start函数先将所有的中断都设置在Supervisor mode，然后设置 SIE 寄存器来接收外部设备、软件和定时器中断，之后初始化定时器。
void
start()
{
// set M Previous Privilege mode to Supervisor, for mret.
unsigned long x = r_mstatus();
x &= ~MSTATUS_MPP_MASK;
x |= MSTATUS_MPP_S;
w_mstatus(x);
// set M Exception Program Counter to main, for mret.
// requires gcc -mcmodel=medany
w_mepc((uint64)main);
// disable paging for now.
w_satp(0);
// delegate all interrupts and exceptions to supervisor mode.
w_medeleg(0xffff);
w_mideleg(0xffff);
w_sie(r_sie() | SIE_SEIE | SIE_STIE | SIE_SSIE);
// configure Physical Memory Protection to give supervisor mode
// access to all of physical memory.
w_pmpaddr0(0x3fffffffffffffull);
w_pmpcfg0(0xf);
// ask for clock interrupts.
timerinit();
// keep each CPU's hartid in its tp register, for cpuid().
int id = r_mhartid();
w_tp(id);
// switch to supervisor mode and jump to main().
asm volatile("mret");
}
main函数中，console是第一个外设。
void
main()
{
if(cpuid() == 0){
consoleinit();
...
plicinit(); // set up interrupt controller
plicinithart(); // ask PLIC for device interrupts
...
}
scheduler();
}
void
plicinithart(void)
{
int hart = cpuid();
// set enable bits for this hart's S-mode
// for the uart and virtio disk.
*(uint32*)PLIC_SENABLE(hart) = (1 << UART0_IRQ) | (1 << VIRTIO0_IRQ);
// set this hart's S-mode priority threshold to 0.
*(uint32*)PLIC_SPRIORITY(hart) = 0;
}
plicinit由0号 CPU 运行。之后，每个 CPU 的核都需要调用plicinithart函数表明可以处理哪些外设中断。上面每个 CPU 核都可以处理来自 UART 和 VIRTIO 的中断，中断的优先级被设置为0。
目前，生成中断的外部设备和传递中断给 CPU 的 PLIC 都设置好了，但 CPU 还没有设置好接收中断，需要设置 SSTATUS 寄存器。在main的最后调用了scheduler函数。
void
scheduler(void)
{
struct proc *p;
struct cpu *c = mycpu();
c->proc = 0;
for(;;){
// The most recent process to run may have had interrupts
// turned off; enable them to avoid a deadlock if all
// processes are waiting.
intr_on();
int found = 0;
for(p = proc; p < &proc[NPROC]; p++) {
acquire(&p->lock);
if(p->state == RUNNABLE) {
// Switch to chosen process. It is the process's job
// to release its lock and then reacquire it
// before jumping back to us.
p->state = RUNNING;
c->proc = p;
swtch(&c->context, &p->context);
// Process is done running for now.
// It should have changed its p->state before coming back.
c->proc = 0;
found = 1;
}
release(&p->lock);
}
if(found == 0) {
// nothing to run; stop running on this core until an interrupt.
intr_on();
asm volatile("wfi");
}
}
}
scheduler函数主要是运行进程。但在实际运行进程前会执行intr_on函数设置 SSTATUS 寄存器来使 CPU 能接收中断。
int
filewrite(struct file *f, uint64 addr, int n)
{
int r, ret = 0;
if(f->writable == 0)
return -1;
if(f->type == FD_PIPE){
ret = pipewrite(f->pipe, addr, n);
} else if(f->type == FD_DEVICE){
if(f->major < 0 || f->major >= NDEV || !devsw[f->major].write)
return -1;
ret = devsw[f->major].write(1, addr, n);
} else if(f->type == FD_INODE){
// write a few blocks at a time to avoid exceeding
// the maximum log transaction size, including
// i-node, indirect block, allocation blocks,
// and 2 blocks of slop for non-aligned writes.
// this really belongs lower down, since writei()
// might be writing a device like the console.
int max = ((MAXOPBLOCKS-1-1-2) / 2) * BSIZE;
int i = 0;
while(i < n){
int n1 = n - i;
if(n1 > max)
n1 = max;
begin_op();
ilock(f->ip);
if ((r = writei(f->ip, 1, addr + i, f->off, n1)) > 0)
f->off += r;
iunlock(f->ip);
end_op();
if(r != n1){
// error from writei
break;
}
i += r;
}
ret = (i == n ? n : -1);
} else {
panic("filewrite");
}
return ret;
}
void
uartputc(int c)
{
acquire(&uart_tx_lock);
if(panicked){
for(;;)
;
}
while(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
// buffer is full.
// wait for uartstart() to open up space in the buffer.
sleep(&uart_tx_r, &uart_tx_lock);
}
uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
uart_tx_w += 1;
uartstart();
release(&uart_tx_lock);
}
#define UART_TX_BUF_SIZE 32
char uart_tx_buf[UART_TX_BUF_SIZE];
uint64 uart_tx_w; // write next to uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE]
uint64 uart_tx_r; // read next from uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE]
在例子里，Shell 是 producer，uartputc首先判断环形 buffer 是否已满，如果已满，则会sleep一段时间，把 CPU 出让给其他进程。这里提示符$是我们送出的第一个字符，因此代码会往下走，把字符送到 buffer 中，更新写指针，然后调用uartstart函数。
void
uartstart()
{
while(1){
if(uart_tx_w == uart_tx_r){
// transmit buffer is empty.
ReadReg(ISR);
return;
}
if((ReadReg(LSR) & LSR_TX_IDLE) == 0){
// the UART transmit holding register is full,
// so we cannot give it another byte.
// it will interrupt when it's ready for a new byte.
return;
}
int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];
uart_tx_r += 1;
// maybe uartputc() is waiting for space in the buffer.
wakeup(&uart_tx_r);
WriteReg(THR, c);
}
}
int
devintr()
{
uint64 scause = r_scause();
if(scause == 0x8000000000000009L){
// this is a supervisor external interrupt, via PLIC.
// irq indicates which device interrupted.
int irq = plic_claim();
if(irq == UART0_IRQ){
uartintr();
} else if(irq == VIRTIO0_IRQ){
virtio_disk_intr();
} else if(irq){
printf("unexpected interrupt irq=%d\n", irq);
}
// the PLIC allows each device to raise at most one
// interrupt at a time; tell the PLIC the device is
// now allowed to interrupt again.
if(irq)
plic_complete(irq);
return 1;
} else if(scause == 0x8000000000000005L){
// timer interrupt.
clockintr();
return 2;
} else {
return 0;
}
}
plic_claim中，当前 CPU 核会告诉 PLIC 自己要处理中断，PLIC_SCLAIM会将中断号返回。对于 UART来说，返回的中断号为10。
int
plic_claim(void)
{
int hart = cpuid();
int irq = *(uint32*)PLIC_SCLAIM(hart);
return irq;
}
// read one input character from the UART.
// return -1 if none is waiting.
int
uartgetc(void)
{
if(ReadReg(LSR) & 0x01){
// input data is ready.
return ReadReg(RHR);
} else {
return -1;
}
}
// handle a uart interrupt, raised because input has
// arrived, or the uart is ready for more output, or
// both. called from devintr().
void
uartintr(void)
{
// read and process incoming characters.
while(1){
int c = uartgetc();
if(c == -1)
break;
consoleintr(c);
}
// send buffered characters.
acquire(&uart_tx_lock);
uartstart();
release(&uart_tx_lock);
}
