Fix user process bug on RV32.

Set sstatus.SIE = 0 on the initial TrapFrame, to prevent interrupt on switching.

kernel/src/arch/riscv32/boot/trap.asm

@@ -62,7 +62,7 @@ _save_context:
     lw s1, 32*4(sp)             # s1 = sstatus
     lw s2, 33*4(sp)             # s2 = sepc
     andi s0, s1, 1 << 8
-    bnez s0, _restore_context   # back to U-mode? (sstatus.SPP = 1)
+    bnez s0, _restore_context   # back to S-mode? (sstatus.SPP = 1)
 _save_kernel_sp:
     addi s0, sp, 36*4
     csrw 0x140, s0              # sscratch = kernel-sp

kernel/src/arch/riscv32/context.rs

@@ -30,7 +30,8 @@ impl TrapFrame {
         tf.x[2] = sp;
         tf.sepc = entry_addr;
         tf.sstatus = sstatus::read();
-        tf.sstatus.set_spie(false);     // Enable interrupt
+        tf.sstatus.set_spie(true);
+        tf.sstatus.set_sie(false);
         tf.sstatus.set_spp(sstatus::SPP::User);
         tf
     }