linux 内核启动 arm64 汇编 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 <[e-mail]>
 * Will Deacon <[e-mail]>
 *
 * 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 <asm/assembler.h>
#include <asm/ptrace.h>
#include <asm/asm-offsets.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>


/*
 * swapper_pg_dir is the virtual address of the initial page table. We place
 * the page tables 3 * PAGE_SIZE below KERNEL_RAM_VADDR. The idmap_pg_dir has
 * 2 pages and is placed below swapper_pg_dir.
 */
#define KERNEL_RAM_VADDR (PAGE_OFFSET + TEXT_OFFSET)


#if (KERNEL_RAM_VADDR & 0xfffff) != 0x80000
#error KERNEL_RAM_VADDR must start at 0xXXX80000
#endif


#define SWAPPER_DIR_SIZE (3 * PAGE_SIZE)
#define IDMAP_DIR_SIZE (2 * PAGE_SIZE)


.globl swapper_pg_dir
.equ swapper_pg_dir, KERNEL_RAM_VADDR - SWAPPER_DIR_SIZE


.globl idmap_pg_dir
.equ idmap_pg_dir, swapper_pg_dir - IDMAP_DIR_SIZE


.macro pgtbl, ttb0, ttb1, phys
add \ttb1, \phys, #TEXT_OFFSET - SWAPPER_DIR_SIZE
sub \ttb0, \ttb1, #IDMAP_DIR_SIZE
.endm


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


#define KERNEL_START KERNEL_RAM_VADDR
#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.
*/
b stext// branch to kernel start, magic
.long 0// reserved
.quad TEXT_OFFSET// Image load offset from start of RAM
.quad 0// reserved
.quad 0// reserved


ENTRY(stext)
mov x21, x0// x21=FDT
bl __calc_phys_offset// x24=PHYS_OFFSET, x28=PHYS_OFFSET-PAGE_OFFSET
bl el2_setup// Drop to EL1
mrs x22, midr_el1// x22=cpuid
mov x0, x22
bl lookup_processor_type
mov x23, x0// x23=current cpu_table
cbz x23, __error_p// invalid processor (x23=0)?
bl __vet_fdt
bl __create_page_tables// x25=TTBR0, x26=TTBR1
/*
* The following calls CPU specific code in a position independent
* manner. See arch/arm64/mm/proc.S for details. x23 = base of
* cpu_info structure selected by lookup_processor_type above.
* On return, the CPU will be ready for the MMU to be turned on and
* the TCR will have been set.
*/
ldr x27, __switch_data// address to jump to after
// MMU has been enabled
adr lr, __enable_mmu// return (PIC) address
ldr x12, [x23, #CPU_INFO_SETUP]
add x12, x12, x28// __virt_to_phys
br x12// initialise processor
ENDPROC(stext)


/*
 * If we're fortunate enough to boot at EL2, ensure that the world is
 * sane before dropping to EL1.
 */
ENTRY(el2_setup)
mrs x0, CurrentEL
cmp x0, #PSR_MODE_EL2t
ccmp x0, #PSR_MODE_EL2h, #0x4, ne
ldr x0, =__boot_cpu_mode// Compute __boot_cpu_mode
add x0, x0, x28
b.eq 1f
str wzr, [x0]// Remember we don't have EL2...
ret


/* Hyp configuration. */
1: ldr w1, =BOOT_CPU_MODE_EL2
str w1, [x0, #4]// This CPU has EL2
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


/* 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
movk x0, #0x30d0, lsl #16
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 */
adr x0, __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
eret
ENDPROC(el2_setup)


/*
 * 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
ENTRY(__boot_cpu_mode)
.long BOOT_CPU_MODE_EL2
.long 0
.popsection


.align 3
2: .quad .
.quad PAGE_OFFSET


#ifdef CONFIG_SMP
.pushsection    .smp.pen.text, "ax"
.align 3
1: .quad .
.quad secondary_holding_pen_release


/*
* 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 __calc_phys_offset// x24=phys offset
bl el2_setup// Drop to EL1
mrs x0, mpidr_el1
ldr     x1, =MPIDR_HWID_BITMASK
and x0, x0, x1
adr x1, 1b
ldp x2, x3, [x1]
sub x1, x1, x2
add x3, x3, x1
pen: ldr x4, [x3]
cmp x4, x0
b.eq secondary_startup
wfe
b pen
ENDPROC(secondary_holding_pen)
.popsection


ENTRY(secondary_startup)
/*
* Common entry point for secondary CPUs.
*/
mrs x22, midr_el1// x22=cpuid
mov x0, x22
bl lookup_processor_type
mov x23, x0// x23=current cpu_table
cbz x23, __error_p// invalid processor (x23=0)?


pgtbl x25, x26, x24// x25=TTBR0, x26=TTBR1
ldr x12, [x23, #CPU_INFO_SETUP]
add x12, x12, x28// __virt_to_phys
blr x12// 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 */


/*
 * Setup common bits before finally enabling the MMU. Essentially this is just
 * loading the page table pointer and vector base registers.
 *
 * On entry to this code, x0 must contain the SCTLR_EL1 value for turning on
 * the MMU.
 */
__enable_mmu:
ldr x5, =vectors
msr vbar_el1, x5
msr ttbr0_el1, x25// load TTBR0
msr ttbr1_el1, x26// load TTBR1
isb
b __turn_mmu_on
ENDPROC(__enable_mmu)


/*
 * Enable the MMU. This completely changes the structure of the visible memory
 * space. You will not be able to trace execution through this.
 *
 *  x0  = system control register
 *  x27 = *virtual* address to jump to upon completion
 *
 * other registers depend on the function called upon completion
 */
.align 6
__turn_mmu_on:
msr sctlr_el1, x0
isb
br x27
ENDPROC(__turn_mmu_on)


/*
 * Calculate the start of physical memory.
 */
__calc_phys_offset:
adr x0, 1f
ldp x1, x2, [x0]
sub x28, x0, x1// x28 = PHYS_OFFSET - PAGE_OFFSET
add x24, x2, x28// x24 = PHYS_OFFSET
ret
ENDPROC(__calc_phys_offset)


.align 3
1: .quad .
.quad PAGE_OFFSET


/*
 * Macro to populate the PGD for the corresponding block entry in the next
 * level (tbl) for the given virtual address.
 *
 * Preserves: pgd, tbl, virt
 * Corrupts: tmp1, tmp2
 */
.macro create_pgd_entry, pgd, tbl, virt, tmp1, tmp2
lsr \tmp1, \virt, #PGDIR_SHIFT
and \tmp1, \tmp1, #PTRS_PER_PGD - 1// PGD index
orr \tmp2, \tbl, #3// PGD entry table type
str \tmp2, [\pgd, \tmp1, lsl #3]
.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, idmap=0
lsr \phys, \phys, #BLOCK_SHIFT
.if \idmap
and \start, \phys, #PTRS_PER_PTE - 1// table index
.else
lsr \start, \start, #BLOCK_SHIFT
and \start, \start, #PTRS_PER_PTE - 1// table index
.endif
orr \phys, \flags, \phys, lsl #BLOCK_SHIFT// table entry
.ifnc \start,\end
lsr \end, \end, #BLOCK_SHIFT
and \end, \end, #PTRS_PER_PTE - 1// table end index
.endif
9999: str \phys, [\tbl, \start, lsl #3] // store the entry
.ifnc \start,\end
add \start, \start, #1// next entry
add \phys, \phys, #BLOCK_SIZE// next block
cmp \start, \end
b.ls 9999b
.endif
.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)
 *   - UART mapping if CONFIG_EARLY_PRINTK is enabled (TTBR1)
 */
__create_page_tables:
pgtbl x25, x26, x24// idmap_pg_dir and swapper_pg_dir addresses


/*
* 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.
*/
add x0, x25, #PAGE_SIZE// section table address
adr x3, __turn_mmu_on// virtual/physical address
create_pgd_entry x25, x0, x3, x5, x6
create_block_map x0, x7, x3, x5, x5, idmap=1


/*
* Map the kernel image (starting with PHYS_OFFSET).
*/
add x0, x26, #PAGE_SIZE// section table address
mov x5, #PAGE_OFFSET
create_pgd_entry x26, x0, x5, x3, x6
ldr x6, =KERNEL_END - 1
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:
#ifdef CONFIG_EARLY_PRINTK
/*
* Create the pgd entry for the UART mapping. The full mapping is done
* later based earlyprintk kernel parameter.
*/
ldr x5, =EARLYCON_IOBASE// UART virtual address
add x0, x26, #2 * PAGE_SIZE// section table address
create_pgd_entry x26, x0, x5, x6, x7
#endif
ret
ENDPROC(__create_page_tables)
.ltorg


.align 3
.type __switch_data, %object
__switch_data:
.quad __mmap_switched
.quad __data_loc// x4
.quad _data// x5
.quad __bss_start// x6
.quad _end// x7
.quad processor_id// x4
.quad __fdt_pointer// x5
.quad memstart_addr// x6
.quad init_thread_union + THREAD_START_SP // sp


/*
 * The following fragment of code is executed with the MMU on in MMU mode, and
 * uses absolute addresses; this is not position independent.
 */
__mmap_switched:
adr x3, __switch_data + 8


ldp x4, x5, [x3], #16
ldp x6, x7, [x3], #16
cmp x4, x5// Copy data segment if needed
1: ccmp x5, x6, #4, ne
b.eq 2f
ldr x16, [x4], #8
str x16, [x5], #8
b 1b
2:
1: cmp x6, x7
b.hs 2f
str xzr, [x6], #8// Clear BSS
b 1b
2:
ldp x4, x5, [x3], #16
ldr x6, [x3], #8
ldr x16, [x3]
mov sp, x16
str x22, [x4]// Save processor ID
str x21, [x5]// Save FDT pointer
str x24, [x6]// Save PHYS_OFFSET
mov x29, #0
b start_kernel
ENDPROC(__mmap_switched)


/*
 * Exception handling. Something went wrong and we can't proceed. We ought to
 * tell the user, but since we don't have any guarantee that we're even
 * running on the right architecture, we do virtually nothing.
 */
__error_p:
ENDPROC(__error_p)


__error:
1: nop
b 1b
ENDPROC(__error)


/*
 * This function gets the processor ID in w0 and searches the cpu_table[] for
 * a match. It returns a pointer to the struct cpu_info it found. The
 * cpu_table[] must end with an empty (all zeros) structure.
 *
 * This routine can be called via C code and it needs to work with the MMU
 * both disabled and enabled (the offset is calculated automatically).
 */
ENTRY(lookup_processor_type)
adr x1, __lookup_processor_type_data
ldp x2, x3, [x1]
sub x1, x1, x2// get offset between VA and PA
add x3, x3, x1// convert VA to PA
1:
ldp w5, w6, [x3]// load cpu_id_val and cpu_id_mask
cbz w5, 2f// end of list?
and w6, w6, w0
cmp w5, w6
b.eq 3f
add x3, x3, #CPU_INFO_SZ
b 1b
2:
mov x3, #0// unknown processor
3:
mov x0, x3
ret
ENDPROC(lookup_processor_type)


.align 3
.type __lookup_processor_type_data, %object
__lookup_processor_type_data:
.quad .
.quad cpu_table
.size __lookup_processor_type_data, . - __lookup_processor_type_data


/*
 * 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)