1. Lecture 7 Interrupt ,Trap and System Call
Oct. 27, 2015
Kyu Ho Park
Lecture 7 Interrupt ,Trap and System Call

2. interrupt
There are 2 kinds of interrupts; 1. Synchronous interrupts often called exceptions. [Generated by the CPU] 2. Asynchronous interrupts often called interrupts. [Generated by other H/W devices]

3. interrupt
-External interrupt: issued by peripheral devices. -Trap: It is also called exception handling such as ‘divide by zero’, ‘segmentation faults’, ‘page faults’ and ‘system calls’. (trap is called also ‘software interrupt) -Interrupts and Traps are all handled in the same way at Linux.

4. Kernel H/W Interrupt handling (x86 CPU)
sys_call_table
H/W
IDT(IVT)
sys_restart_syscall
sys_exit
sys_write
idt_table
devide_error
debug
nmi
int3
system_call .
CPU 1 . 2 .
action 3
PIC N .
action
irq_desc
timer_interrupt
floppy_intrrupt
floppy_interrupt 32 . .
do_IRQ .
128
action .
action . .
223
255
0~31 entry of IDT are for trap handler

5. Interrupt numbers 0~19 : Intel CPU uses.
20~31: Reserved for Intel CPU.
32~255: Allocated to OS.

6. Interrupt Vector Table
Process
Interrupt
ISR
Disable Interrupt
Save Processor states
Interrupt handling
Return from Interrupt
Restore Processor States
Enable Interrupt

7. irq handling procedure
common_interrupt: SAVE_ALL call do_IRQ jump ret_from_intr SAVE_ALL: pushl %es; pushl %ds; pushl %eax; pushl %ebp; pushl %edi; pushl %esi; pushl %edx; pushl %ecx; movl $(_USER_DS), %edx; movl %edx, %ds; movl %edx, %es;

8. interrupt
Interrupt: -external interrupt [ret_from_intr( )] -trap -fault: page fault [ret_from_exception( )] -trap: system call [ret_from_sys_call( )] -abort: divide by zero

9. Interrupt handling
Exception ‘divide_by_zero(DVZ)’  IDT(Interrupt Description Table)
DVZ handling routine
Interrupt  IDT  irq_desc  irq service routine
System call  IDT  sys_call_table sys_call service routine

10. Installing interrupt handler
Declared in <linux/interrupt.h> int request_irq(unsigned int irq, void (*handler)(int, void *, struct pt_regs *), unsigned long flags, const char *dev_name,void *dev_id); , where irq: IRQ number, handler:interrupt handling function, flags: SA_INTERRUPT : Interrupt disables on the current processor, SA_SHIRQ: share the same IRQ numbers which means the interrupt can be shared between devices, SA_SAMPLE_RANDOM: if your device generates interrupts at truly random times, you should set this flags. dev_name: the string is passed to request_irq is used in /proc/interrupts to show the owner of the interrupt. dev_id: pointer used for shared interrupt lines. If the interrupt is not shared , set it to NULL. return value : 0: successful or a negative error code. void free_irq(unsigned int irq, void *dev_id);

11. Interrupt handler function
irqreturn_t int_interrupt(int irq, void *dev_id, struct pt_reg *regs) { … return IRQ_HANDLED; } - regs: all register values of CPU are stored at regs.

12. Kernel H/W Interrupt handling (x86 CPU)
sys_call_table
H/W
IDT(IVT)
sys_restart_syscall
sys_exit
sys_write
idt_table
devide_error
debug
nmi
int3
system_call .
CPU 1 . 2 .
action 3
PIC N .
action
irq_desc
timer_interrupt
floppy_intrrupt
floppy_interrupt 32 . .
do_IRQ .
128
action .
action . .
223
255
0~31 entry of IDT are for trap handler

13. request_irq( irq, int_handler,
request_irq( irq, int_handler, ..) function registers the IRQ number and its handler to irq_descriptor table.
free_irq( ) removes the irq_handler from the ire_descriptor table.
When a interrupt occurs, do_IRQ( ) function is called and it calls the interrupt handler registered in irq_descriptor table.

14. How to check the status of interrupt
#cat /proc/interrupts

15. External interrupt

16. Exception Vector Table

17. Interrupt handling flow

18. intr handling procedure
ARM CPU Interrupt Vector Table(IVT) _start: b reset b undefined _instruction b software_interrupt b prefetch_abort b data_abort b not_used b IRQ b FIQ

21. Registration and freeing irq_handler
int request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags,const char *name, void *dev); void free_irq(unsigned int, void *); EX: if (request_irq(IRQ_EINT1,&button_isr,SA_INTERRUPT, "INT_BUTTON_TEST", NULL)) printk(KERN_ERR "INT_BUTTON_TEST: IRQ1 Registration filed!\n"); free_irq(IRQ_EINT1, NULL);

22. isr_test_button.c(1)

23. isr_test_button.c(3)

24. System calls
System calls are function invocations made from user space into kernel in order to request some service or resource from OS.
CPU provides 4 privilege levels(Linux uses only level 0 and 3).
return
Level 0 1 2 3
User space
Kernel space
System call

25. system_call handling (x86)
user task main( ) {… fork( ); } libc.a - fork( ) { movl 2,%eax int $0x80
Kernel
ENTRY(system_call) /*arch/x86/kernel/entry_32.S*/
SAVE_ALL -
call *SYMBOL_NAME(sys_call_table)(,%eax,4)
ret_from_sys_call(schedule,signal,……)
IDT
0x0
divide_error( )
nmi( )
0x80
system_call( )
sys_call_table 1
sys_exit( )
sys_fork( )
/*kernel/fork.c*/ 2
sys_fork( ) 3
sys_read( )
sys_write( )

26. System Call Interface for user applications
To access kernel or hardware devices
We will implement it in the Linux source code
Which you’ve already used in Project 2
Only for 64-bit VM
USER
Application
System
Call
Kernel
System Call Interface
Access
Hardware
Hardware
Device

27. How to add a new system call
1. Add a entry at “arch/x86/syscalls/syscall_64.tbl”. 2. Add a entry at the last line of the file “include/linux/syscalls.h”. 3. Create a file and implement the system call handler at “kernel/mysyscall.c”. 4. Add mysyscall.o to “obj-y” at “kernel/Makefile”. 5. Compile , install and reboot the kernel.

28. How to Add System Call (1)
arch/x86/syscalls/syscall_64.tbl
Add entry
Remember the number
(it should be less then 512, and should not be same as others)

29. How to Add System Call (2)
include/linux/syscalls.h
Add entry at the last part of the file
Use function name as previous step

30. How to Add System Call (3)
kernel/mysyscall.c
Create file and implement system call handler
Use function name as previous step

31. How to Add System Call (4)
kernel/Makefile
Add entry to “obj-y”
Use name from the created file name

32. How to Add System Call (5)
Compile and install
# make -j4 bzImage
# make install
Reboot

33. How to Use New System Call (1)
Write test program in the user level
Use same number as before

34. How to Use New System Call (2)
Compile and run

35. Make Your Own System Call
Make a system call
User gives two integer values
System call handler
Prints information including your student ID
Returns results of addition of two integer values given by user
User prints returned value
Example result

36. System call implementation for ARM CPU
1. Allocation of system call number /home/beagleboard/linuxkernel/stable-kernel/linux-stable/include/asm-generic/unistd.h #define __NR_newsyscall XXX 2. Register the new system call to sys_call_table /home/beagleboard/linuxkernel/stable-kernel/linux-stable/arch/arm/kernel/calls.S CALL(sys_newsyscall) 3. /home/beagleboard/linuxkernel/stable-kernel/linux-stable/kernel#vi newsyscall.c #include <linux/kernel.h> asmlinkage int sys_newsyscall( ) { printk(“newsyscall: Hello!\n”); return 21; } 4. Register the function on /linux-stable/include/linux/syscall.h asmlinkage long sys_newsyscall( ); 5. vi kernel/Makefile obj-y =………………. newsyscall.o 6. Make bzImage