EABI和OABI

相信很多学习嵌入式linux的人都碰到过这样一个问题：

初学linux的时候大家都用的工具链版本多是3.4.5或3.4.2，名字为arm-linux-gcc或arm-softfloat-linux-gnu-gcc，可突然有一天发现这几个版本的编译器无法编译最新的内核了，并且发现人们都换了工具链了（arm-none-linux-gnueabi-gcc），怎么办，换呗。于是也都换成了这个工具链，编译内核，制作跟文件系统，按部就班的做，和以前没什么区别，很多人甚至想这些操作我都做烂了，闭着眼睛都能完成，一切和以前也没什么区别。可是紧接着问题来了，当内核启动到最后阶段挂载文件系统是出现如下错误：

kernel panic:attempted to kill init

或者什么错误都不提示，只是永远进入不了终端。

这是什么问题呢，答案是大家需要在内核配置的时候选择上如下内容：

make menuconfig
        Kernel Features --->
                [*] Use the ARM EABI to compile the kernel
                [*] Allow old ABI binaries to run with this kernel (EXPERIMENTAL)

选上这两项重新编译内核，发现上面提到的问题就解决了，为什么呢，这两个选项是什么东西呢。在这里得提到几个概念：

ABI:application binary interface
        OABI:old application binary interface
        EABI:extended application binary interface

但上面两项选中后在内核的配置文件.config中CONFIG_AEABI和CONFIG_OABI_COMPAT会被设置为“y”，CONFIG_AEABI表示现在内核为EABI，CONFIG_OABI_COMPAT表示兼容OABI。

为什么这两个选项会影响到我们的系统启动呢。

这两个选项可以选择任意一个也可以都选。这里涉及到两个结构sys_call_table和sys_oabi_call_table，这两个表是一个内核的跳转表，存放的是系统调用的指针。当CONFIG_AEABI和CONFIG_OABI_COMPAT都选中的时候，当应用程序使用OABI的是时候调用sys_oabi_call_table中的系统调用，应用程序使用EABI时使用sys_call_table中的系统调用。而如果只选CONFIG_AEABI则使用sys_call_table。两个都不选则使用sys_call_table。应为CONFIG_OABI_COMPAT对CONFIG_AEABI有依赖关系所以不能只选择CONFIG_OABI_COMPAT。

可以参考如下代码

arch/arm/kernel/entry-common.S
        #if defined(CONFIG_OABI_COMPAT)

        /*
                * If we have CONFIG_OABI_COMPAT then we need to look at the swi
                * value to determine if it is an EABI or an old ABI call.
                */
        #ifdef CONFIG_ARM_THUMB
                tst r8, #PSR_T_BIT
                movne    r10, #0         @ no thumb OABI emulation
                ldreq        r10, [lr, #-4]        @ get SWI instruction
        #else
                ldr    r10, [lr, #-4]        @ get SWI instruction
            A710( and ip, r10, #0x0f000000        @ check for SWI )
            A710( teq ip, #0x0f000000 )
            A710( bne .Larm710bug )
        #endif
        #ifdef CONFIG_CPU_ENDIAN_BE8
                rev    r10, r10        @ little endian instruction
        #endif
        #elif defined(CONFIG_AEABI)
                /*
                * Pure EABI user space always put syscall number into scno (r7).
                */
            A710( ldr      ip, [lr, #-4]        @ get SWI instruction )
            A710( and    ip, ip, #0x0f000000        @ check for SWI )
            A710( teq      ip, #0x0f000000 )
            A710( bne    .Larm710bug )
        #elif defined(CONFIG_ARM_THUMB)
                /* Legacy ABI only, possibly thumb mode. */
                tst r8, #PSR_T_BIT        @ this is SPSR from save_user_regs
                addne    scno, r7, #__NR_SYSCALL_BASE        @ put OS number in
                ldreq    scno, [lr, #-4]
        #else
                /* Legacy ABI only. */
                ldr    scno, [lr, #-4] @ get SWI instruction
            A710( and    ip, scno, #0x0f000000        @ check for SWI )
            A710( teq    ip, #0x0f000000 )
            A710( bne    .Larm710bug )
        #endif
        #ifdef CONFIG_ALIGNMENT_TRAP
                ldr    ip, __cr_alignment
                ldr    ip, [ip]
                mcr    p15, 0, ip, c1, c0        @ update control register
        #endif
                enable_irq
                get_thread_info tsk
                adr tbl, sys_call_table        @ load syscall table pointer
                ldr    ip, [tsk, #TI_FLAGS]        @ check for syscall tracing
        #if defined(CONFIG_OABI_COMPAT)
                /*
                * If the swi argument is zero, this is an EABI call and we do nothing.
                *
                * If this is an old ABI call, get the syscall number into scno and
                * get the old ABI syscall table address.
                */
                bics r10, r10, #0xff000000
                eorne    scno, r10, #__NR_OABI_SYSCALL_BASE
                ldrne      tbl, =sys_oabi_call_table
        #elif !defined(CONFIG_AEABI)
                bic    scno, scno, #0xff000000    @ mask off SWI op-code
                eor    scno, scno, #__NR_SYSCALL_BASE @ check OS number
        #endif