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# 陷入，中断和驱动程序
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运行进程时，cpu 一直处于一个大循环中：取指，更新 PC，执行，取指……。但有些情况下用户程序需要进入内核，而不是
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执行下一条用户指令。一种情况是系统调用，当用户程序执行ecall指令要求内核为其做某事时。另一种情况是异常：一条指令（用户或内核）做了一些非法的事情，如除以零或使用无效的虚拟地址。第三种情况是设备中断，当一个设备发出需要注意的信号时，例如当磁盘硬件完成一个读写请求时。
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我们使用trap作为这些情况的通用术语。
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## Traps from user space
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在用户空间执行时，如果用户程序进行了系统调用（ecall指令），或者做了一些非法的事情，或者设备中断，都可能发生trap。来自用户空间的trap的处理路径是uservec（kernel/uservec.S:16），然后是usertrap（kernel/trap.c:37）；返回时是usertrapret（kernel/trap.c:90），然后是userret（kernel/trampoline.S:16）。
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首先是uservec，这是一个汇编程序
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```asm
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uservec:
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csrrd $t0, LOONGARCH_CSR_SAVE0
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#
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# trap.c sets eentry to point here, so
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# traps from user space start here,
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# in privilege0, but with a
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# user page table.
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#
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# CSR.SAVE points to where the process's p->trapframe is
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# mapped into user space, at TRAPFRAME.
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#
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# swap a0 and SAVE1
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# so that a0 is TRAPFRAME
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csrwr $a0, LOONGARCH_CSR_SAVE1
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# save the user registers in TRAPFRAME
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st.d $ra, $a0, 0
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st.d $tp, $a0, 8
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st.d $sp, $a0, 16
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st.d $a1, $a0, 32
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st.d $a2, $a0, 40
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st.d $a3, $a0, 48
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st.d $a4, $a0, 56
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st.d $a5, $a0, 64
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st.d $a6, $a0, 72
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st.d $a7, $a0, 80
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st.d $t0, $a0, 88
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st.d $t1, $a0, 96
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st.d $t2, $a0, 104
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st.d $t3, $a0, 112
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st.d $t4, $a0, 120
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st.d $t5, $a0, 128
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st.d $t6, $a0, 136
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st.d $t7, $a0, 144
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st.d $t8, $a0, 152
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st.d $r21, $a0,160
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st.d $fp, $a0, 168
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st.d $s0, $a0, 176
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st.d $s1, $a0, 184
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st.d $s2, $a0, 192
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st.d $s3, $a0, 200
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st.d $s4, $a0, 208
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st.d $s5, $a0, 216
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st.d $s6, $a0, 224
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st.d $s7, $a0, 232
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st.d $s8, $a0, 240
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# save the user a0 in p->trapframe->a0
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csrrd $t0, LOONGARCH_CSR_SAVE1
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st.d $t0, $a0, 24
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# restore kernel stack pointer from p->trapframe->kernel_sp
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ld.d $sp, $a0, 248
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# make tp hold the current hartid, from p->trapframe->kernel_hartid
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ld.d $tp, $a0, 264
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# restore kernel page table from p->trapframe->kernel_pgdl
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ld.d $t1, $a0, 272
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csrwr $t1, LOONGARCH_CSR_PGDL
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invtlb 0x0,$zero,$zero
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# jump to usertrap(), which does not return
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b usertrap
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```
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实现了一系列的上下文切换操作，将用户态的寄存器状态保存到一个特定的数据结构（通常称为 trap frame）中，然后切换到内核态。
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```c
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//
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// handle an interrupt, exception, or system call from user space.
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// called from trampoline.S
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//
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void
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usertrap(void)
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{
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int which_dev = 0;
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if((r_csr_prmd() & PRMD_PPLV) == 0)
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panic("usertrap: not from user mode");
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struct proc *p = myproc();
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// save user program counter.
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p->trapframe->era = r_csr_era();
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if(((r_csr_estat()& CSR_ESTAT_ECODE) >> 16) == 0xb){
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// system call
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if(p->killed)
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exit(-1);
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// sepc points to the ecall instruction,
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// but we want to return to the next instruction.
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p->trapframe->era += 4;
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// an interrupt will change sstatus &c registers,
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// so don't enable until done with those registers.
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intr_on();
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syscall();
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} else if((which_dev = devintr()) != 0){
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// ok
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}
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else {
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printf("usertrap(): unexpected trapcause %p pid=%d\n", r_csr_estat(), p->pid);
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printf(" era=%p badi=%p\n", r_csr_era(), r_csr_badi());
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p->killed = 1;
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}
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if(p->killed)
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exit(-1);
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// give up the CPU if this is a timer interrupt.
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if(which_dev == 2)
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{
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myproc()->user_time++;
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yield();
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}
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usertrapret();
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}
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```
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从 `usertrap` 的代码可以看出，总共有两种不同的用户中断（对应if 和 else if）
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第一个是syscall，第二个是设备中断。
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我们先来看syscall
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### syscall
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syscall的处理函数是 `syscall` ，它的实现在 `kernel/syscall.c` 中
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```c
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static uint64 (*syscalls[])(void) = {
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[SYS_fork] sys_fork,
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[SYS_exit] sys_exit,
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[SYS_wait] sys_wait,
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[SYS_pipe] sys_pipe,
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[SYS_read] sys_read,
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[SYS_kill] sys_kill,
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[SYS_exec] sys_exec,
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[SYS_fstat] sys_fstat,
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[SYS_chdir] sys_chdir,
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[SYS_dup] sys_dup,
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[SYS_getpid] sys_getpid,
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[SYS_sbrk] sys_sbrk,
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[SYS_sleep] sys_sleep,
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[SYS_uptime] sys_uptime,
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// [SYS_open] sys_open,
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[SYS_write] sys_write,
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// [SYS_mknod] sys_mknod,
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// [SYS_unlink] sys_unlink,
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[SYS_unlinkat] sys_unlinkat,
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[SYS_link] sys_link,
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// [SYS_mkdir] sys_mkdir,
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[SYS_mkdirat] sys_mkdirat,
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[SYS_close] sys_close,
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[SYS_openat] sys_openat,
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[SYS_uname] sys_uname,
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[SYS_getppid] sys_getppid,
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[SYS_brk] sys_brk,
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[SYS_clone] sys_clone,
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[SYS_execve] sys_execve,
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[SYS_gettimeofday] sys_gettimeofday,
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[SYS_dup3] sys_dup3,
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[SYS_times] sys_times,
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[SYS_wait4] sys_wait4,
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[SYS_sched_yield] sys_sched_yield,
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[SYS_nanosleep] sys_nanosleep,
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[SYS_getdents] sys_getdents,
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[SYS_getcwd] sys_getcwd,
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// [SYS_mknod] sys_mknod,
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[SYS_mmap] sys_mmap,
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[SYS_munmap] sys_munmap,
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[SYS_mount] sys_mount,
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[SYS_umount] sys_umount,
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};
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void
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syscall(void)
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{
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int num;
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struct proc *p = myproc();
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num = p->trapframe->a7;
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// int a2 = p->trapframe->a2;
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if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
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// printf("\na2: %d\n",a2);
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// printf("\nsyscall: %d, proc name: %s, proc stat: %d\n", num, p->name, p->state);
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p->trapframe->a0 = syscalls[num]();
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} else {
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printf("%d %s: unknown sys call %d\n",
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p->pid, p->name, num);
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p->trapframe->a0 = -1;
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}
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}
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```
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这里 `syscalls` 为函数指针数组，数组的下标对应系统调用号，数组的值对应系统调用的处理函数。当用户程序执行ecall指令时，会将系统调用号保存到寄存器a7中，然后调用 `syscall` 函数，该函数会根据系统调用号调用相应的处理函数(如sys_exec等)。
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这就是系统调用的过程。
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### 设备中断
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设备中断的处理函数是 `devintr` ，它的实现在 `kernel/trap.c` 中
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```c
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int
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devintr()
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{
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uint32 estat = r_csr_estat();
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uint32 ecfg = r_csr_ecfg();
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if(estat & HWI_VEC & ecfg){
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// this is a supervisor external interrupt, via PLIC.
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// irq indicates which device interrupted.
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uint64 irq = extioi_claim();
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if(irq & (1UL << UART0_IRQ)){
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uartintr();
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// tell the apic the device is
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// now allowed to interrupt again.
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extioi_complete(1UL << UART0_IRQ);
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} else if(irq){
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printf("unexpected interrupt irq=%d\n", irq);
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apic_complete(irq);
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extioi_complete(irq);
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}
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return 1;
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} else if(estat & ecfg & TI_VEC){
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// software interrupt from a machine-mode timer interrupt,
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// forwarded by timervec in kernelvec.S.
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proc_tick();
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if(cpuid() == 0){
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clockintr();
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}
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w_csr_ticlr(r_csr_ticlr() | CSR_TICLR_CLR);
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return 2;
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} else {
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return 0;
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}
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}
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```
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设备中断有时钟中断还有来自uart的中断，我们先来看uart的handler函数
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```c
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// handle a uart interrupt, raised because input has
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// arrived, or the uart is ready for more output, or
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// both. called from trap.c.
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void
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uartintr(void)
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{
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// read and process incoming characters.
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while(1){
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int c = uartgetc();
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if(c == -1)
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break;
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consoleintr(c);
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}
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// send buffered characters.
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acquire(&uart_tx_lock);
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uartstart();
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release(&uart_tx_lock);
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}
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```
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这个函数的效果是把输入在console里的字符重新回显在console里。方便我们输入。
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我们再看时钟中断
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```c
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void
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clockintr()
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{
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acquire(&tickslock);
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ticks++;
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wakeup(&ticks);
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release(&tickslock);
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}
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```
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每一次时钟中断，都把ticks加一，然后唤醒所有等待ticks的进程。以防止一个
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进程占用cpu太久。 |
