在entry.S中，定義了異常向量表，從代碼中我們可以知道以下信息：

• 該表的基地址在vectors處(在開機的時候，會將其寫入到vbar_el1中)
• 這個表以".align 11"方式對齊，即這張表至少占據2^11 size
• kernel_entry是一個用于定義向量表offset的宏，在kernel_entry中，是以“.align 7”方式對齊的，即2^8=0x80方式對齊，與ARMV8中的向量表offset一致
• armv8的向量表中又16個offset，在Linux Kernel中，僅實現了6個
• Linux Kernel中沒有實現 FIQ

(linux_5.9/arch/arm64/kernel/entry.S)/** Exception vectors.*/.pushsection ".entry.text", "ax".align 11 SYM_CODE_START(vectors)kernel_ventry 1, sync_invalid // Synchronous EL1tkernel_ventry 1, irq_invalid // IRQ EL1tkernel_ventry 1, fiq_invalid // FIQ EL1tkernel_ventry 1, error_invalid // Error EL1tkernel_ventry 1, sync // Synchronous EL1hkernel_ventry 1, irq // IRQ EL1hkernel_ventry 1, fiq_invalid // FIQ EL1hkernel_ventry 1, error // Error EL1hkernel_ventry 0, sync // Synchronous 64-bit EL0kernel_ventry 0, irq // IRQ 64-bit EL0kernel_ventry 0, fiq_invalid // FIQ 64-bit EL0kernel_ventry 0, error // Error 64-bit EL0#ifdef CONFIG_COMPATkernel_ventry 0, sync_compat, 32 // Synchronous 32-bit EL0kernel_ventry 0, irq_compat, 32 // IRQ 32-bit EL0kernel_ventry 0, fiq_invalid_compat, 32 // FIQ 32-bit EL0kernel_ventry 0, error_compat, 32 // Error 32-bit EL0 #elsekernel_ventry 0, sync_invalid, 32 // Synchronous 32-bit EL0kernel_ventry 0, irq_invalid, 32 // IRQ 32-bit EL0kernel_ventry 0, fiq_invalid, 32 // FIQ 32-bit EL0kernel_ventry 0, error_invalid, 32 // Error 32-bit EL0 #endif SYM_CODE_END(vectors)

我們再來看看kernel_ventry宏，它是用于定義向量表offset的宏，，從代碼中我們可以知道以下信息：

• kernel_ventry定義的offset以“.align 7”方式對齊的，即2^8=0x80方式對齊，與ARMV8中的向量表offset一致
• 該宏定義最終調用了“b el()\el()_\label”，即：
  – kernel_ventry 1, sync //調用了 el1_sync
  – kernel_ventry 1, irq //調用了 el1_irq
  – kernel_ventry 1, error //調用了 el1_error
  – kernel_ventry 0, sync //調用了 el0_sync
  – kernel_ventry 0, irq //調用了 el0_irq
  – kernel_ventry 0, error //調用了 el0_error

(linux_5.9/arch/arm64/kernel/entry.S).macro kernel_ventry, el, label, regsize = 64.align 7 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0.if \el == 0 alternative_if ARM64_UNMAP_KERNEL_AT_EL0.if \regsize == 64mrs x30, tpidrro_el0msr tpidrro_el0, xzr.elsemov x30, xzr.endif alternative_else_nop_endif.endif #endifsub sp, sp, #S_FRAME_SIZE #ifdef CONFIG_VMAP_STACK/** Test whether the SP has overflowed, without corrupting a GPR.* Task and IRQ stacks are aligned so that SP & (1 << THREAD_SHIFT)* should always be zero.*/add sp, sp, x0 // sp' = sp + x0sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sptbnz x0, #THREAD_SHIFT, 0fsub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = spb el\()\el\()_\label0:/** Either we've just detected an overflow, or we've taken an exception* while on the overflow stack. Either way, we won't return to* userspace, and can clobber EL0 registers to free up GPRs.*//* Stash the original SP (minus S_FRAME_SIZE) in tpidr_el0. */msr tpidr_el0, x0/* Recover the original x0 value and stash it in tpidrro_el0 */sub x0, sp, x0msr tpidrro_el0, x0/* Switch to the overflow stack */adr_this_cpu sp, overflow_stack + OVERFLOW_STACK_SIZE, x0/** Check whether we were already on the overflow stack. This may happen* after panic() re-enables interrupts.*/mrs x0, tpidr_el0 // sp of interrupted contextsub x0, sp, x0 // delta with top of overflow stacktst x0, #~(OVERFLOW_STACK_SIZE - 1) // within range?b.ne __bad_stack // no? -> bad stack pointer/* We were already on the overflow stack. Restore sp/x0 and carry on. */sub sp, sp, x0mrs x0, tpidrro_el0#endifb el\()\el\()_\label.endm

在__primary_switched函數中，將向量表的基地址vectors寫入到了vbar_el1中.

(linux_5.9/arch/arm64/kernel/entry.S)SYM_FUNC_START_LOCAL(__primary_switched)adrp x4, init_thread_unionadd sp, x4, #THREAD_SIZEadr_l x5, init_taskmsr sp_el0, x5 // Save thread_info#ifdef CONFIG_ARM64_PTR_AUTH__ptrauth_keys_init_cpu x5, x6, x7, x8 #endifadr_l x8, vectors // load VBAR_EL1 with virtualmsr vbar_el1, x8 // vector table addressisbstp xzr, x30, [sp, #-16]!mov x29, sp#ifdef CONFIG_SHADOW_CALL_STACKadr_l scs_sp, init_shadow_call_stack // Set shadow call stack #endifstr_l x21, __fdt_pointer, x5 // Save FDT pointerldr_l x4, kimage_vaddr // Save the offset betweensub x4, x4, x0 // the kernel virtual andstr_l x4, kimage_voffset, x5 // physical mappings// Clear BSSadr_l x0, __bss_startmov x1, xzradr_l x2, __bss_stopsub x2, x2, x0bl __pi_memsetdsb ishst // Make zero page visible to PTW#ifdef CONFIG_KASANbl kasan_early_init #endif #ifdef CONFIG_RANDOMIZE_BASEtst x23, ~(MIN_KIMG_ALIGN - 1) // already running randomized?b.ne 0fmov x0, x21 // pass FDT address in x0bl kaslr_early_init // parse FDT for KASLR optionscbz x0, 0f // KASLR disabled? just proceedorr x23, x23, x0 // record KASLR offsetldp x29, x30, [sp], #16 // we must enable KASLR, returnret // to __primary_switch() 0: #endifadd sp, sp, #16mov x29, #0mov x30, #0b start_kernel SYM_FUNC_END(__primary_switched)

而在開機時的在__primary_switched函數中被調用的流程如下所示

_head —> primary_entry —> __primary_switch —> __primary_switched —> 將向量表基地址vectors寫入到vbar_el1寄存器

相關代碼：

(linux_5.9/arch/arm64/kernel/head.S)SYM_CODE_START(primary_entry)bl preserve_boot_argsbl el2_setup // Drop to EL1, w0=cpu_boot_modeadrp x23, __PHYS_OFFSETand x23, x23, MIN_KIMG_ALIGN - 1 // KASLR offset, defaults to 0bl set_cpu_boot_mode_flagbl __create_page_tables/** 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.*/bl __cpu_setup // initialise processorb __primary_switch SYM_CODE_END(primary_entry)__HEAD _head:/** DO NOT MODIFY. Image header expected by Linux boot-loaders.*/ #ifdef CONFIG_EFI/** This add instruction has no meaningful effect except that* its opcode forms the magic "MZ" signature required by UEFI.*/add x13, x18, #0x16b primary_entry #elseb primary_entry // branch to kernel start, magic.long 0 // reserved #endif ......

總結

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