Enforce 64-bit timer manipulation

The initial code was overly complex and also exhibited slightly slower
performance.

device.h

@@ -185,5 +185,5 @@ typedef struct {
 #if SEMU_HAS(VIRTIOBLK)
     virtio_blk_state_t vblk;
 #endif
-    uint32_t timer_lo, timer_hi;
+    uint64_t timer;
 } emu_state_t;

main.c

@@ -162,8 +162,8 @@ static inline sbi_ret_t handle_sbi_ecall_TIMER(vm_t *vm, int32_t fid)
     emu_state_t *data = (emu_state_t *) vm->priv;
     switch (fid) {
     case SBI_TIMER__SET_TIMER:
-        data->timer_lo = vm->x_regs[RV_R_A0];
-        data->timer_hi = vm->x_regs[RV_R_A1];
+        data->timer = (((uint64_t) vm->x_regs[RV_R_A1]) << 32) |
+                      (uint64_t) (vm->x_regs[RV_R_A0]);
         return (sbi_ret_t){SBI_SUCCESS, 0};
     default:
         return (sbi_ret_t){SBI_ERR_NOT_SUPPORTED, 0};
@@ -406,7 +406,7 @@ static int semu_start(int argc, char **argv)
     atexit(unmap_files);
 
     /* Set up RISC-V hart */
-    emu.timer_hi = emu.timer_lo = 0xFFFFFFFF;
+    emu.timer = 0xFFFFFFFFFFFFFFFF;
     vm.s_mode = true;
     vm.x_regs[RV_R_A0] = 0; /* hart ID. i.e., cpuid */
     vm.x_regs[RV_R_A1] = dtb_addr;
@@ -446,8 +446,7 @@ static int semu_start(int argc, char **argv)
 #endif
         }
 
-        if (vm.insn_count_hi > emu.timer_hi ||
-            (vm.insn_count_hi == emu.timer_hi && vm.insn_count > emu.timer_lo))
+        if (vm.insn_count > emu.timer)
             vm.sip |= RV_INT_STI_BIT;
         else
             vm.sip &= ~RV_INT_STI_BIT;

riscv.c

@@ -439,7 +439,7 @@ static void csr_read(vm_t *vm, uint16_t addr, uint32_t *value)
              * and writes should set the value after the increment. However,
              * we do not expose any way to write the counters.
              */
-            *value = (addr & (1 << 7)) ? vm->insn_count_hi : vm->insn_count;
+            *value = vm->insn_count >> ((addr & (1 << 7)) ? 32 : 0);
         }
         return;
     }
@@ -806,9 +806,8 @@ void vm_step(vm_t *vm)
         return;
 
     vm->pc += 4;
+    /* Assume no integer overflow */
     vm->insn_count++;
-    if (!vm->insn_count)
-        vm->insn_count_hi++;
 
     uint32_t insn_opcode = insn & MASK(7), value;
     switch (insn_opcode) {

riscv.h

@@ -68,7 +68,7 @@ struct __vm_internal {
      * utilized in these capacities and should not be modified between logical
      * resets.
      */
-    uint32_t insn_count, insn_count_hi;
+    uint64_t insn_count;
 
     /* Instruction execution state must be set to "NONE" for instruction
      * execution to continue. If the state is not "NONE," the vm_step()