timer interrupt in the kernel -> yield

2 files changed, 15 insertions, 4 deletions

diff --git a/kernel/kernelvec.S b/kernel/kernelvec.S
index 4f52688..e9b0ced 100644
--- a/kernel/kernelvec.S
+++ b/kernel/kernelvec.S
@@ -47,7 +47,7 @@ kernelvec:
         ld ra, 0(sp)
         ld sp, 8(sp)
         ld gp, 16(sp)
-        ld tp, 24(sp)
+        // not this, in case we moved CPUs: ld tp, 24(sp)
         ld t0, 32(sp)
         ld t1, 40(sp)
         ld t2, 48(sp)
diff --git a/kernel/trap.c b/kernel/trap.c
index 586f123..06fc4b3 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -121,12 +121,14 @@ usertrapret(void)
   ((void (*)(uint64,uint64))TRAMPOLINE)(TRAMPOLINE - PGSIZE, satp);
 }
 
-// interrupts and exceptions from kernel code go here,
+// interrupts and exceptions from kernel code go here via kernelvec,
 // on whatever the current kernel stack is.
 // must be 4-byte aligned to fit in stvec.
 void 
 kerneltrap()
 {
+  int which_dev = 0;
+  uint64 sepc = r_sepc();
   uint64 sstatus = r_sstatus();
   uint64 scause = r_scause();
   
@@ -135,11 +137,20 @@ kerneltrap()
   if(intr_get() != 0)
     panic("kerneltrap: interrupts enabled");
 
-  if(devintr() == 0){
-    printf("scause 0x%p\n", scause);
+  if((which_dev = devintr()) == 0){
+    printf("scause %p\n", scause);
     printf("sepc=%p stval=%p\n", r_sepc(), r_stval());
     panic("kerneltrap");
   }
+
+  // give up the CPU if this is a timer interrupt.
+  if(which_dev == 2 && myproc() != 0 && myproc()->state == RUNNING)
+    yield();
+
+  // the yield() may have caused some traps to occur,
+  // so restore trap registers for use by kernelvec.S's sepc instruction.
+  w_sepc(sepc);
+  w_sstatus(sstatus);
 }
 
 // check if it's an external interrupt or software interrupt,