【转载】kernel_entry和kernel_exit

 

版权声明：本文为CSDN博主「liuhangtiant」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/liuhangtiant/article/details/90456136

 

 

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。