1 files changed, 184 insertions, 0 deletions
diff --git a/kernel/trap.c b/kernel/trap.c
new file mode 100644
index 0000000..050a94d
--- /dev/null
+++ b/kernel/trap.c
@@ -0,0 +1,184 @@
+#include "types.h"
+#include "param.h"
+#include "memlayout.h"
+#include "riscv.h"
+#include "proc.h"
+#include "spinlock.h"
+#include "defs.h"
+
+struct spinlock tickslock;
+uint ticks;
+
+extern char trampout[], trampin[];
+
+// in kernelvec.S, calls kerneltrap().
+void kernelvec();
+
+extern int devintr();
+
+void
+trapinit(void)
+{
+  initlock(&tickslock, "time");
+}
+
+// set up to take exceptions and traps while in the kernel.
+void
+trapinithart(void)
+{
+  w_stvec((uint64)kernelvec);
+}
+
+//
+// handle an interrupt, exception, or system call from user space.
+// called from trampoline.S
+//
+void
+usertrap(void)
+{
+  int which_dev = 0;
+  
+  if((r_sstatus() & SSTATUS_SPP) != 0)
+    panic("usertrap: not from user mode");
+
+  // send interrupts and exceptions to kerneltrap(),
+  // since we're now in the kernel.
+  w_stvec((uint64)kernelvec);
+
+  struct proc *p = myproc();
+  
+  // save user program counter.
+  p->tf->epc = r_sepc();
+
+  intr_on();
+  
+  if(r_scause() == 8){
+    // system call
+
+    // sepc points to the ecall instruction,
+    // but we want to return to the next instruction.
+    p->tf->epc += 4;
+
+    syscall();
+  } else if((which_dev = devintr()) != 0){
+    // ok
+  } else {
+    printf("usertrap(): unexpected scause 0x%p pid=%d\n", r_scause(), p->pid);
+    printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
+    p->killed = 1;
+  }
+
+  if(p->killed)
+    exit();
+
+  // give up the CPU if this is a timer interrupt.
+  if(which_dev == 2)
+    yield();
+
+  usertrapret();
+}
+
+//
+// return to user space
+//
+void
+usertrapret(void)
+{
+  struct proc *p = myproc();
+
+  // turn off interrupts, since we're switching
+  // now from kerneltrap() to usertrap().
+  intr_off();
+
+  // send interrupts and exceptions to trampoline.S
+  w_stvec(TRAMPOLINE + (trampin - trampout));
+
+  // set up values that trampoline.S will need when
+  // the process next re-enters the kernel.
+  p->tf->kernel_satp = r_satp();
+  p->tf->kernel_sp = (uint64)p->kstack + PGSIZE;
+  p->tf->kernel_trap = (uint64)usertrap;
+  p->tf->hartid = r_tp();
+
+  // set up the registers that trampoline.S's sret will use
+  // to get to user space.
+  
+  // set S Previous Privilege mode to User.
+  unsigned long x = r_sstatus();
+  x &= ~SSTATUS_SPP; // clear SPP to 0 for user mode
+  x |= SSTATUS_SPIE; // enable interrupts in user mode
+  w_sstatus(x);
+
+  // set S Exception Program Counter to the saved user pc.
+  w_sepc(p->tf->epc);
+
+  // tell trampline.S the user page table to switch to.
+  uint64 satp = MAKE_SATP(p->pagetable);
+
+  // jump to trampoline.S at the top of memory, which 
+  // switches to the user page table, restores user registers,
+  // and switches to user mode with sret.
+  ((void (*)(uint64,uint64))TRAMPOLINE)(TRAMPOLINE - PGSIZE, satp);
+}
+
+// interrupts and exceptions from kernel code go here,
+// on whatever the current kernel stack is.
+// must be 4-byte aligned to fit in stvec.
+void 
+kerneltrap()
+{
+  uint64 sstatus = r_sstatus();
+  uint64 scause = r_scause();
+  
+  if((sstatus & SSTATUS_SPP) == 0)
+    panic("kerneltrap: not from supervisor mode");
+  if(intr_get() != 0)
+    panic("kerneltrap: interrupts enabled");
+
+  if(devintr() == 0){
+    printf("scause 0x%p\n", scause);
+    printf("sepc=%p stval=%p\n", r_sepc(), r_stval());
+    panic("kerneltrap");
+  }
+}
+
+// check if it's an external interrupt or software interrupt,
+// and handle it.
+// returns 2 if timer interrupt,
+// 1 if other device,
+// 0 if not recognized.
+int
+devintr()
+{
+  uint64 scause = r_scause();
+
+  if((scause & 0x8000000000000000L) &&
+     (scause & 0xff) == 9){
+    // supervisor external interrupt, via PLIC.
+    int irq = plic_claim();
+
+    if(irq == UART0_IRQ){
+      uartintr();
+    }
+
+    plic_complete(irq);
+    return 1;
+  } else if(scause == 0x8000000000000001){
+    // software interrupt from a machine-mode timer interrupt.
+
+    if(cpuid() == 0){
+      acquire(&tickslock);
+      ticks++;
+      wakeup(&ticks);
+      release(&tickslock);
+    }
+    
+    // acknowledge.
+    w_sip(r_sip() & ~2);
+
+    return 2;
+  } else {
+    return 0;
+  }
+}
+