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中断和中断处理 Part 1.
Introduction
这是 linux 内核揭密 新章节的第一部分。我们已经在这本书前面的章节中走过了漫长的道路。开始于内核初始化的第一步，结束于启动第一个 init 程序 。我们见证了一系列与各种内核子系统相关的初始化步骤，但是我们并没有深入这些子系统。在这一章节中，我们将会试着去了解这些内核子系统是如何工作和实现的。就像你在这章标题中看到的，第一个子系统是中断。
什么是中断？
我们已经在这本书的很多地方听到过 中断（interrupts） 这个词，也看到过很多关于中断的例子。在这一节中我们将会从下面的主题开始：
- 什么是
中断？ - 什么是
中断处理？
我们将会继续深入探讨 中断 的细节和 Linux 内核如何处理他们。
所以，首先什么是中断？中断就是当软件或者硬件需要使用 CPU 时引发的 事件（event） 。比如，当我们在键盘上按下一个键的时候，我们下一步期望做什么？操作系统和电脑应该怎么做？做一个简单的假设，每一个物理硬件都有一根连接 CPU 的中断线，设备可以通过它对 CPU 发起中断信号。但是中断并不是直接通知给 CPU。在老机器上中断通知给 PIC ，它是一个顺序处理各种设备的各种中断请求的芯片。在新机器上，则是高级程序中断控制器（Advanced Programmable Interrupt Controller），即我们熟知的 APIC。一个 APIC 包括两个独立的设备：
Local APICI/O APIC
第一个设备 - Local APIC 存在于每个CPU核心中，Local APIC 负责处理 CPU-specific 的中断配置。Local APIC 常被用于管理来自 APIC 时钟（APIC-timer），热敏元件和其他与 I/O 设备连接的设备的中断。
第二个设备 - I/O APIC 提供了多核处理器的中断管理。它被用来在所有的 CPU 核心中分发外部中断。更多关于 local 和 I/O APIC 的内容将会在这一节的下面讲到。就如你所知道的，中断可以在任何时间发生。当一个中断发生时，操作系统必须立刻处理它。但是 处理一个中断 是什么意思呢？当一个中断发生时，操作系统必须确保下面的步骤：
- 内核必须暂停执行当前进程(取代当前的任务)；
- 内核必须搜索中断处理程序并且转交控制权(执行中断处理程序)；
- 中断处理程序结束之后，被中断的进程能够恢复执行。
当然，在这个中断处理程序中会涉及到很多错综复杂的过程。但是上面 3 条是这个程序的基本骨架。
每个中断处理程序的地址都保存在一个特殊的位置，这个位置被称为 中断描述符表（Interrupt Descriptor Table） 或者 IDT。处理器使用一个唯一的数字来识别中断和异常的类型，这个数字被称为 中断标识码（vector number）。一个中断标识码就是一个 IDT 的标识。中断标识码范围是有限的，从 0 到 255。你可以在 Linux 内核源码中找到下面的中断标识码范围检查代码：
BUG_ON((unsigned)n > 0xFF);
你可以在 Linux 内核源码中关于中断设置的地方找到这个检查(例如：set_intr_gate, void set_system_intr_gate 在 arch/x86/include/asm/desc.h中)。最开始从 0 到 31 的 32 个中断标识码被处理器保留，用作处理架构定义的异常和中断。你可以在 Linux 内核初始化程序的第二部分 - 早期中断和异常处理中找到这个表和关于这些中断标识码的描述。从 32 到 255 的中断标识码设计为用户定义中断并且不被系统保留。这些中断通常分配给外部 I/O 设备，使这些设备可以发送中断给处理器。
现在，我们来讨论中断的类型。笼统地来讲，我们可以把中断分为两个主要类型：
- 外部或者硬件引起的中断；
- 软件引起的中断。
第一种类型 - 外部中断，由 Local APIC 或者与 Local APIC 连接的处理器针脚接收。第二种类型 - 软件引起的中断，由处理器特殊的情况引起(有时使用特殊架构的指令)。一个常见的关于特殊情况的例子就是 除零。另一个例子就是使用 系统调用（syscall） 退出程序。
就如之前提到过的，中断可以在任何时间因为超出代码和 CPU 控制的原因而发生。另一方面，异常和程序执行 同步（synchronous） ，并且可以被分为 3 类：
故障（Faults）陷入（Traps）终止（Aborts）
故障 是在执行一个“不完善的”指令（可以在之后被修正）之前被报告的异常。如果发生了，它允许被中断的程序继续执行。
接下来的 陷入 是一个在执行了 陷入 指令后立刻被报告的异常。陷入同样允许被中断的程序继续执行，就像 故障 一样。
最后的 终止 是一个从不报告引起异常的精确指令的异常，并且不允许被中断的程序继续执行。
我们已经从前面的部分知道，中断可以分为 可屏蔽的（maskable） 和 不可屏蔽的（non-maskable）。可屏蔽的中断可以被阻塞，使用 x86_64 的指令 - sti 和 cli。我们可以在 Linux 内核代码中找到他们：
static inline void native_irq_disable(void)
{
asm volatile("cli": : :"memory");
}
and
static inline void native_irq_enable(void)
{
asm volatile("sti": : :"memory");
}
这两个指令修改了在中断寄存器中的 IF 标识位。 sti 指令设置 IF 标识，cli 指令清除这个标识。不可屏蔽的中断总是被报告。通常，任何硬件上的失败都映射为不可屏蔽中断。
如果多个异常或者中断同时发生，处理器以事先设定好的中断优先级处理他们。我们可以定义下面表中的从最低到最高的优先级：
+----------------------------------------------------------------+
| | |
| Priority | Description |
| | |
+--------------+-------------------------------------------------+
| | Hardware Reset and Machine Checks |
| 1 | - RESET |
| | - Machine Check |
+--------------+-------------------------------------------------+
| | Trap on Task Switch |
| 2 | - T flag in TSS is set |
| | |
+--------------+-------------------------------------------------+
| | External Hardware Interventions |
| | - FLUSH |
| 3 | - STOPCLK |
| | - SMI |
| | - INIT |
+--------------+-------------------------------------------------+
| | Traps on the Previous Instruction |
| 4 | - Breakpoints |
| | - Debug Trap Exceptions |
+--------------+-------------------------------------------------+
| 5 | Nonmaskable Interrupts |
+--------------+-------------------------------------------------+
| 6 | Maskable Hardware Interrupts |
+--------------+-------------------------------------------------+
| 7 | Code Breakpoint Fault |
+--------------+-------------------------------------------------+
| 8 | Faults from Fetching Next Instruction |
| | Code-Segment Limit Violation |
| | Code Page Fault |
+--------------+-------------------------------------------------+
| | Faults from Decoding the Next Instruction |
| | Instruction length > 15 bytes |
| 9 | Invalid Opcode |
| | Coprocessor Not Available |
| | |
+--------------+-------------------------------------------------+
| 10 | Faults on Executing an Instruction |
| | Overflow |
| | Bound error |
| | Invalid TSS |
| | Segment Not Present |
| | Stack fault |
| | General Protection |
| | Data Page Fault |
| | Alignment Check |
| | x87 FPU Floating-point exception |
| | SIMD floating-point exception |
| | Virtualization exception |
+--------------+-------------------------------------------------+
现在我们了解了一些关于各种类型的中断和异常的内容，是时候转到更实用的部分了。我们从 中断描述符表 开始。就如之前所提到的，IDT 保存了中断和异常处理程序的入口指针。IDT 是一个类似于 全局描述符表（Global Descriptor Table）的结构，我们在内核启动程序的第二部分已经介绍过。但是他们确实有一些不同，IDT 的表项被称为 门（gates），而不是 描述符（descriptors）。
- 中断门
- 任务门
- 陷阱门
in the x86 architecture. Only long mode interrupt gates and trap gates can be referenced in the x86_64. Like the Global Descriptor Table, the Interrupt Descriptor table is an array of 8-byte gates on x86 and an array of 16-byte gates on x86_64. We can remember from the second part of the Kernel booting process, that Global Descriptor Table must contain NULL descriptor as its first element. Unlike the Global Descriptor Table, the Interrupt Descriptor Table may contain a gate; it is not mandatory. For example, you may remember that we have loaded the Interrupt Descriptor table with the NULL gates only in the earlier part while transitioning into protected mode:
/*
* Set up the IDT
*/
static void setup_idt(void)
{
static const struct gdt_ptr null_idt = {0, 0};
asm volatile("lidtl %0" : : "m" (null_idt));
}
from the arch/x86/boot/pm.c. The Interrupt Descriptor table can be located anywhere in the linear address space and the base address of it must be aligned on an 8-byte boundary on x86 or 16-byte boundary on x86_64. The base address of the IDT is stored in the special register - IDTR. There are two instructions on x86-compatible processors to modify the IDTR register:
LIDTSIDT
The first instruction LIDT is used to load the base-address of the IDT i.e. the specified operand into the IDTR. The second instruction SIDT is used to read and store the contents of the IDTR into the specified operand. The IDTR register is 48-bits on the x86 and contains the following information:
+-----------------------------------+----------------------+
| | |
| Base address of the IDT | Limit of the IDT |
| | |
+-----------------------------------+----------------------+
47 16 15 0
Looking at the implementation of setup_idt, we have prepared a null_idt and loaded it to the IDTR register with the lidt instruction. Note that null_idt has gdt_ptr type which is defined as:
struct gdt_ptr {
u16 len;
u32 ptr;
} __attribute__((packed));
Here we can see the definition of the structure with the two fields of 2-bytes and 4-bytes each (a total of 48-bits) as we can see in the diagram. Now let's look at the IDT entries structure. The IDT entries structure is an array of the 16-byte entries which are called gates in the x86_64. They have the following structure:
127 96
+-------------------------------------------------------------------------------+
| |
| Reserved |
| |
+--------------------------------------------------------------------------------
95 64
+-------------------------------------------------------------------------------+
| |
| Offset 63..32 |
| |
+-------------------------------------------------------------------------------+
63 48 47 46 44 42 39 34 32
+-------------------------------------------------------------------------------+
| | | D | | | | | | |
| Offset 31..16 | P | P | 0 |Type |0 0 0 | 0 | 0 | IST |
| | | L | | | | | | |
-------------------------------------------------------------------------------+
31 16 15 0
+-------------------------------------------------------------------------------+
| | |
| Segment Selector | Offset 15..0 |
| | |
+-------------------------------------------------------------------------------+
To form an index into the IDT, the processor scales the exception or interrupt vector by sixteen. The processor handles the occurrence of exceptions and interrupts just like it handles calls of a procedure when it sees the call instruction. A processor uses an unique number or vector number of the interrupt or the exception as the index to find the necessary Interrupt Descriptor Table entry. Now let's take a closer look at an IDT entry.
As we can see, IDT entry on the diagram consists of the following fields:
0-15bits - offset from the segment selector which is used by the processor as the base address of the entry point of the interrupt handler;16-31bits - base address of the segment select which contains the entry point of the interrupt handler;IST- a new special mechanism in thex86_64, will see it later;DPL- Descriptor Privilege Level;P- Segment Present flag;48-63bits - second part of the handler base address;64-95bits - third part of the base address of the handler;96-127bits - and the last bits are reserved by the CPU.
And the last Type field describes the type of the IDT entry. There are three different kinds of handlers for interrupts:
- Interrupt gate
- Trap gate
- Task gate
The IST or Interrupt Stack Table is a new mechanism in the x86_64. It is used as an alternative to the the legacy stack-switch mechanism. Previously The x86 architecture provided a mechanism to automatically switch stack frames in response to an interrupt. The IST is a modified version of the x86 Stack switching mode. This mechanism unconditionally switches stacks when it is enabled and can be enabled for any interrupt in the IDT entry related with the certain interrupt (we will soon see it). From this we can understand that IST is not necessary for all interrupts. Some interrupts can continue to use the legacy stack switching mode. The IST mechanism provides up to seven IST pointers in the Task State Segment or TSS which is the special structure which contains information about a process. The TSS is used for stack switching during the execution of an interrupt or exception handler in the Linux kernel. Each pointer is referenced by an interrupt gate from the IDT.
The Interrupt Descriptor Table represented by the array of the gate_desc structures:
extern gate_desc idt_table[];
where gate_desc is:
#ifdef CONFIG_X86_64
...
...
...
typedef struct gate_struct64 gate_desc;
...
...
...
#endif
and gate_struct64 defined as:
struct gate_struct64 {
u16 offset_low;
u16 segment;
unsigned ist : 3, zero0 : 5, type : 5, dpl : 2, p : 1;
u16 offset_middle;
u32 offset_high;
u32 zero1;
} __attribute__((packed));
Each active thread has a large stack in the Linux kernel for the x86_64 architecture. The stack size is defined as THREAD_SIZE and is equal to:
#define PAGE_SHIFT 12
#define PAGE_SIZE (_AC(1,UL) << PAGE_SHIFT)
...
...
...
#define THREAD_SIZE_ORDER (2 + KASAN_STACK_ORDER)
#define THREAD_SIZE (PAGE_SIZE << THREAD_SIZE_ORDER)
The PAGE_SIZE is 4096-bytes and the THREAD_SIZE_ORDER depends on the KASAN_STACK_ORDER. As we can see, the KASAN_STACK depends on the CONFIG_KASAN kernel configuration parameter and is defined as:
#ifdef CONFIG_KASAN
#define KASAN_STACK_ORDER 1
#else
#define KASAN_STACK_ORDER 0
#endif
KASan is a runtime memory debugger. So... the THREAD_SIZE will be 16384 bytes if CONFIG_KASAN is disabled or 32768 if this kernel configuration option is enabled. These stacks contain useful data as long as a thread is alive or in a zombie state. While the thread is in user-space, the kernel stack is empty except for the thread_info structure (details about this structure are available in the fourth part of the Linux kernel initialization process) at the bottom of the stack. The active or zombie threads aren't the only threads with their own stack. There also exist specialized stacks that are associated with each available CPU. These stacks are active when the kernel is executing on that CPU. When the user-space is executing on the CPU, these stacks do not contain any useful information. Each CPU has a few special per-cpu stacks as well. The first is the interrupt stack used for the external hardware interrupts. Its size is determined as follows:
#define IRQ_STACK_ORDER (2 + KASAN_STACK_ORDER)
#define IRQ_STACK_SIZE (PAGE_SIZE << IRQ_STACK_ORDER)
or 16384 bytes. The per-cpu interrupt stack represented by the irq_stack_union union in the Linux kernel for x86_64:
union irq_stack_union {
char irq_stack[IRQ_STACK_SIZE];
struct {
char gs_base[40];
unsigned long stack_canary;
};
};
The first irq_stack field is a 16 kilobytes array. Also you can see that irq_stack_union contains a structure with the two fields:
gs_base- Thegsregister always points to the bottom of theirqstackunion. On thex86_64, thegsregister is shared by per-cpu area and stack canary (more aboutper-cpuvariables you can read in the special part). All per-cpu symbols are zero based and thegspoints to the base of the per-cpu area. You already know that segmented memory model is abolished in the long mode, but we can set the base address for the two segment registers -fsandgswith the Model specific registers and these registers can be still be used as address registers. If you remember the first part of the Linux kernel initialization process, you can remember that we have set thegsregister:
movl $MSR_GS_BASE,%ecx
movl initial_gs(%rip),%eax
movl initial_gs+4(%rip),%edx
wrmsr
where initial_gs points to the irq_stack_union:
GLOBAL(initial_gs)
.quad INIT_PER_CPU_VAR(irq_stack_union)
stack_canary- Stack canary for the interrupt stack is astack protectorto verify that the stack hasn't been overwritten. Note thatgs_baseis a 40 bytes array.GCCrequires that stack canary will be on the fixed offset from the base of thegsand its value must be40for thex86_64and20for thex86.
The irq_stack_union is the first datum in the percpu area, we can see it in the System.map:
0000000000000000 D __per_cpu_start
0000000000000000 D irq_stack_union
0000000000004000 d exception_stacks
0000000000009000 D gdt_page
...
...
...
We can see its definition in the code:
DECLARE_PER_CPU_FIRST(union irq_stack_union, irq_stack_union) __visible;
Now, it's time to look at the initialization of the irq_stack_union. Besides the irq_stack_union definition, we can see the definition of the following per-cpu variables in the arch/x86/include/asm/processor.h:
DECLARE_PER_CPU(char *, irq_stack_ptr);
DECLARE_PER_CPU(unsigned int, irq_count);
The first is the irq_stack_ptr. From the variable's name, it is obvious that this is a pointer to the top of the stack. The second - irq_count is used to check if a CPU is already on an interrupt stack or not. Initialization of the irq_stack_ptr is located in the setup_per_cpu_areas function in arch/x86/kernel/setup_percpu.c:
void __init setup_per_cpu_areas(void)
{
...
...
#ifdef CONFIG_X86_64
for_each_possible_cpu(cpu) {
...
...
...
per_cpu(irq_stack_ptr, cpu) =
per_cpu(irq_stack_union.irq_stack, cpu) +
IRQ_STACK_SIZE - 64;
...
...
...
#endif
...
...
}
Here we go over all the CPUs one-by-one and setup irq_stack_ptr. This turns out to be equal to the top of the interrupt stack minus 64. Why 64?TODO arch/x86/kernel/cpu/common.c source code file is following:
void load_percpu_segment(int cpu)
{
...
...
...
loadsegment(gs, 0);
wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
}
and as we already know the gs register points to the bottom of the interrupt stack:
movl $MSR_GS_BASE,%ecx
movl initial_gs(%rip),%eax
movl initial_gs+4(%rip),%edx
wrmsr
GLOBAL(initial_gs)
.quad INIT_PER_CPU_VAR(irq_stack_union)
Here we can see the wrmsr instruction which loads the data from edx:eax into the Model specific register pointed by the ecx register. In our case the model specific register is MSR_GS_BASE which contains the base address of the memory segment pointed by the gs register. edx:eax points to the address of the initial_gs which is the base address of our irq_stack_union.
We already know that x86_64 has a feature called Interrupt Stack Table or IST and this feature provides the ability to switch to a new stack for events non-maskable interrupt, double fault and etc... There can be up to seven IST entries per-cpu. Some of them are:
DOUBLEFAULT_STACKNMI_STACKDEBUG_STACKMCE_STACK
or
#define DOUBLEFAULT_STACK 1
#define NMI_STACK 2
#define DEBUG_STACK 3
#define MCE_STACK 4
All interrupt-gate descriptors which switch to a new stack with the IST are initialized with the set_intr_gate_ist function. For example:
set_intr_gate_ist(X86_TRAP_NMI, &nmi, NMI_STACK);
...
...
...
set_intr_gate_ist(X86_TRAP_DF, &double_fault, DOUBLEFAULT_STACK);
where &nmi and &double_fault are addresses of the entries to the given interrupt handlers:
asmlinkage void nmi(void);
asmlinkage void double_fault(void);
defined in the arch/x86/kernel/entry_64.S
idtentry double_fault do_double_fault has_error_code=1 paranoid=2
...
...
...
ENTRY(nmi)
...
...
...
END(nmi)
When an interrupt or an exception occurs, the new ss selector is forced to NULL and the ss selector’s rpl field is set to the new cpl. The old ss, rsp, register flags, cs, rip are pushed onto the new stack. In 64-bit mode, the size of interrupt stack-frame pushes is fixed at 8-bytes, so we will get the following stack:
+---------------+
| |
| SS | 40
| RSP | 32
| RFLAGS | 24
| CS | 16
| RIP | 8
| Error code | 0
| |
+---------------+
If the IST field in the interrupt gate is not 0, we read the IST pointer into rsp. If the interrupt vector number has an error code associated with it, we then push the error code onto the stack. If the interrupt vector number has no error code, we go ahead and push the dummy error code on to the stack. We need to do this to ensure stack consistency. Next we load the segment-selector field from the gate descriptor into the CS register and must verify that the target code-segment is a 64-bit mode code segment by the checking bit 21 i.e. the L bit in the Global Descriptor Table. Finally we load the offset field from the gate descriptor into rip which will be the entry-point of the interrupt handler. After this the interrupt handler begins to execute. After an interrupt handler finishes its execution, it must return control to the interrupted process with the iret instruction. The iret instruction unconditionally pops the stack pointer (ss:rsp) to restore the stack of the interrupted process and does not depend on the cpl change.
That's all.
Conclusion
It is the end of the first part about interrupts and interrupt handling in the Linux kernel. We saw some theory and the first steps of the initialization of stuff related to interrupts and exceptions. In the next part we will continue to dive into interrupts and interrupts handling - into the more practical aspects of it.
If you have any questions or suggestions write me a comment or ping me at twitter.
Please note that English is not my first language, And I am really sorry for any inconvenience. If you find any mistakes please send me a PR to linux-insides.