target/ppc: Decrementer fix BookE semantics

The decrementer store function has logic that short-cuts the timer if a
very small value is stored (0, 1, or 2) and raises an interrupt
directly. There are two problem with this on BookE.

First is that BookE says a decrementer interrupt should not be raised
on a store of 0, only of a decrement from 1. Second is that raising
the irq directly will bypass the auto-reload logic in the booke decr
timer function, breaking autoreload when 1 or 2 is stored.

Fix this by removing that small-value special case. It makes this
tricky logic even more difficult to reason about, and it hardly matters
for performance.

Cc: sdicaro@DDCI.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230530131214.373524-2-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

hw/ppc/ppc.c

@@ -811,20 +811,15 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
     }
 
     /*
-     * Going from 2 -> 1, 1 -> 0 or 0 -> -1 is the event to generate a DEC
-     * interrupt.
-     *
-     * If we get a really small DEC value, we can assume that by the time we
-     * handled it we should inject an interrupt already.
+     * Going from 1 -> 0 or 0 -> -1 is the event to generate a DEC interrupt.
      *
      * On MSB level based DEC implementations the MSB always means the interrupt
      * is pending, so raise it on those.
      *
      * On MSB edge based DEC implementations the MSB going from 0 -> 1 triggers
      * an edge interrupt, so raise it here too.
      */
-    if ((value < 3) ||
-        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && signed_value < 0) ||
+    if (((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && signed_value < 0) ||
         ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && signed_value < 0
           && signed_decr >= 0)) {
         (*raise_excp)(cpu);