linux-insides/Interrupts/linux-interrupts-4.md

Interrupts and Interrupt Handling. Part 4.

Initialization of non-early interrupt gates

This is fourth part about an interrupts and exceptions handling in the Linux kernel and in the previous part we saw first early #DB and #BP exceptions handlers from the arch/x86/kernel/traps.c. We stopped on the right after the early_trap_init function that called in the setup_arch function which defined in the arch/x86/kernel/setup.c. In this part we will continue to dive into an interrupts and exceptions handling in the Linux kernel for x86_64 and continue to do it from the place where we left off in the last part. First thing which is related to the interrupts and exceptions handling is the setup of the #PF or page fault handler with the early_trap_pf_init function. Let's start from it.

Early page fault handler

The early_trap_pf_init function defined in the arch/x86/kernel/traps.c. It uses set_intr_gate macro that fills Interrupt Descriptor Table with the given entry:

void __init early_trap_pf_init(void)
{
#ifdef CONFIG_X86_64
         set_intr_gate(X86_TRAP_PF, page_fault);
#endif
}

This macro defined in the arch/x86/include/asm/desc.h. We already saw macros like this in the previous part - set_system_intr_gate and set_intr_gate_ist. This macro checks that given vector number is not greater than 255 (maximum vector number) and calls _set_gate function as set_system_intr_gate and set_intr_gate_ist did it:

#define set_intr_gate(n, addr)                                  \
do {                                                            \
        BUG_ON((unsigned)n > 0xFF);                             \
        _set_gate(n, GATE_INTERRUPT, (void *)addr, 0, 0,        \
                  __KERNEL_CS);                                 \
        _trace_set_gate(n, GATE_INTERRUPT, (void *)trace_##addr,\
                        0, 0, __KERNEL_CS);                     \
} while (0)

The set_intr_gate macro takes two parameters:

• vector number of a interrupt;
• address of an interrupt handler;

In our case they are:

• X86_TRAP_PF - 14;
• page_fault - the interrupt handler entry point.

The X86_TRAP_PF is the element of enum which defined in the arch/x86/include/asm/traprs.h:

enum {
	...
	...
	...
	...
	X86_TRAP_PF,            /* 14, Page Fault */
	...
	...
	...
}

When the early_trap_pf_init will be called, the set_intr_gate will be expanded to the call of the _set_gate which will fill the IDT with the handler for the page fault. Now let's look on the implementation of the page_fault handler. The page_fault handler defined in the arch/x86/entry/entry_64.S assembly source code file as all exceptions handlers. Let's look on it:

trace_idtentry page_fault do_page_fault has_error_code=1

We saw in the previous part how #DB and #BP handlers defined. They were defined with the idtentry macro, but here we can see trace_idtentry. This macro defined in the same source code file and depends on the CONFIG_TRACING kernel configuration option:

#ifdef CONFIG_TRACING
.macro trace_idtentry sym do_sym has_error_code:req
idtentry trace(\sym) trace(\do_sym) has_error_code=\has_error_code
idtentry \sym \do_sym has_error_code=\has_error_code
.endm
#else
.macro trace_idtentry sym do_sym has_error_code:req
idtentry \sym \do_sym has_error_code=\has_error_code
.endm
#endif

We will not dive into exceptions Tracing now. If CONFIG_TRACING is not set, we can see that trace_idtentry macro just expands to the normal idtentry. We already saw implementation of the idtentry macro in the previous part, so let's start from the page_fault exception handler.

As we can see in the idtentry definition, the handler of the page_fault is do_page_fault function which defined in the arch/x86/mm/fault.c and as all exceptions handlers it takes two arguments:

• regs - pt_regs structure that holds state of an interrupted process;
• error_code - error code of the page fault exception.

Let's look inside this function. First of all we read content of the cr2 control register:

dotraplinkage void notrace
do_page_fault(struct pt_regs *regs, unsigned long error_code)
{
	unsigned long address = read_cr2();
	...
	...
	...
}

This register contains a linear address which caused page fault. In the next step we make a call of the exception_enter function from the include/linux/context_tracking.h. The exception_enter and exception_exit are functions from context tracking subsystem in the Linux kernel used by the RCU to remove its dependency on the timer tick while a processor runs in userspace. Almost in every exception handler we will see similar code:

enum ctx_state prev_state;
prev_state = exception_enter();
...
... // exception handler here
...
exception_exit(prev_state);

The exception_enter function checks that context tracking is enabled with the context_tracking_is_enabled and if it is in enabled state, we get previous context with the this_cpu_read (more about this_cpu_* operations you can read in the Documentation). After this it calls context_tracking_user_exit function which informs the context tracking that the processor is exiting userspace mode and entering the kernel:

static inline enum ctx_state exception_enter(void)
{
        enum ctx_state prev_ctx;

        if (!context_tracking_is_enabled())
                return 0;

        prev_ctx = this_cpu_read(context_tracking.state);
        context_tracking_user_exit();

        return prev_ctx;
}

The state can be one of the:

enum ctx_state {
    IN_KERNEL = 0,
	IN_USER,
} state;

And in the end we return previous context. Between the exception_enter and exception_exit we call actual page fault handler:

__do_page_fault(regs, error_code, address);

The __do_page_fault is defined in the same source code file as do_page_fault - arch/x86/mm/fault.c. In the beginning of the __do_page_fault we check state of the kmemcheck checker. The kmemcheck detects warns about some uses of uninitialized memory. We need to check it because page fault can be caused by kmemcheck:

if (kmemcheck_active(regs))
		kmemcheck_hide(regs);
	prefetchw(&mm->mmap_sem);

After this we can see the call of the prefetchw which executes instruction with the same name which fetches X86_FEATURE_3DNOW to get exclusive cache line. The main purpose of prefetching is to hide the latency of a memory access. In the next step we check that we got page fault not in the kernel space with the following condition:

if (unlikely(fault_in_kernel_space(address))) {
...
...
...
}

where fault_in_kernel_space is:

static int fault_in_kernel_space(unsigned long address)
{
        return address >= TASK_SIZE_MAX;
}

The TASK_SIZE_MAX macro expands to the:

#define TASK_SIZE_MAX   ((1UL << 47) - PAGE_SIZE)

or 0x00007ffffffff000. Pay attention on unlikely macro. There are two macros in the Linux kernel:

#define likely(x)      __builtin_expect(!!(x), 1)
#define unlikely(x)    __builtin_expect(!!(x), 0)

You can often find these macros in the code of the Linux kernel. Main purpose of these macros is optimization. Sometimes this situation is that we need to check the condition of the code and we know that it will rarely be true or false. With these macros we can tell to the compiler about this. For example

static int proc_root_readdir(struct file *file, struct dir_context *ctx)
{
        if (ctx->pos < FIRST_PROCESS_ENTRY) {
                int error = proc_readdir(file, ctx);
                if (unlikely(error <= 0))
                        return error;
...
...
...
}

Here we can see proc_root_readdir function which will be called when the Linux VFS needs to read the root directory contents. If condition marked with unlikely, compiler can put false code right after branching. Now let's back to the our address check. Comparison between the given address and the 0x00007ffffffff000 will give us to know, was page fault in the kernel mode or user mode. After this check we know it. After this __do_page_fault routine will try to understand the problem that provoked page fault exception and then will pass address to the appropriate routine. It can be kmemcheck fault, spurious fault, kprobes fault and etc. Will not dive into implementation details of the page fault exception handler in this part, because we need to know many different concepts which are provided by the Linux kernel, but will see it in the chapter about the memory management in the Linux kernel.

Back to start_kernel

There are many different function calls after the early_trap_pf_init in the setup_arch function from different kernel subsystems, but there are no one interrupts and exceptions handling related. So, we have to go back where we came from - start_kernel function from the init/main.c. The first things after the setup_arch is the trap_init function from the arch/x86/kernel/traps.c. This function makes initialization of the remaining exceptions handlers (remember that we already setup 3 handlers for the #DB - debug exception, #BP - breakpoint exception and #PF - page fault exception). The trap_init function starts from the check of the Extended Industry Standard Architecture:

#ifdef CONFIG_EISA
        void __iomem *p = early_ioremap(0x0FFFD9, 4);

        if (readl(p) == 'E' + ('I'<<8) + ('S'<<16) + ('A'<<24))
                EISA_bus = 1;
        early_iounmap(p, 4);
#endif

Note that it depends on the CONFIG_EISA kernel configuration parameter which represents EISA support. Here we use early_ioremap function to map I/O memory on the page tables. We use readl function to read first 4 bytes from the mapped region and if they are equal to EISA string we set EISA_bus to one. In the end we just unmap previously mapped region. More about early_ioremap you can read in the part which describes Fix-Mapped Addresses and ioremap.

After this we start to fill the Interrupt Descriptor Table with the different interrupt gates. First of all we set #DE or Divide Error and #NMI or Non-maskable Interrupt:

set_intr_gate(X86_TRAP_DE, divide_error);
set_intr_gate_ist(X86_TRAP_NMI, &nmi, NMI_STACK);

We use set_intr_gate macro to set the interrupt gate for the #DE exception and set_intr_gate_ist for the #NMI. You can remember that we already used these macros when we have set the interrupts gates for the page fault handler, debug handler and etc, you can find explanation of it in the previous part. After this we setup exception gates for the following exceptions:

set_system_intr_gate(X86_TRAP_OF, &overflow);
set_intr_gate(X86_TRAP_BR, bounds);
set_intr_gate(X86_TRAP_UD, invalid_op);
set_intr_gate(X86_TRAP_NM, device_not_available);

Here we can see:

• #OF or Overflow exception. This exception indicates that an overflow trap occurred when an special INTO instruction was executed;
• #BR or BOUND Range exceeded exception. This exception indicates that a BOUND-range-exceed fault occurred when a BOUND instruction was executed;
• #UD or Invalid Opcode exception. Occurs when a processor attempted to execute invalid or reserved opcode, processor attempted to execute instruction with invalid operand(s) and etc;
• #NM or Device Not Available exception. Occurs when the processor tries to execute x87 FPU floating point instruction while EM flag in the control register cr0 was set.

In the next step we set the interrupt gate for the #DF or Double fault exception:

set_intr_gate_ist(X86_TRAP_DF, &double_fault, DOUBLEFAULT_STACK);

This exception occurs when processor detected a second exception while calling an exception handler for a prior exception. In usual way when the processor detects another exception while trying to call an exception handler, the two exceptions can be handled serially. If the processor cannot handle them serially, it signals the double-fault or #DF exception.

The following set of the interrupt gates is:

set_intr_gate(X86_TRAP_OLD_MF, &coprocessor_segment_overrun);
set_intr_gate(X86_TRAP_TS, &invalid_TSS);
set_intr_gate(X86_TRAP_NP, &segment_not_present);
set_intr_gate_ist(X86_TRAP_SS, &stack_segment, STACKFAULT_STACK);
set_intr_gate(X86_TRAP_GP, &general_protection);
set_intr_gate(X86_TRAP_SPURIOUS, &spurious_interrupt_bug);
set_intr_gate(X86_TRAP_MF, &coprocessor_error);
set_intr_gate(X86_TRAP_AC, &alignment_check);

Here we can see setup for the following exception handlers:

• #CSO or Coprocessor Segment Overrun - this exception indicates that math coprocessor of an old processor detected a page or segment violation. Modern processors do not generate this exception
• #TS or Invalid TSS exception - indicates that there was an error related to the Task State Segment.
• #NP or Segment Not Present exception indicates that the present flag of a segment or gate descriptor is clear during attempt to load one of cs, ds, es, fs, or gs register.
• #SS or Stack Fault exception indicates one of the stack related conditions was detected, for example a not-present stack segment is detected when attempting to load the ss register.
• #GP or General Protection exception indicates that the processor detected one of a class of protection violations called general-protection violations. There are many different conditions that can cause general-protection exception. For example loading the ss, ds, es, fs, or gs register with a segment selector for a system segment, writing to a code segment or a read-only data segment, referencing an entry in the Interrupt Descriptor Table (following an interrupt or exception) that is not an interrupt, trap, or task gate and many many more.
• Spurious Interrupt - a hardware interrupt that is unwanted.
• #MF or x87 FPU Floating-Point Error exception caused when the x87 FPU has detected a floating point error.
• #AC or Alignment Check exception Indicates that the processor detected an unaligned memory operand when alignment checking was enabled.

After that we setup this exception gates, we can see setup of the Machine-Check exception:

#ifdef CONFIG_X86_MCE
	set_intr_gate_ist(X86_TRAP_MC, &machine_check, MCE_STACK);
#endif

Note that it depends on the CONFIG_X86_MCE kernel configuration option and indicates that the processor detected an internal machine error or a bus error, or that an external agent detected a bus error. The next exception gate is for the SIMD Floating-Point exception:

set_intr_gate(X86_TRAP_XF, &simd_coprocessor_error);

which indicates the processor has detected an SSE or SSE2 or SSE3 SIMD floating-point exception. There are six classes of numeric exception conditions that can occur while executing an SIMD floating-point instruction:

• Invalid operation
• Divide-by-zero
• Denormal operand
• Numeric overflow
• Numeric underflow
• Inexact result (Precision)

In the next step we fill the used_vectors array which defined in the arch/x86/include/asm/desc.h header file and represents bitmap:

DECLARE_BITMAP(used_vectors, NR_VECTORS);

of the first 32 interrupts (more about bitmaps in the Linux kernel you can read in the part which describes cpumasks and bitmaps)

for (i = 0; i < FIRST_EXTERNAL_VECTOR; i++)
	set_bit(i, used_vectors)

where FIRST_EXTERNAL_VECTOR is:

#define FIRST_EXTERNAL_VECTOR           0x20

After this we setup the interrupt gate for the ia32_syscall and add 0x80 to the used_vectors bitmap:

#ifdef CONFIG_IA32_EMULATION
        set_system_intr_gate(IA32_SYSCALL_VECTOR, ia32_syscall);
        set_bit(IA32_SYSCALL_VECTOR, used_vectors);
#endif

There is CONFIG_IA32_EMULATION kernel configuration option on x86_64 Linux kernels. This option provides ability to execute 32-bit processes in compatibility-mode. In the next parts we will see how it works, in the meantime we need only to know that there is yet another interrupt gate in the IDT with the vector number 0x80. In the next step we maps IDT to the fixmap area:

__set_fixmap(FIX_RO_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
idt_descr.address = fix_to_virt(FIX_RO_IDT);

and write its address to the idt_descr.address (more about fix-mapped addresses you can read in the second part of the Linux kernel memory management chapter). After this we can see the call of the cpu_init function that defined in the arch/x86/kernel/cpu/common.c. This function makes initialization of the all per-cpu state. In the beginning of the cpu_init we do the following things: First of all we wait while current cpu is initialized and than we call the cr4_init_shadow function which stores shadow copy of the cr4 control register for the current cpu and load CPU microcode if need with the following function calls:

wait_for_master_cpu(cpu);
cr4_init_shadow();
load_ucode_ap();

Next we get the Task State Segment for the current cpu and orig_ist structure which represents origin Interrupt Stack Table values with the:

t = &per_cpu(cpu_tss, cpu);
oist = &per_cpu(orig_ist, cpu);

As we got values of the Task State Segment and Interrupt Stack Table for the current processor, we clear following bits in the cr4 control register:

cr4_clear_bits(X86_CR4_VME|X86_CR4_PVI|X86_CR4_TSD|X86_CR4_DE);

with this we disable vm86 extension, virtual interrupts, timestamp (RDTSC can only be executed with the highest privilege) and debug extension. After this we reload the Global Descriptor Table and Interrupt Descriptor table with the:

	switch_to_new_gdt(cpu);
	loadsegment(fs, 0);
	load_current_idt();

After this we setup array of the Thread-Local Storage Descriptors, configure NX and load CPU microcode. Now is time to setup and load per-cpu Task State Segments. We are going in a loop through the all exception stack which is N_EXCEPTION_STACKS or 4 and fill it with Interrupt Stack Tables:

	if (!oist->ist[0]) {
		char *estacks = per_cpu(exception_stacks, cpu);

		for (v = 0; v < N_EXCEPTION_STACKS; v++) {
			estacks += exception_stack_sizes[v];
			oist->ist[v] = t->x86_tss.ist[v] =
					(unsigned long)estacks;
			if (v == DEBUG_STACK-1)
				per_cpu(debug_stack_addr, cpu) = (unsigned long)estacks;
		}
	}

As we have filled Task State Segments with the Interrupt Stack Tables we can set TSS descriptor for the current processor and load it with the:

set_tss_desc(cpu, t);
load_TR_desc();

where set_tss_desc macro from the arch/x86/include/asm/desc.h writes given descriptor to the Global Descriptor Table of the given processor:

#define set_tss_desc(cpu, addr) __set_tss_desc(cpu, GDT_ENTRY_TSS, addr)
static inline void __set_tss_desc(unsigned cpu, unsigned int entry, void *addr)
{
        struct desc_struct *d = get_cpu_gdt_table(cpu);
        tss_desc tss;
        set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS,
                              IO_BITMAP_OFFSET + IO_BITMAP_BYTES +
                              sizeof(unsigned long) - 1);
        write_gdt_entry(d, entry, &tss, DESC_TSS);
}

and load_TR_desc macro expands to the ltr or Load Task Register instruction:

#define load_TR_desc()                          native_load_tr_desc()
static inline void native_load_tr_desc(void)
{
        asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
}

In the end of the trap_init function we can see the following code:

set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
...
...
...
#ifdef CONFIG_X86_64
        memcpy(&nmi_idt_table, &idt_table, IDT_ENTRIES * 16);
        set_nmi_gate(X86_TRAP_DB, &debug);
        set_nmi_gate(X86_TRAP_BP, &int3);
#endif

Here we copy idt_table to the nmi_dit_table and setup exception handlers for the #DB or Debug exception and #BR or Breakpoint exception. You can remember that we already set these interrupt gates in the previous part, so why do we need to setup it again? We setup it again because when we initialized it before in the early_trap_init function, the Task State Segment was not ready yet, but now it is ready after the call of the cpu_init function.

That's all. Soon we will consider all handlers of these interrupts/exceptions.

Conclusion

It is the end of the fourth part about interrupts and interrupt handling in the Linux kernel. We saw the initialization of the Task State Segment in this part and initialization of the different interrupt handlers as Divide Error, Page Fault exception and etc. You can note that we saw just initialization stuff, and will dive into details about handlers for these exceptions. In the next part we will start to do 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 PR to linux-insides.

Links

• page fault
• Interrupt Descriptor Table
• Tracing
• cr2
• RCU
• this_cpu_* operations
• kmemcheck
• prefetchw
• 3DNow
• CPU caches
• VFS
• Linux kernel memory management
• Fix-Mapped Addresses and ioremap
• Extended Industry Standard Architecture
• INT isntruction
• INTO
• BOUND
• opcode
• control register
• x87 FPU
• MCE exception
• SIMD
• cpumasks and bitmaps
• NX
• Task State Segment
• Previous part