zircon 启动代码

本文链接： https://blog.csdn.net/tiantao2012/article/details/93329106
zircon 是fuchsia的内核，其base 版本是little kernel.
zircon目前支持x86和arm64 两种架构，其arm64架构的入口地址为
zircon-master\kernel\arch\arm64\start.S
其入口函数如下：
FUNCTION(_start)
    /* Save the Boot info for the primary CPU only */
    mrs     cpuid, mpidr_el1
    ubfx    cpuid, cpuid, #0, #15 /* mask Aff0 and Aff1 fields */
    cbnz    cpuid, .Lno_save_bootinfo
    /* save x0 in zbi_paddr */
    adrp    tmp, zbi_paddr
    str     x0, [tmp, #:lo12:zbi_paddr]
    /* save entry point physical address in kernel_entry_paddr */
    adrp    tmp, kernel_entry_paddr
    adr     tmp2, _start
    str     tmp2, [tmp, #:lo12:kernel_entry_paddr]
    adrp    tmp2, arch_boot_el
    mrs     x2, CurrentEL
    str     x2, [tmp2, #:lo12:arch_boot_el]
.Lno_save_bootinfo:

    bl      arm64_elX_to_el1
    bl      arch_invalidate_cache_all
#使能cache从而让atomics和spinlocks工作
    /* enable caches so atomics and spinlocks work */
    mrs     tmp, sctlr_el1
    orr     tmp, tmp, #(1<<12) /* Enable icache */
    orr     tmp, tmp, #(1<<2)  /* Enable dcache/ucache */
    msr     sctlr_el1, tmp
从这段代码可以知道所有cpu在zircon中的入口函数都是_start这个函数，如果是cpu o的话，需要保存BootLoader
传递过来的boot info，而其他cpu 则不用保存bootinfo，所以这里有个跳转口.Lno_save_bootinfo。如果是非0 cpu的话
则直接跳到no_save_bootinfo执行
.Lmmu_on_vaddr:
    /* Disable trampoline page-table in ttbr0 */
    movlit  tmp, MMU_TCR_FLAGS_KERNEL
    msr     tcr_el1, tmp
    isb

    /* Invalidate TLB */
    tlbi    vmalle1
    isb
#通过cpuid来判断释放是cpu 0，如果是cpu 0的话，则继续往下执行，如果是非cpu 0，则跳到secondary_boot 开始执行
    cbnz    cpuid, .Lsecondary_boot

    // set up the boot stack for real
    adr_global tmp, boot_cpu_kstack_end
    mov     sp, tmp

    // Set the thread pointer early so compiler-generated references
    // to the stack-guard and unsafe-sp slots work.  This is not a
    // real 'struct thread' yet, just a pointer to (past, actually)
    // the two slots used by the ABI known to the compiler.  This avoids
    // having to compile-time disable safe-stack and stack-protector
    // code generation features for all the C code in the bootstrap
    // path, which (unlike on x86, e.g.) is enough to get annoying.
    adr_global tmp, boot_cpu_fake_thread_pointer_location
    msr     tpidr_el1, tmp

    // set the per cpu pointer for cpu 0
    adr_global x18, arm64_percpu_array

    // Choose a good (ideally random) stack-guard value as early as possible.
    bl      choose_stack_guard
    mrs     tmp, tpidr_el1
    str     x0, [tmp, #ZX_TLS_STACK_GUARD_OFFSET]
    // Don't leak the value to other code.
    mov     x0, xzr
#开始进入c函数
    bl  lk_main
    b   .
#非bootcpu 从这里执行，最终跳到arm64_secondary_entry

.Lsecondary_boot:
    bl      arm64_get_secondary_sp
    cbz     x0, .Lunsupported_cpu_trap
    mov     sp, x0
    msr     tpidr_el1, x1

    bl      arm64_secondary_entry

.Lunsupported_cpu_trap:
    wfe
    b       .Lunsupported_cpu_trap
END_FUNCTION(_start)
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