch/i386/asm-offsets.c 

/* 
 * Generate definitions needed by assembly language modules. 
 * This code generates raw asm output which is post-processed to extract 
 * and format the required data. 
 */ 

#include  
#include  
#include  

/* Use marker if you need to separate the values later */ 

#define DEFINE(sym, val, marker) \ 
  asm volatile("\n-> " #sym " %0 " #val " " #marker : : "i" (val)) 

#define BLANK() asm volatile("\n->" : : ) 

int 
main(void) 
{ 
  DEFINE(state,        offsetof(struct task_struct, state),); 
  DEFINE(flags,        offsetof(struct task_struct, flags),); 
  DEFINE(sigpending,   offsetof(struct task_struct, sigpending),); 
  DEFINE(addr_limit,   offsetof(struct task_struct, addr_limit),); 
  DEFINE(exec_domain,  offsetof(struct task_struct, exec_domain),); 
  DEFINE(need_resched, offsetof(struct task_struct, need_resched),); 
  DEFINE(tsk_ptrace,   offsetof(struct task_struct, ptrace),); 
  DEFINE(processor,    offsetof(struct task_struct, processor),); 
  BLANK(); 
  DEFINE(ENOSYS,       ENOSYS,); 
  return 0; 
} 

asm-offsets.s to asm-offsets.h. 

# Convert raw asm offsets into something that can be included as 
# assembler definitions.  It converts 
#   -> symbol $value source 
# into 
#   symbol = value /* 0xvalue source */ 

echo '#ifndef __ASM_OFFSETS_H__' 
echo '#define __ASM_OFFSETS_H__' 
echo '/*' 
echo ' * DO NOT MODIFY' 
echo ' *' 
echo " * This file was generated by arch/$(ARCH)/Makefile.in." 
echo ' *' 
echo ' */' 
echo '' 
awk ' 
  /^->$/{printf("\n")} 
  /^-> /{ 
    sym = $2; 
    val = $3; 
    sub(/^\$/, "", val); 
    $1 = ""; 
    $2 = ""; 
    $3 = ""; 
    printf("%-20s = %3d\t/* 0x%x\t%s */\n", sym, val, val, $0) 
  } 
' 
echo '#endif' 

Generated arch/i386/asm-offsets.h 

#ifndef __ASM_OFFSETS_H__ 
#define __ASM_OFFSETS_H__ 
/* 
 * DO NOT MODIFY 
 * 
 * This file was generated by arch/i386/Makefile.in. 
 * 
 */ 

state                =   0      /* 0x0     offsetof(struct task_struct, state) */ 
flags                =   4      /* 0x4     offsetof(struct task_struct, flags) */ 
sigpending           =   8      /* 0x8     offsetof(struct task_struct, sigpending) */ 
addr_limit           =  12      /* 0xc     offsetof(struct task_struct, addr_limit) */ 
exec_domain          =  16      /* 0x10    offsetof(struct task_struct, exec_domain) */ 
need_resched         =  20      /* 0x14    offsetof(struct task_struct, need_resched) */ 
tsk_ptrace           =  24      /* 0x18    offsetof(struct task_struct, ptrace) */ 
processor            =  52      /* 0x34    offsetof(struct task_struct, processor) */ 

ENOSYS               =  38      /* 0x26    ENOSYS */ 
#endif 


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