linux-brain/arch/arm64/kernel/head.S

/*
 * Low-level CPU initialisation
 * Based on arch/arm/kernel/head.S
 *
 * Copyright (C) 1994-2002 Russell King
 * Copyright (C) 2003-2012 ARM Ltd.
 * Authors:	Catalin Marinas <catalin.marinas@arm.com>
 *		Will Deacon <will.deacon@arm.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/linkage.h>
#include <linux/init.h>
#include <linux/irqchip/arm-gic-v3.h>

#include <asm/assembler.h>
#include <asm/ptrace.h>
#include <asm/asm-offsets.h>
#include <asm/cache.h>
#include <asm/cputype.h>
#include <asm/memory.h>
#include <asm/thread_info.h>
#include <asm/pgtable-hwdef.h>
#include <asm/pgtable.h>
#include <asm/page.h>
#include <asm/virt.h>

#define __PHYS_OFFSET	(KERNEL_START - TEXT_OFFSET)

#if (TEXT_OFFSET & 0xfff) != 0
#error TEXT_OFFSET must be at least 4KB aligned
#elif (PAGE_OFFSET & 0x1fffff) != 0
#error PAGE_OFFSET must be at least 2MB aligned
#elif TEXT_OFFSET > 0x1fffff
#error TEXT_OFFSET must be less than 2MB
#endif

#ifdef CONFIG_ARM64_64K_PAGES
#define BLOCK_SHIFT	PAGE_SHIFT
#define BLOCK_SIZE	PAGE_SIZE
#define TABLE_SHIFT	PMD_SHIFT
#else
#define BLOCK_SHIFT	SECTION_SHIFT
#define BLOCK_SIZE	SECTION_SIZE
#define TABLE_SHIFT	PUD_SHIFT
#endif

#define KERNEL_START	_text
#define KERNEL_END	_end

/*
 * Initial memory map attributes.
 */
#ifndef CONFIG_SMP
#define PTE_FLAGS	PTE_TYPE_PAGE | PTE_AF
#define PMD_FLAGS	PMD_TYPE_SECT | PMD_SECT_AF
#else
#define PTE_FLAGS	PTE_TYPE_PAGE | PTE_AF | PTE_SHARED
#define PMD_FLAGS	PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S
#endif

#ifdef CONFIG_ARM64_64K_PAGES
#define MM_MMUFLAGS	PTE_ATTRINDX(MT_NORMAL) | PTE_FLAGS
#else
#define MM_MMUFLAGS	PMD_ATTRINDX(MT_NORMAL) | PMD_FLAGS
#endif

/*
 * Kernel startup entry point.
 * ---------------------------
 *
 * The requirements are:
 *   MMU = off, D-cache = off, I-cache = on or off,
 *   x0 = physical address to the FDT blob.
 *
 * This code is mostly position independent so you call this at
 * __pa(PAGE_OFFSET + TEXT_OFFSET).
 *
 * Note that the callee-saved registers are used for storing variables
 * that are useful before the MMU is enabled. The allocations are described
 * in the entry routines.
 */
	__HEAD

	/*
	 * DO NOT MODIFY. Image header expected by Linux boot-loaders.
	 */
#ifdef CONFIG_EFI
efi_head:
	/*
	 * This add instruction has no meaningful effect except that
	 * its opcode forms the magic "MZ" signature required by UEFI.
	 */
	add	x13, x18, #0x16
	b	stext
#else
	b	stext				// branch to kernel start, magic
	.long	0				// reserved
#endif
	.quad	_kernel_offset_le		// Image load offset from start of RAM, little-endian
	.quad	_kernel_size_le			// Effective size of kernel image, little-endian
	.quad	_kernel_flags_le		// Informative flags, little-endian
	.quad	0				// reserved
	.quad	0				// reserved
	.quad	0				// reserved
	.byte	0x41				// Magic number, "ARM\x64"
	.byte	0x52
	.byte	0x4d
	.byte	0x64
#ifdef CONFIG_EFI
	.long	pe_header - efi_head		// Offset to the PE header.
#else
	.word	0				// reserved
#endif

#ifdef CONFIG_EFI
	.globl	stext_offset
	.set	stext_offset, stext - efi_head
	.align 3
pe_header:
	.ascii	"PE"
	.short 	0
coff_header:
	.short	0xaa64				// AArch64
	.short	2				// nr_sections
	.long	0 				// TimeDateStamp
	.long	0				// PointerToSymbolTable
	.long	1				// NumberOfSymbols
	.short	section_table - optional_header	// SizeOfOptionalHeader
	.short	0x206				// Characteristics.
						// IMAGE_FILE_DEBUG_STRIPPED |
						// IMAGE_FILE_EXECUTABLE_IMAGE |
						// IMAGE_FILE_LINE_NUMS_STRIPPED
optional_header:
	.short	0x20b				// PE32+ format
	.byte	0x02				// MajorLinkerVersion
	.byte	0x14				// MinorLinkerVersion
	.long	_end - stext			// SizeOfCode
	.long	0				// SizeOfInitializedData
	.long	0				// SizeOfUninitializedData
	.long	efi_stub_entry - efi_head	// AddressOfEntryPoint
	.long	stext_offset			// BaseOfCode

extra_header_fields:
	.quad	0				// ImageBase
	.long	0x1000				// SectionAlignment
	.long	PECOFF_FILE_ALIGNMENT		// FileAlignment
	.short	0				// MajorOperatingSystemVersion
	.short	0				// MinorOperatingSystemVersion
	.short	0				// MajorImageVersion
	.short	0				// MinorImageVersion
	.short	0				// MajorSubsystemVersion
	.short	0				// MinorSubsystemVersion
	.long	0				// Win32VersionValue

	.long	_end - efi_head			// SizeOfImage

	// Everything before the kernel image is considered part of the header
	.long	stext_offset			// SizeOfHeaders
	.long	0				// CheckSum
	.short	0xa				// Subsystem (EFI application)
	.short	0				// DllCharacteristics
	.quad	0				// SizeOfStackReserve
	.quad	0				// SizeOfStackCommit
	.quad	0				// SizeOfHeapReserve
	.quad	0				// SizeOfHeapCommit
	.long	0				// LoaderFlags
	.long	0x6				// NumberOfRvaAndSizes

	.quad	0				// ExportTable
	.quad	0				// ImportTable
	.quad	0				// ResourceTable
	.quad	0				// ExceptionTable
	.quad	0				// CertificationTable
	.quad	0				// BaseRelocationTable

	// Section table
section_table:

	/*
	 * The EFI application loader requires a relocation section
	 * because EFI applications must be relocatable.  This is a
	 * dummy section as far as we are concerned.
	 */
	.ascii	".reloc"
	.byte	0
	.byte	0			// end of 0 padding of section name
	.long	0
	.long	0
	.long	0			// SizeOfRawData
	.long	0			// PointerToRawData
	.long	0			// PointerToRelocations
	.long	0			// PointerToLineNumbers
	.short	0			// NumberOfRelocations
	.short	0			// NumberOfLineNumbers
	.long	0x42100040		// Characteristics (section flags)


	.ascii	".text"
	.byte	0
	.byte	0
	.byte	0        		// end of 0 padding of section name
	.long	_end - stext		// VirtualSize
	.long	stext_offset		// VirtualAddress
	.long	_edata - stext		// SizeOfRawData
	.long	stext_offset		// PointerToRawData

	.long	0		// PointerToRelocations (0 for executables)
	.long	0		// PointerToLineNumbers (0 for executables)
	.short	0		// NumberOfRelocations  (0 for executables)
	.short	0		// NumberOfLineNumbers  (0 for executables)
	.long	0xe0500020	// Characteristics (section flags)

	/*
	 * EFI will load stext onwards at the 4k section alignment
	 * described in the PE/COFF header. To ensure that instruction
	 * sequences using an adrp and a :lo12: immediate will function
	 * correctly at this alignment, we must ensure that stext is
	 * placed at a 4k boundary in the Image to begin with.
	 */
	.align 12
#endif

ENTRY(stext)
	bl	preserve_boot_args
	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
	adrp	x24, __PHYS_OFFSET
	bl	set_cpu_boot_mode_flag

	bl	__vet_fdt
	bl	__create_page_tables		// x25=TTBR0, x26=TTBR1
	/*
	 * The following calls CPU setup code, see arch/arm64/mm/proc.S for
	 * details.
	 * On return, the CPU will be ready for the MMU to be turned on and
	 * the TCR will have been set.
	 */
	ldr	x27, =__mmap_switched		// address to jump to after
						// MMU has been enabled
	adr_l	lr, __enable_mmu		// return (PIC) address
	b	__cpu_setup			// initialise processor
ENDPROC(stext)

/*
 * Preserve the arguments passed by the bootloader in x0 .. x3
 */
preserve_boot_args:
	mov	x21, x0				// x21=FDT

	adr_l	x0, boot_args			// record the contents of
	stp	x21, x1, [x0]			// x0 .. x3 at kernel entry
	stp	x2, x3, [x0, #16]

	dmb	sy				// needed before dc ivac with
						// MMU off

	add	x1, x0, #0x20			// 4 x 8 bytes
	b	__inval_cache_range		// tail call
ENDPROC(preserve_boot_args)

/*
 * Determine validity of the x21 FDT pointer.
 * The dtb must be 8-byte aligned and live in the first 512M of memory.
 */
__vet_fdt:
	tst	x21, #0x7
	b.ne	1f
	cmp	x21, x24
	b.lt	1f
	mov	x0, #(1 << 29)
	add	x0, x0, x24
	cmp	x21, x0
	b.ge	1f
	ret
1:
	mov	x21, #0
	ret
ENDPROC(__vet_fdt)
/*
 * Macro to create a table entry to the next page.
 *
 *	tbl:	page table address
 *	virt:	virtual address
 *	shift:	#imm page table shift
 *	ptrs:	#imm pointers per table page
 *
 * Preserves:	virt
 * Corrupts:	tmp1, tmp2
 * Returns:	tbl -> next level table page address
 */
	.macro	create_table_entry, tbl, virt, shift, ptrs, tmp1, tmp2
	lsr	\tmp1, \virt, #\shift
	and	\tmp1, \tmp1, #\ptrs - 1	// table index
	add	\tmp2, \tbl, #PAGE_SIZE
	orr	\tmp2, \tmp2, #PMD_TYPE_TABLE	// address of next table and entry type
	str	\tmp2, [\tbl, \tmp1, lsl #3]
	add	\tbl, \tbl, #PAGE_SIZE		// next level table page
	.endm

/*
 * Macro to populate the PGD (and possibily PUD) for the corresponding
 * block entry in the next level (tbl) for the given virtual address.
 *
 * Preserves:	tbl, next, virt
 * Corrupts:	tmp1, tmp2
 */
	.macro	create_pgd_entry, tbl, virt, tmp1, tmp2
	create_table_entry \tbl, \virt, PGDIR_SHIFT, PTRS_PER_PGD, \tmp1, \tmp2
#if SWAPPER_PGTABLE_LEVELS == 3
	create_table_entry \tbl, \virt, TABLE_SHIFT, PTRS_PER_PTE, \tmp1, \tmp2
#endif
	.endm

/*
 * Macro to populate block entries in the page table for the start..end
 * virtual range (inclusive).
 *
 * Preserves:	tbl, flags
 * Corrupts:	phys, start, end, pstate
 */
	.macro	create_block_map, tbl, flags, phys, start, end
	lsr	\phys, \phys, #BLOCK_SHIFT
	lsr	\start, \start, #BLOCK_SHIFT
	and	\start, \start, #PTRS_PER_PTE - 1	// table index
	orr	\phys, \flags, \phys, lsl #BLOCK_SHIFT	// table entry
	lsr	\end, \end, #BLOCK_SHIFT
	and	\end, \end, #PTRS_PER_PTE - 1		// table end index
9999:	str	\phys, [\tbl, \start, lsl #3]		// store the entry
	add	\start, \start, #1			// next entry
	add	\phys, \phys, #BLOCK_SIZE		// next block
	cmp	\start, \end
	b.ls	9999b
	.endm

/*
 * Setup the initial page tables. We only setup the barest amount which is
 * required to get the kernel running. The following sections are required:
 *   - identity mapping to enable the MMU (low address, TTBR0)
 *   - first few MB of the kernel linear mapping to jump to once the MMU has
 *     been enabled, including the FDT blob (TTBR1)
 *   - pgd entry for fixed mappings (TTBR1)
 */
__create_page_tables:
	adrp	x25, idmap_pg_dir
	adrp	x26, swapper_pg_dir
	mov	x27, lr

	/*
	 * Invalidate the idmap and swapper page tables to avoid potential
	 * dirty cache lines being evicted.
	 */
	mov	x0, x25
	add	x1, x26, #SWAPPER_DIR_SIZE
	bl	__inval_cache_range

	/*
	 * Clear the idmap and swapper page tables.
	 */
	mov	x0, x25
	add	x6, x26, #SWAPPER_DIR_SIZE
1:	stp	xzr, xzr, [x0], #16
	stp	xzr, xzr, [x0], #16
	stp	xzr, xzr, [x0], #16
	stp	xzr, xzr, [x0], #16
	cmp	x0, x6
	b.lo	1b

	ldr	x7, =MM_MMUFLAGS

	/*
	 * Create the identity mapping.
	 */
	mov	x0, x25				// idmap_pg_dir
	adrp	x3, KERNEL_START		// __pa(KERNEL_START)

#ifndef CONFIG_ARM64_VA_BITS_48
#define EXTRA_SHIFT	(PGDIR_SHIFT + PAGE_SHIFT - 3)
#define EXTRA_PTRS	(1 << (48 - EXTRA_SHIFT))

	/*
	 * If VA_BITS < 48, it may be too small to allow for an ID mapping to be
	 * created that covers system RAM if that is located sufficiently high
	 * in the physical address space. So for the ID map, use an extended
	 * virtual range in that case, by configuring an additional translation
	 * level.
	 * First, we have to verify our assumption that the current value of
	 * VA_BITS was chosen such that all translation levels are fully
	 * utilised, and that lowering T0SZ will always result in an additional
	 * translation level to be configured.
	 */
#if VA_BITS != EXTRA_SHIFT
#error "Mismatch between VA_BITS and page size/number of translation levels"
#endif

	/*
	 * Calculate the maximum allowed value for TCR_EL1.T0SZ so that the
	 * entire kernel image can be ID mapped. As T0SZ == (64 - #bits used),
	 * this number conveniently equals the number of leading zeroes in
	 * the physical address of KERNEL_END.
	 */
	adrp	x5, KERNEL_END
	clz	x5, x5
	cmp	x5, TCR_T0SZ(VA_BITS)	// default T0SZ small enough?
	b.ge	1f			// .. then skip additional level

	str_l	x5, idmap_t0sz, x6

	create_table_entry x0, x3, EXTRA_SHIFT, EXTRA_PTRS, x5, x6
1:
#endif

	create_pgd_entry x0, x3, x5, x6
	mov	x5, x3				// __pa(KERNEL_START)
	adr_l	x6, KERNEL_END			// __pa(KERNEL_END)
	create_block_map x0, x7, x3, x5, x6

	/*
	 * Map the kernel image (starting with PHYS_OFFSET).
	 */
	mov	x0, x26				// swapper_pg_dir
	mov	x5, #PAGE_OFFSET
	create_pgd_entry x0, x5, x3, x6
	ldr	x6, =KERNEL_END			// __va(KERNEL_END)
	mov	x3, x24				// phys offset
	create_block_map x0, x7, x3, x5, x6

	/*
	 * Map the FDT blob (maximum 2MB; must be within 512MB of
	 * PHYS_OFFSET).
	 */
	mov	x3, x21				// FDT phys address
	and	x3, x3, #~((1 << 21) - 1)	// 2MB aligned
	mov	x6, #PAGE_OFFSET
	sub	x5, x3, x24			// subtract PHYS_OFFSET
	tst	x5, #~((1 << 29) - 1)		// within 512MB?
	csel	x21, xzr, x21, ne		// zero the FDT pointer
	b.ne	1f
	add	x5, x5, x6			// __va(FDT blob)
	add	x6, x5, #1 << 21		// 2MB for the FDT blob
	sub	x6, x6, #1			// inclusive range
	create_block_map x0, x7, x3, x5, x6
1:
	/*
	 * Since the page tables have been populated with non-cacheable
	 * accesses (MMU disabled), invalidate the idmap and swapper page
	 * tables again to remove any speculatively loaded cache lines.
	 */
	mov	x0, x25
	add	x1, x26, #SWAPPER_DIR_SIZE
	dmb	sy
	bl	__inval_cache_range

	mov	lr, x27
	ret
ENDPROC(__create_page_tables)
	.ltorg

/*
 * The following fragment of code is executed with the MMU enabled.
 */
	.set	initial_sp, init_thread_union + THREAD_START_SP
__mmap_switched:
	adr_l	x6, __bss_start
	adr_l	x7, __bss_stop

1:	cmp	x6, x7
	b.hs	2f
	str	xzr, [x6], #8			// Clear BSS
	b	1b
2:
	adr_l	sp, initial_sp, x4
	str_l	x21, __fdt_pointer, x5		// Save FDT pointer
	str_l	x24, memstart_addr, x6		// Save PHYS_OFFSET
	mov	x29, #0
	b	start_kernel
ENDPROC(__mmap_switched)

/*
 * end early head section, begin head code that is also used for
 * hotplug and needs to have the same protections as the text region
 */
	.section ".text","ax"
/*
 * If we're fortunate enough to boot at EL2, ensure that the world is
 * sane before dropping to EL1.
 *
 * Returns either BOOT_CPU_MODE_EL1 or BOOT_CPU_MODE_EL2 in x20 if
 * booted in EL1 or EL2 respectively.
 */
ENTRY(el2_setup)
	mrs	x0, CurrentEL
	cmp	x0, #CurrentEL_EL2
	b.ne	1f
	mrs	x0, sctlr_el2
CPU_BE(	orr	x0, x0, #(1 << 25)	)	// Set the EE bit for EL2
CPU_LE(	bic	x0, x0, #(1 << 25)	)	// Clear the EE bit for EL2
	msr	sctlr_el2, x0
	b	2f
1:	mrs	x0, sctlr_el1
CPU_BE(	orr	x0, x0, #(3 << 24)	)	// Set the EE and E0E bits for EL1
CPU_LE(	bic	x0, x0, #(3 << 24)	)	// Clear the EE and E0E bits for EL1
	msr	sctlr_el1, x0
	mov	w20, #BOOT_CPU_MODE_EL1		// This cpu booted in EL1
	isb
	ret

	/* Hyp configuration. */
2:	mov	x0, #(1 << 31)			// 64-bit EL1
	msr	hcr_el2, x0

	/* Generic timers. */
	mrs	x0, cnthctl_el2
	orr	x0, x0, #3			// Enable EL1 physical timers
	msr	cnthctl_el2, x0
	msr	cntvoff_el2, xzr		// Clear virtual offset

#ifdef CONFIG_ARM_GIC_V3
	/* GICv3 system register access */
	mrs	x0, id_aa64pfr0_el1
	ubfx	x0, x0, #24, #4
	cmp	x0, #1
	b.ne	3f

	mrs_s	x0, ICC_SRE_EL2
	orr	x0, x0, #ICC_SRE_EL2_SRE	// Set ICC_SRE_EL2.SRE==1
	orr	x0, x0, #ICC_SRE_EL2_ENABLE	// Set ICC_SRE_EL2.Enable==1
	msr_s	ICC_SRE_EL2, x0
	isb					// Make sure SRE is now set
	msr_s	ICH_HCR_EL2, xzr		// Reset ICC_HCR_EL2 to defaults

3:
#endif

	/* Populate ID registers. */
	mrs	x0, midr_el1
	mrs	x1, mpidr_el1
	msr	vpidr_el2, x0
	msr	vmpidr_el2, x1

	/* sctlr_el1 */
	mov	x0, #0x0800			// Set/clear RES{1,0} bits
CPU_BE(	movk	x0, #0x33d0, lsl #16	)	// Set EE and E0E on BE systems
CPU_LE(	movk	x0, #0x30d0, lsl #16	)	// Clear EE and E0E on LE systems
	msr	sctlr_el1, x0

	/* Coprocessor traps. */
	mov	x0, #0x33ff
	msr	cptr_el2, x0			// Disable copro. traps to EL2

#ifdef CONFIG_COMPAT
	msr	hstr_el2, xzr			// Disable CP15 traps to EL2
#endif

	/* Stage-2 translation */
	msr	vttbr_el2, xzr

	/* Hypervisor stub */
	adrp	x0, __hyp_stub_vectors
	add	x0, x0, #:lo12:__hyp_stub_vectors
	msr	vbar_el2, x0

	/* spsr */
	mov	x0, #(PSR_F_BIT | PSR_I_BIT | PSR_A_BIT | PSR_D_BIT |\
		      PSR_MODE_EL1h)
	msr	spsr_el2, x0
	msr	elr_el2, lr
	mov	w20, #BOOT_CPU_MODE_EL2		// This CPU booted in EL2
	eret
ENDPROC(el2_setup)

/*
 * Sets the __boot_cpu_mode flag depending on the CPU boot mode passed
 * in x20. See arch/arm64/include/asm/virt.h for more info.
 */
ENTRY(set_cpu_boot_mode_flag)
	adr_l	x1, __boot_cpu_mode
	cmp	w20, #BOOT_CPU_MODE_EL2
	b.ne	1f
	add	x1, x1, #4
1:	str	w20, [x1]			// This CPU has booted in EL1
	dmb	sy
	dc	ivac, x1			// Invalidate potentially stale cache line
	ret
ENDPROC(set_cpu_boot_mode_flag)

/*
 * We need to find out the CPU boot mode long after boot, so we need to
 * store it in a writable variable.
 *
 * This is not in .bss, because we set it sufficiently early that the boot-time
 * zeroing of .bss would clobber it.
 */
	.pushsection	.data..cacheline_aligned
	.align	L1_CACHE_SHIFT
ENTRY(__boot_cpu_mode)
	.long	BOOT_CPU_MODE_EL2
	.long	BOOT_CPU_MODE_EL1
	.popsection

#ifdef CONFIG_SMP
	/*
	 * This provides a "holding pen" for platforms to hold all secondary
	 * cores are held until we're ready for them to initialise.
	 */
ENTRY(secondary_holding_pen)
	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
	bl	set_cpu_boot_mode_flag
	mrs	x0, mpidr_el1
	ldr     x1, =MPIDR_HWID_BITMASK
	and	x0, x0, x1
	adr_l	x3, secondary_holding_pen_release
pen:	ldr	x4, [x3]
	cmp	x4, x0
	b.eq	secondary_startup
	wfe
	b	pen
ENDPROC(secondary_holding_pen)

	/*
	 * Secondary entry point that jumps straight into the kernel. Only to
	 * be used where CPUs are brought online dynamically by the kernel.
	 */
ENTRY(secondary_entry)
	bl	el2_setup			// Drop to EL1
	bl	set_cpu_boot_mode_flag
	b	secondary_startup
ENDPROC(secondary_entry)

ENTRY(secondary_startup)
	/*
	 * Common entry point for secondary CPUs.
	 */
	adrp	x25, idmap_pg_dir
	adrp	x26, swapper_pg_dir
	bl	__cpu_setup			// initialise processor

	ldr	x21, =secondary_data
	ldr	x27, =__secondary_switched	// address to jump to after enabling the MMU
	b	__enable_mmu
ENDPROC(secondary_startup)

ENTRY(__secondary_switched)
	ldr	x0, [x21]			// get secondary_data.stack
	mov	sp, x0
	mov	x29, #0
	b	secondary_start_kernel
ENDPROC(__secondary_switched)
#endif	/* CONFIG_SMP */

/*
 * Enable the MMU.
 *
 *  x0  = SCTLR_EL1 value for turning on the MMU.
 *  x27 = *virtual* address to jump to upon completion
 *
 * other registers depend on the function called upon completion
 */
__enable_mmu:
	ldr	x5, =vectors
	msr	vbar_el1, x5
	msr	ttbr0_el1, x25			// load TTBR0
	msr	ttbr1_el1, x26			// load TTBR1
	isb
	msr	sctlr_el1, x0
	isb
	br	x27
ENDPROC(__enable_mmu)