前言
本文介绍kernel_entry和kernel_exit两个宏，这两个函数并不单单是从用户空间进入内核空间以及从内核空间退出到用户空间才会用到。所有的异常处理函数首先会调用kernel_entry保存现场，最后调用kernel_exit恢复现场。
kernel_entry
.macro kernel_entry, el, regsize = 64
sub sp, sp, #S_FRAME_SIZE
.if \regsize == 32
mov w0, w0 // zero upper 32 bits of x0
.endif
stp x0, x1, [sp, #16 * 0]
stp x2, x3, [sp, #16 * 1]
stp x4, x5, [sp, #16 * 2]
stp x6, x7, [sp, #16 * 3]
stp x8, x9, [sp, #16 * 4]
stp x10, x11, [sp, #16 * 5]
stp x12, x13, [sp, #16 * 6]
stp x14, x15, [sp, #16 * 7]
stp x16, x17, [sp, #16 * 8]
stp x18, x19, [sp, #16 * 9]
stp x20, x21, [sp, #16 * 10]
stp x22, x23, [sp, #16 * 11]
stp x24, x25, [sp, #16 * 12]
stp x26, x27, [sp, #16 * 13]
stp x28, x29, [sp, #16 * 14]
.if \el == 0
mrs x21, sp_el0
mov tsk, sp
and tsk, tsk, #~(THREAD_SIZE - 1) // Ensure MDSCR_EL1.SS is clear,
ldr x19, [tsk, #TI_FLAGS] // since we can unmask debug
disable_step_tsk x19, x20 // exceptions when scheduling.
mov x29, xzr // fp pointed to user-space
.else
add x21, sp, #S_FRAME_SIZE
get_thread_info tsk
/* Save the task's original addr_limit and set USER_DS (TASK_SIZE_64) */
ldr x20, [tsk, #TI_ADDR_LIMIT]
str x20, [sp, #S_ORIG_ADDR_LIMIT]
mov x20, #TASK_SIZE_64
str x20, [tsk, #TI_ADDR_LIMIT]
/* No need to reset PSTATE.UAO, hardware's already set it to 0 for us */
.endif /* \el == 0 */
mrs x22, elr_el1
mrs x23, spsr_el1
stp lr, x21, [sp, #S_LR]
stp x22, x23, [sp, #S_PC]
/*
* Set syscallno to -1 by default (overridden later if real syscall).
*/
.if \el == 0
mvn x21, xzr
str x21, [sp, #S_SYSCALLNO]
.endif
/*
* Set sp_el0 to current thread_info.
*/
.if \el == 0
msr sp_el0, tsk
.endif
/*
* Registers that may be useful after this macro is invoked:
*
* x21 - aborted SP
* x22 - aborted PC
* x23 - aborted PSTATE
*/
.endm
该宏有两个输入参数，el表示从哪一个exception level进入该异常，从用户空间进入该异常el=0，从内核空间进入该异常，el=1；regsize表示寄存器位宽，当内核空间运行在AARCH64是，用户空间可以运行在AARCH64或者AARCH32，所以从用户空间进入异常需要知道用户空间运行在AARCH64还是AARCH32。第一步是将通用寄存器X0-X29入栈。第二步如果是从用户空间进入该异常的话，需要先保存用户空间的栈指针，因为用户空间的栈指针会另做他用，用于存放tsk的地址；如果是从内核空间进入该异常的话，可以直接从SP_EL0获得tsk，因为内核空间就是通过SP_EL0存放tsk的。关于tsk是什么东西？有什么用途？我会专门写一些列进程管理的文章来阐述。第三步保存EL1的LR,ELR,SPSR等寄存器，也是为了恢复现场使用的。第四步，如果是从用户空间进入该异常的话，会将系统调用number设置为-1，后面真正的系统调用处理函数会将其设置为真是的系统调用number。第五步，将tsk存放到sp_el0。
kernel_exit
··· .macro kernel_exit, el .if \el != 0 /* Restore the task’s original addr_limit. */ ldr x20, [sp, #S_ORIG_ADDR_LIMIT] str x20, [tsk, #TI_ADDR_LIMIT]
/* No need to restore UAO, it will be restored from SPSR_EL1 */
.endif
ldp x21, x22, [sp, #S_PC] // load ELR, SPSR
.if \el == 0
ct_user_enter
ldr x23, [sp, #S_SP] // load return stack pointer
msr sp_el0, x23
#ifdef CONFIG_ARM64_ERRATUM_845719
alternative_if ARM64_WORKAROUND_845719
tbz x22, #4, 1f
#ifdef CONFIG_PID_IN_CONTEXTIDR
mrs x29, contextidr_el1
msr contextidr_el1, x29
#else
msr contextidr_el1, xzr
#endif
1:
alternative_else_nop_endif
#endif
.endif
msr elr_el1, x21 // set up the return data
msr spsr_el1, x22
ldp x0, x1, [sp, #16 * 0]
ldp x2, x3, [sp, #16 * 1]
ldp x4, x5, [sp, #16 * 2]
ldp x6, x7, [sp, #16 * 3]
ldp x8, x9, [sp, #16 * 4]
ldp x10, x11, [sp, #16 * 5]
ldp x12, x13, [sp, #16 * 6]
ldp x14, x15, [sp, #16 * 7]
ldp x16, x17, [sp, #16 * 8]
ldp x18, x19, [sp, #16 * 9]
ldp x20, x21, [sp, #16 * 10]
ldp x22, x23, [sp, #16 * 11]
ldp x24, x25, [sp, #16 * 12]
ldp x26, x27, [sp, #16 * 13]
ldp x28, x29, [sp, #16 * 14]
ldr lr, [sp, #S_LR]
add sp, sp, #S_FRAME_SIZE // restore sp
eret // return to kernel
.endm
··· kernel_exit基本上是kernel_entry的逆过程，我们不在赘述，有一点需要说明的是eret指令用于退出异常。
