hw/ppc: Always store the decrementer value

When writing a value to the decrementer that raises an exception, the
irq is raised, but the value is not stored so the store doesn't appear
to have changed the register when it is read again.

Always store the write value to the register.

Fixes: e81a982 ("PPC: Clean up DECR implementation")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/ppc/ppc.c

@@ -841,6 +841,16 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
         return;
     }
 
+    /*
+     * Calculate the next decrementer event and set a timer.
+     * decr_next is in timebase units to keep rounding simple. Note it is
+     * not adjusted by tb_offset because if TB changes via tb_offset changing,
+     * decrementer does not change, so not directly comparable with TB.
+     */
+    now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+    next = ns_to_tb(tb_env->decr_freq, now) + value;
+    *nextp = next; /* nextp is in timebase units */
+
     /*
      * Going from 1 -> 0 or 0 -> -1 is the event to generate a DEC interrupt.
      *
@@ -862,16 +872,6 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
         (*lower_excp)(cpu);
     }
 
-    /*
-     * Calculate the next decrementer event and set a timer.
-     * decr_next is in timebase units to keep rounding simple. Note it is
-     * not adjusted by tb_offset because if TB changes via tb_offset changing,
-     * decrementer does not change, so not directly comparable with TB.
-     */
-    now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
-    next = ns_to_tb(tb_env->decr_freq, now) + value;
-    *nextp = next;
-
     /* Adjust timer */
     timer_mod(timer, tb_to_ns_round_up(tb_env->decr_freq, next));
 }