don't enable interrupts until done with sstatus, scause, &c

1 files changed, 6 insertions, 4 deletions

diff --git a/kernel/trap.c b/kernel/trap.c
index 06fc4b3..835a3b0 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -37,7 +37,7 @@ void
 usertrap(void)
 {
   int which_dev = 0;
-  
+
   if((r_sstatus() & SSTATUS_SPP) != 0)
     panic("usertrap: not from user mode");
 
@@ -49,8 +49,6 @@ usertrap(void)
   
   // save user program counter.
   p->tf->epc = r_sepc();
-
-  intr_on();
   
   if(r_scause() == 8){
     // system call
@@ -59,11 +57,15 @@ usertrap(void)
     // but we want to return to the next instruction.
     p->tf->epc += 4;
 
+    // an interrupt will change sstatus &c registers,
+    // so don't enable until done with those registers.
+    intr_on();
+
     syscall();
   } else if((which_dev = devintr()) != 0){
     // ok
   } else {
-    printf("usertrap(): unexpected scause 0x%p pid=%d\n", r_scause(), p->pid);
+    printf("usertrap(): unexpected scause %p pid=%d\n", r_scause(), p->pid);
     printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
     p->killed = 1;
   }