[答疑]-ATF中异常向量表为何没有实现“Current Exception level with SP_ELx, x＞0.“

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先看下ARM官方文档中所定义的向量表

第一行描述: 没有发生Exception切换，且SP使用的是SP_EL0
第二行描述: 没有发生Exception切换，且SP使用的是SP_ELx(x=1,2,3)
第三行描述: 发生l Exception切换，且target level使用的是aarch64；
第四行描述: 发生l Exception切换，且target level使用的是aarch32；

在宏内核体系中，第一行是不会用到的。不考虑aarch32，第四行也用不着；
如果SCR_EL3.FIQ=1，那么cpu在EL3时来的FIQ，将使用第二行向量表；
如果SCR_EL3.FIQ=1，那么cpu在EL0/1时来的FIQ，将使用第三行向量表；

而我们在看下ATF的向量表定义的代码，发现第二行的向量表没有实现，这是为什么呢？
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	.section	.vectors, "ax"; .align 11
	.align	7
runtime_exceptions:
	/* -----------------------------------------------------
	 * Current EL with _sp_el0 : 0x0 - 0x200
	 * -----------------------------------------------------
	 */
sync_exception_sp_el0:
	/* -----------------------------------------------------
	 * We don't expect any synchronous exceptions from EL3
	 * -----------------------------------------------------
	 */
	bl	report_unhandled_exception
	check_vector_size sync_exception_sp_el0

	.align	7
	/* -----------------------------------------------------
	 * EL3 code is non-reentrant. Any asynchronous exception
	 * is a serious error. Loop infinitely.
	 * -----------------------------------------------------
	 */
irq_sp_el0:
	bl	report_unhandled_interrupt
	check_vector_size irq_sp_el0

	.align	7
fiq_sp_el0:
	bl	report_unhandled_interrupt
	check_vector_size fiq_sp_el0

	.align	7
serror_sp_el0:
	bl	report_unhandled_exception
	check_vector_size serror_sp_el0

	/* -----------------------------------------------------
	 * Current EL with SPx: 0x200 - 0x400
	 * -----------------------------------------------------
	 */
	.align	7
sync_exception_sp_elx:
	/* -----------------------------------------------------
	 * This exception will trigger if anything went wrong
	 * during a previous exception entry or exit or while
	 * handling an earlier unexpected synchronous exception.
	 * There is a high probability that SP_EL3 is corrupted.
	 * -----------------------------------------------------
	 */
	bl	report_unhandled_exception
	check_vector_size sync_exception_sp_elx

	.align	7
irq_sp_elx:
	bl	report_unhandled_interrupt
	check_vector_size irq_sp_elx

	.align	7
fiq_sp_elx:
	bl	report_unhandled_interrupt
	check_vector_size fiq_sp_elx

	.align	7
serror_sp_elx:
	bl	report_unhandled_exception
	check_vector_size serror_sp_elx

	/* -----------------------------------------------------
	 * Lower EL using AArch64 : 0x400 - 0x600
	 * -----------------------------------------------------
	 */
	.align	7
sync_exception_aarch64:
	/* -----------------------------------------------------
	 * This exception vector will be the entry point for
	 * SMCs and traps that are unhandled at lower ELs most
	 * commonly. SP_EL3 should point to a valid cpu context
	 * where the general purpose and system register state
	 * can be saved.
	 * -----------------------------------------------------
	 */
	handle_sync_exception
	check_vector_size sync_exception_aarch64

	.align	7
	/* -----------------------------------------------------
	 * Asynchronous exceptions from lower ELs are not
	 * currently supported. Report their occurrence.
	 * -----------------------------------------------------
	 */
irq_aarch64:
	handle_interrupt_exception irq_aarch64
	check_vector_size irq_aarch64

	.align	7
fiq_aarch64:
	handle_interrupt_exception fiq_aarch64
	check_vector_size fiq_aarch64

	.align	7
serror_aarch64:
	bl	report_unhandled_exception
	check_vector_size serror_aarch64

	/* -----------------------------------------------------
	 * Lower EL using AArch32 : 0x600 - 0x800
	 * -----------------------------------------------------
	 */
	.align	7
sync_exception_aarch32:
	/* -----------------------------------------------------
	 * This exception vector will be the entry point for
	 * SMCs and traps that are unhandled at lower ELs most
	 * commonly. SP_EL3 should point to a valid cpu context
	 * where the general purpose and system register state
	 * can be saved.
	 * -----------------------------------------------------
	 */
	handle_sync_exception
	check_vector_size sync_exception_aarch32

	.align	7
	/* -----------------------------------------------------
	 * Asynchronous exceptions from lower ELs are not
	 * currently supported. Report their occurrence.
	 * -----------------------------------------------------
	 */
irq_aarch32:
	handle_interrupt_exception irq_aarch32
	check_vector_size irq_aarch32

	.align	7
fiq_aarch32:
	handle_interrupt_exception fiq_aarch32
	check_vector_size fiq_aarch32

	.align	7
serror_aarch32:
	bl	report_unhandled_exception
	check_vector_size serror_aarch32

在ATF/docs/firmware-design.md中找到了答案, 原来是在进入ATF之前，disable了所有的exception，ATF又没有修改PSTATE.DAIF，所有在ATF Runtime时 irq/fiq/serror/svnc都是disabled。所以异常向量表的第二行，也就用不着了。

Required CPU state when calling bl31_entrypoint() during cold boot


This function must only be called by the primary CPU.


On entry to this function the calling primary CPU must be executing in AArch64
EL3, little-endian data access, and all interrupt sources masked:

• PSTATE.EL = 3
• PSTATE.RW = 1
• PSTATE.DAIF = 0xf
• SCTLR_EL3.EE = 0