Interrupts (中断)

Interrupt

---

• When an interrupt occurs, Program Counter jumps to Interrupt Handler & execute
• We have a Interrupt Controller like the RISCV CLINT in RISCV to handle interrupt sources that share the same interrupt pin

Mechanism

1. Causes Process (进程) to temporarily suspend
2. Save the state info of the process into the corresponding Process Control Block (PCB)
3. Executes the Interrupt Handler
4. When Interrupt Handler is done, the running Process (进程) is restarted and the state is restored from the PCB

Free the CPU, no more Polling

We don’t need to get CPU to keep Polling for response which may take a long time to produce. Instead, send a notification to the CPU via interrupt. Thus, CPU is able to do other stuff while waiting for a response

Better Security

Programs in User Space switching Privilege Level themselves is dangerous. With Software Interrupt, the CPU is preconfigured by Kernel with where in the kernel code to jump to

Generally can't be interrupted

Interrupts are allowed to run to completion without being interrupted by other sources. For example, if a keyboard’s interrupt is interrupted, we may lose the input from the keyboard

However, there are exceptions (This part I haven’t explored yet)

Busy State

As long as the Interrupt Handler is running, other interrupts may not be handled, and interrupts are generally not queued

But Interrupt Priorities exist in some systems to handle Simultaneous Interrupts

Tip

Interrupts should complete quickly to prevent the above problems

Trap Interrupt

Used to switch from User Mode to Kernel Mode

See Trap Interrupt (陷入) for more details

Turn off interrupt in XV6-RISCV

This is achieved by writing to the Control & Status Registers (CSRs) - sstatus. The code snippets below going down the Abstraction (抽象)

intr_off() is the entry point

spinlock.c

// XV6-RISCV kernel codes, spinlock.c
 
// ...
void
push_off(void)
{
  // ..
  intr_off();
  // ...
}
// ...

We can see from code snippet below, to disable the interrupt is setting the second bit from right on the sstatus register

riscv.h

// XV6-RISCV kernel codes, riscv.h
 
// ...
#define SSTATUS_SIE (1L << 1) // Supervisor Interrupt Enable
 
// ...
// disable device interrupts
static inline void
intr_off()
{
  w_sstatus(r_sstatus() & ~SSTATUS_SIE);
}
// ...

Eventually, it boils down to Assembly language

riscv.h

// XV6-RISCV kernel codes, riscv.h
 
// ...
static inline void 
w_sstatus(uint64 x)
{
  asm volatile("csrw sstatus, %0" : : "r" (x));
}
// ..

Software Interrupt

---

• Known as upcalls, or Signal in Unix, or asynchronous events in Windows
• Process (进程) being told when some event that deserves immediate attention has occurred, to virtualise some part of the kernel, so that applications can behave more like kernel, follow the Synchronous Sequential Circuit behaviour

Hardware interrupts (外中断)

---

• Raised by hardware devices. Follow the Asynchronous Sequential Circuit behaviour, can occur at any time
• One example is Timer Chip used for Preemptive Multitasking

Alarm Signal

Sent after a specified number of seconds has elapsed to notify a process of an event