接上文。

2.5 初始化cpu状态（__cpu_setup）

虽然在异常初始化流程中已经设置了sctlr_el1等系统控制寄存器，但在打开mmu前还需要其它一些准备工作。

......
# arch/arm64/mm/proc.S
/*
 *	__cpu_setup
 *
 *	Initialise the processor for turning the MMU on.
 *
 * Output:
 *	Return in x0 the value of the SCTLR_EL1 register.
 */
	.pushsection ".idmap.text", "awx"          '放在.idmap.text段中'
SYM_FUNC_START(__cpu_setup)
	tlbi	vmalle1				// Invalidate local TLB
	dsb	nsh

	mov	x1, #3 << 20
	msr	cpacr_el1, x1			// Enable FP/ASIMD             ---（2.5.1）
	mov	x1, #1 << 12			// Reset mdscr_el1 and disable ---（2.5.2）
	msr	mdscr_el1, x1			// access to the DCC from EL0  
	isb					// Unmask debug exceptions now,
	enable_dbg				// since this is per-cpu       ---（2.5.3）
	reset_pmuserenr_el0 x1			// Disable PMU access from EL0 关闭EL0访问PMU
	reset_amuserenr_el0 x1			// Disable AMU access from EL0 关闭EL0访问AMU

	/*
	 * Default values for VMSA control registers. These will be adjusted
	 * below depending on detected CPU features.
	 */
        //name .req register name: 为寄存器定义一个别名
	mair	.req	x17                          //定义X17寄存器的别名  mair	     
	tcr	.req	x16                          //定义X16寄存器的别名  tcr	
	mov_q	mair, MAIR_EL1_SET                   //                       //---（2.5.4）
	mov_q	tcr, TCR_TxSZ(VA_BITS) | TCR_CACHE_FLAGS | TCR_SMP_FLAGS | \  //---（2.5.5）
			TCR_TG_FLAGS | TCR_KASLR_FLAGS | TCR_ASID16 | \
			TCR_TBI0 | TCR_A1 | TCR_KASAN_SW_FLAGS

#ifdef CONFIG_ARM64_MTE                                                       //---（2.5.6）
	/*
	 * Update MAIR_EL1, GCR_EL1 and TFSR*_EL1 if MTE is supported
	 * (ID_AA64PFR1_EL1[11:8] > 1).
	 */
	mrs	x10, ID_AA64PFR1_EL1
	ubfx	x10, x10, #ID_AA64PFR1_MTE_SHIFT, #4
	cmp	x10, #ID_AA64PFR1_MTE
	b.lt	1f

	/* Normal Tagged memory type at the corresponding MAIR index */
	mov	x10, #MAIR_ATTR_NORMAL_TAGGED
	bfi	mair, x10, #(8 *  MT_NORMAL_TAGGED), #8

	mov	x10, #KERNEL_GCR_EL1
	msr_s	SYS_GCR_EL1, x10

	/*
	 * If GCR_EL1.RRND=1 is implemented the same way as RRND=0, then
	 * RGSR_EL1.SEED must be non-zero for IRG to produce
	 * pseudorandom numbers. As RGSR_EL1 is UNKNOWN out of reset, we
	 * must initialize it.
	 */
	mrs	x10, CNTVCT_EL0
	ands	x10, x10, #SYS_RGSR_EL1_SEED_MASK
	csinc	x10, x10, xzr, ne
	lsl	x10, x10, #SYS_RGSR_EL1_SEED_SHIFT
	msr_s	SYS_RGSR_EL1, x10

	/* clear any pending tag check faults in TFSR*_EL1 */
	msr_s	SYS_TFSR_EL1, xzr
	msr_s	SYS_TFSRE0_EL1, xzr

	/* set the TCR_EL1 bits */
	mov_q	x10, TCR_MTE_FLAGS
	orr	tcr, tcr, x10
1:
#endif
	tcr_clear_errata_bits tcr, x9, x5                              // ---（2.5.7）

#ifdef CONFIG_ARM64_VA_BITS_52                                        //假设我使用的48bit
	ldr_l		x9, vabits_actual
	sub		x9, xzr, x9
	add		x9, x9, #64
	tcr_set_t1sz	tcr, x9
#else
	ldr_l		x9, idmap_t0sz                                 // ---（2.5.8-1）idmap_t0sz = TCR_T0SZ(VA_BITS_MIN) = 16                         
#endif
	tcr_set_t0sz	tcr, x9                                        // ---（2.5.8-2）tcr寄存器的T0SZ域

	/*
	 * Set the IPS bits in TCR_EL1.
	 */
	tcr_compute_pa_size tcr, #TCR_IPS_SHIFT, x5, x6                // ---（2.5.9）
#ifdef CONFIG_ARM64_HW_AFDBM                                           // ---（2.5.10）
	/*
	 * Enable hardware update of the Access Flags bit.
	 * Hardware dirty bit management is enabled later,
	 * via capabilities.
	 */
	mrs	x9, ID_AA64MMFR1_EL1
	and	x9, x9, #0xf
	cbz	x9, 1f
	orr	tcr, tcr, #TCR_HA		// hardware Access flag update
1:
#endif	/* CONFIG_ARM64_HW_AFDBM */
	msr	mair_el1, mair                  //Memory Attribute Indirection Register (EL1)真正的设置动作       
	msr	tcr_el1, tcr                    //Translation Control Register (EL1)真正的设置动作 
	/*
	 * Prepare SCTLR
	 */
	mov_q	x0, INIT_SCTLR_EL1_MMU_ON       // ---（2.5.11）
	ret					// return to head.S

	.unreq	mair                            //.unreq用来取消一个寄存器的别名