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IPI

x86 APIC 中断

x86 APIC 中断向量

  • 比如说有以下一些 x86 APIC 中断向量
  • arch/x86/include/asm/irq_vectors.h
#define ERROR_APIC_VECTOR       0xfe
#define RESCHEDULE_VECTOR       0xfd
#define CALL_FUNCTION_VECTOR        0xfc
#define CALL_FUNCTION_SINGLE_VECTOR 0xfb
#define THERMAL_APIC_VECTOR     0xfa
#define THRESHOLD_APIC_VECTOR       0xf9
#define REBOOT_VECTOR           0xf8

声明 x86 APIC 中断向量

  • 声明这些向量
    • arch/x86/include/asm/idtentry.h
/* System vector entry points */
#ifdef CONFIG_X86_LOCAL_APIC
DECLARE_IDTENTRY_SYSVEC(ERROR_APIC_VECTOR,      sysvec_error_interrupt);
DECLARE_IDTENTRY_SYSVEC(SPURIOUS_APIC_VECTOR,       sysvec_spurious_apic_interrupt);
DECLARE_IDTENTRY_SYSVEC(LOCAL_TIMER_VECTOR,     sysvec_apic_timer_interrupt);
DECLARE_IDTENTRY_SYSVEC(X86_PLATFORM_IPI_VECTOR,    sysvec_x86_platform_ipi);
#endif

#ifdef CONFIG_SMP
DECLARE_IDTENTRY(RESCHEDULE_VECTOR,         sysvec_reschedule_ipi);
DECLARE_IDTENTRY_SYSVEC(IRQ_MOVE_CLEANUP_VECTOR,    sysvec_irq_move_cleanup);
DECLARE_IDTENTRY_SYSVEC(REBOOT_VECTOR,          sysvec_reboot);
DECLARE_IDTENTRY_SYSVEC(CALL_FUNCTION_SINGLE_VECTOR,    sysvec_call_function_single);
DECLARE_IDTENTRY_SYSVEC(CALL_FUNCTION_VECTOR,       sysvec_call_function);
#endif
#ifdef CONFIG_X86_LOCAL_APIC
# ifdef CONFIG_X86_MCE_THRESHOLD
DECLARE_IDTENTRY_SYSVEC(THRESHOLD_APIC_VECTOR,      sysvec_threshold);
# endif

# ifdef CONFIG_X86_MCE_AMD
DECLARE_IDTENTRY_SYSVEC(DEFERRED_ERROR_VECTOR,      sysvec_deferred_error);
# endif

# ifdef CONFIG_X86_THERMAL_VECTOR
DECLARE_IDTENTRY_SYSVEC(THERMAL_APIC_VECTOR,        sysvec_thermal);
# endif

# ifdef CONFIG_IRQ_WORK
DECLARE_IDTENTRY_SYSVEC(IRQ_WORK_VECTOR,        sysvec_irq_work);
# endif
#endif
  • 以上声明在 C 代码和汇编代码中分别展开
  • 在 C 代码中的展开
/**
 * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
 *            No error code pushed by hardware
 * @vector: Vector number (ignored for C)
 * @func:   Function name of the entry point
 *
 * Declares three functions:
 * - The ASM entry point: asm_##func
 * - The XEN PV trap entry point: xen_##func (maybe unused)
 * - The C handler called from the ASM entry point
 *
 * Note: This is the C variant of DECLARE_IDTENTRY(). As the name says it
 * declares the entry points for usage in C code. There is an ASM variant
 * as well which is used to emit the entry stubs in entry_32/64.S.
 */
#define DECLARE_IDTENTRY(vector, func)                  \
    asmlinkage void asm_##func(void);               \
    asmlinkage void xen_asm_##func(void);               \
    __visible void func(struct pt_regs *regs)

/**
 * DECLARE_IDTENTRY_SYSVEC - Declare functions for system vector entry points
 * @vector: Vector number (ignored for C)
 * @func:   Function name of the entry point
 *
 * Declares three functions:
 * - The ASM entry point: asm_##func
 * - The XEN PV trap entry point: xen_##func (maybe unused)
 * - The C handler called from the ASM entry point
 *
 * Maps to DECLARE_IDTENTRY().
 */
#define DECLARE_IDTENTRY_SYSVEC(vector, func)               \
    DECLARE_IDTENTRY(vector, func)
  • 在汇编中展开
  • arch/x86/include/asm/idtentry.h
/* System vector entries */
#define DECLARE_IDTENTRY_SYSVEC(vector, func)               \
    idtentry_sysvec vector func
  • arch/x86/entry/entry_64.S
/**
 * idtentry_body - Macro to emit code calling the C function
 * @cfunc:      C function to be called
 * @has_error_code: Hardware pushed error code on stack
 */
.macro idtentry_body cfunc has_error_code:req

    /*
     * Call error_entry() and switch to the task stack if from userspace.
     *
     * When in XENPV, it is already in the task stack, and it can't fault
     * for native_iret() nor native_load_gs_index() since XENPV uses its
     * own pvops for IRET and load_gs_index().  And it doesn't need to
     * switch the CR3.  So it can skip invoking error_entry().
     */
    ALTERNATIVE "call error_entry; movq %rax, %rsp", \
            "call xen_error_entry", X86_FEATURE_XENPV

    ENCODE_FRAME_POINTER
    UNWIND_HINT_REGS

    movq    %rsp, %rdi          /* pt_regs pointer into 1st argument*/

    .if \has_error_code == 1
        movq    ORIG_RAX(%rsp), %rsi    /* get error code into 2nd argument*/
        movq    $-1, ORIG_RAX(%rsp) /* no syscall to restart */
    .endif

    call    \cfunc

    /* For some configurations \cfunc ends up being a noreturn. */
    REACHABLE

    jmp error_return
.endm

/**
 * idtentry - Macro to generate entry stubs for simple IDT entries
 * @vector:     Vector number
 * @asmsym:     ASM symbol for the entry point
 * @cfunc:      C function to be called
 * @has_error_code: Hardware pushed error code on stack
 *
 * The macro emits code to set up the kernel context for straight forward
 * and simple IDT entries. No IST stack, no paranoid entry checks.
 */
.macro idtentry vector asmsym cfunc has_error_code:req
SYM_CODE_START(\asmsym)

    .if \vector == X86_TRAP_BP
        /* #BP advances %rip to the next instruction */
        UNWIND_HINT_IRET_REGS offset=\has_error_code*8 signal=0
    .else
        UNWIND_HINT_IRET_REGS offset=\has_error_code*8
    .endif

    ENDBR
    ASM_CLAC
    cld

    .if \has_error_code == 0
        pushq   $-1         /* ORIG_RAX: no syscall to restart */
    .endif

    .if \vector == X86_TRAP_BP
        /*
         * If coming from kernel space, create a 6-word gap to allow the
         * int3 handler to emulate a call instruction.
         */
        testb   $3, CS-ORIG_RAX(%rsp)
        jnz .Lfrom_usermode_no_gap_\@
        .rept   6
        pushq   5*8(%rsp)
        .endr
        UNWIND_HINT_IRET_REGS offset=8
.Lfrom_usermode_no_gap_\@:
    .endif

    idtentry_body \cfunc \has_error_code

_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm

/*
 * System vectors which invoke their handlers directly and are not
 * going through the regular common device interrupt handling code.
 */
.macro idtentry_sysvec vector cfunc
    idtentry \vector asm_\cfunc \cfunc has_error_code=0
.endm
  • 比如说，DECLARE_IDTENTRY(RESCHEDULE_VECTOR, sysvec_reschedule_ipi) 展开以后会有：
    • 依照模板生成汇编例程 asm_sysvec_reschedule_ipi
    • 声明 C 函数 sysvec_reschedule_ipi()，该函数会被 asm_sysvec_reschedule_ipi 调用

定义 x86 APIC 中断向量

  • 以 Rescheduling interrupts 和 Function call interrupts 为例，定义如下
  • arch/x86/kernel/smp.c
/*
 * Reschedule call back. KVM uses this interrupt to force a cpu out of
 * guest mode.
 */
DEFINE_IDTENTRY_SYSVEC_SIMPLE(sysvec_reschedule_ipi)
{
    ack_APIC_irq();
    trace_reschedule_entry(RESCHEDULE_VECTOR);
    inc_irq_stat(irq_resched_count);
    scheduler_ipi();
    trace_reschedule_exit(RESCHEDULE_VECTOR);
}

DEFINE_IDTENTRY_SYSVEC(sysvec_call_function)
{
    ack_APIC_irq();
    trace_call_function_entry(CALL_FUNCTION_VECTOR);
    inc_irq_stat(irq_call_count);
    generic_smp_call_function_interrupt();
    trace_call_function_exit(CALL_FUNCTION_VECTOR);
}
  • 定义的宏如下：
/**
 * DEFINE_IDTENTRY_SYSVEC - Emit code for system vector IDT entry points
 * @func:   Function name of the entry point
 *
 * irqentry_enter/exit() and irq_enter/exit_rcu() are invoked before the
 * function body. KVM L1D flush request is set.
 *
 * Runs the function on the interrupt stack if the entry hit kernel mode
 */
#define DEFINE_IDTENTRY_SYSVEC(func)                    \
static void __##func(struct pt_regs *regs);             \
                                    \
__visible noinstr void func(struct pt_regs *regs)           \
{                                   \
    irqentry_state_t state = irqentry_enter(regs);          \
                                    \
    instrumentation_begin();                    \
    kvm_set_cpu_l1tf_flush_l1d();                   \
    run_sysvec_on_irqstack_cond(__##func, regs);            \
    instrumentation_end();                      \
    irqentry_exit(regs, state);                 \
}                                   \
                                    \
static noinline void __##func(struct pt_regs *regs)

/**
 * DEFINE_IDTENTRY_SYSVEC_SIMPLE - Emit code for simple system vector IDT
 *                 entry points
 * @func:   Function name of the entry point
 *
 * Runs the function on the interrupted stack. No switch to IRQ stack and
 * only the minimal __irq_enter/exit() handling.
 *
 * Only use for 'empty' vectors like reschedule IPI and KVM posted
 * interrupt vectors.
 */
#define DEFINE_IDTENTRY_SYSVEC_SIMPLE(func)             \
static __always_inline void __##func(struct pt_regs *regs);     \
                                    \
__visible noinstr void func(struct pt_regs *regs)           \
{                                   \
    irqentry_state_t state = irqentry_enter(regs);          \
                                    \
    instrumentation_begin();                    \
    __irq_enter_raw();                      \
    kvm_set_cpu_l1tf_flush_l1d();                   \
    __##func (regs);                        \
    __irq_exit_raw();                       \
    instrumentation_end();                      \
    irqentry_exit(regs, state);                 \
}                                   \
                                    \
static __always_inline void __##func(struct pt_regs *regs)
  • 比如说 DEFINE_IDTENTRY_SYSVEC_SIMPLE(sysvec_reschedule_ipi) 会展开：
    • 按照宏的模板展开的 C 函数 sysvec_reschedule_ipi()，由 asm_sysvec_reschedule_ipi 例程调用
    • 特定的 __sysvec_reschedule_ipi()，由 sysvec_reschedule_ipi() 函数调用

注册向量到 IDT

  • 准备 IDT entries
    • arch/x86/kernel/idt.c
/*
 * The APIC and SMP idt entries
 */
static const __initconst struct idt_data apic_idts[] = {
#ifdef CONFIG_SMP
    INTG(RESCHEDULE_VECTOR,         asm_sysvec_reschedule_ipi),
    INTG(CALL_FUNCTION_VECTOR,      asm_sysvec_call_function),
    INTG(CALL_FUNCTION_SINGLE_VECTOR,   asm_sysvec_call_function_single),
    INTG(IRQ_MOVE_CLEANUP_VECTOR,       asm_sysvec_irq_move_cleanup),
    INTG(REBOOT_VECTOR,         asm_sysvec_reboot),
#endif

#ifdef CONFIG_X86_THERMAL_VECTOR
    INTG(THERMAL_APIC_VECTOR,       asm_sysvec_thermal),
#endif

#ifdef CONFIG_X86_MCE_THRESHOLD
    INTG(THRESHOLD_APIC_VECTOR,     asm_sysvec_threshold),
#endif

#ifdef CONFIG_X86_MCE_AMD
    INTG(DEFERRED_ERROR_VECTOR,     asm_sysvec_deferred_error),
#endif

#ifdef CONFIG_X86_LOCAL_APIC
    INTG(LOCAL_TIMER_VECTOR,        asm_sysvec_apic_timer_interrupt),
    INTG(X86_PLATFORM_IPI_VECTOR,       asm_sysvec_x86_platform_ipi),
# ifdef CONFIG_HAVE_KVM
    INTG(POSTED_INTR_VECTOR,        asm_sysvec_kvm_posted_intr_ipi),
    INTG(POSTED_INTR_WAKEUP_VECTOR,     asm_sysvec_kvm_posted_intr_wakeup_ipi),
    INTG(POSTED_INTR_NESTED_VECTOR,     asm_sysvec_kvm_posted_intr_nested_ipi),
# endif
# ifdef CONFIG_IRQ_WORK
    INTG(IRQ_WORK_VECTOR,           asm_sysvec_irq_work),
# endif
    INTG(SPURIOUS_APIC_VECTOR,      asm_sysvec_spurious_apic_interrupt),
    INTG(ERROR_APIC_VECTOR,         asm_sysvec_error_interrupt),
#endif
};
  • 设置 IDT entries
start_kernel()
-> init_IRQ()
   -> x86_init.irqs.intr_init()
   => native_init_IRQ()
      -> idt_setup_apic_and_irq_gates()
         -> idt_setup_from_table(idt_table, apic_idts, ARRAY_SIZE(apic_idts), true)

static __init void
idt_setup_from_table(gate_desc *idt, const struct idt_data *t, int size, bool sys)
{
    gate_desc desc;

    for (; size > 0; t++, size--) {
        idt_init_desc(&desc, t);
        write_idt_entry(idt, t->vector, &desc);
        if (sys)
            set_bit(t->vector, system_vectors);
    }
}

发送 IPI 中断

  • 通过写 APIC 的 Interrupt Command Register（ICR）寄存器来发送 IPI
  • 对于 xAPIC 是 MMIO 映射的寄存器地址 0xFEE0 0300（0-31 bit）和 0xFEE0 0310（32-63 bit）
  • 对于 x2APIC 是 MSR 地址 0x830

IPI 是否可屏蔽？

  • Linux 中大部分 IPI 都是以 Fix Delivery Mode 发送，行为和外设中断类似，接到 INTR pin，都属于可屏蔽的
  • 以下一些场景发送的 IPI 都是 NMI 的方式，接入 NMI pin
    • crash 时 kdump 发出的 IPI
    • 让 CPU 产生 backtrace，sysrq 有类似的命令
    • 通过 /dev/mcelog 注入 MCE
    • Stop CPU
  • 另外还有一些 IPI 是多处理器启动时 BSP 启动 AP 时用到的 INIT 和 SIPI，从 LAPIC 结构图上看走的也是 NMI pin，应该也属于不可屏蔽的

Check if specific interrupt is enabled ( IPI )

The term IPI designates a category of interrupts, specifically: NMI, SMI, SIPI, INIT and Fixed interrupts. Of these, the NMI, SMI, SIPI and INIT cannot be masked without disabling the LAPIC globally.

The NMI (Non-maskable Interrupt) was designed to be non-maskable from the beginning, as the name implies. There is a hack to mask it: setting the bit7 of port 70h. However, this is a hardware trick, it ties the #NMI (today it is the LINT1 but this is configurable) high. The IPIs are sent through a software message so this trick won't work.

The SMI (System Management Interrupt) is used to enter the SMM (see previous link), a mode designed to be as trasparent to the software as possible. It is not maskable, for what the software is concerned, it doesn't exist.

The INIT and SIPI (Startup-IPI) interrupts are used to reset and wake up a CPU. They cannot be masked by design (the BIOS usually put the APs, Application Processors, to sleep with a cli / hlt sequence).

The Fixed interrupts can be masked with the IF flag or when a higher priority fixed interrupt ISR is executing (just like > the legacy IRQs with the PIC).

It's possible to mask only the Fixed interrupts, something that may actually corresponds to the common understanding of the term IPI, by soft disabling the APIC. This can be done by clearing bit8 of the Spurious Interrupt Vector Register at offset 0f0h from the LAPIC base (the LAPIC base is set in the IA32_APIC_BASE MSR, its address is 1bh). Of course, clearing IF will also do.

Alternatively it's possible to disable the LAPIC entirely (it won't respond to any IPI) by clearing bit11 of the IA32_APIC_BASE MSR.

To check if the IPIs are enable you have to check the IF flag, the Spurious Interrupt Vector Register and the IA32_APIC_BASE MSR.

  • 例如 kdump 会用到的 apic_send_IPI_allbutself(NMI_VECTOR)
//arch/x86/kernel/reboot.c
nmi_shootdown_cpus()
   //arch/x86/kernel/apic/ipi.c
-> apic_send_IPI_allbutself(NMI_VECTOR)
   -> apic->send_IPI_allbutself(vector)
      //arch/x86/kernel/apic/x2apic_phys.c
   => x2apic_send_IPI_allbutself()
      ->  __x2apic_send_IPI_shorthand(vector, APIC_DEST_ALLBUT)
             //
          -> __prepare_ICR(which, vector, 0)
             //arch/x86/include/asm/apic.h
          -> native_x2apic_icr_write(cfg, 0)
  • 对于 NMI_VECTOR 向量，ICR 的 Delivery Mode 类型被设置为了 APIC_DM_NMI
  • arch/x86/kernel/apic/local.h
static inline unsigned int __prepare_ICR(unsigned int shortcut, int vector,
                     unsigned int dest)
{
    unsigned int icr = shortcut | dest;

    switch (vector) {
    default:
        icr |= APIC_DM_FIXED | vector;
        break;
    case NMI_VECTOR:
        icr |= APIC_DM_NMI;
        break;
    }
    return icr;
}

References