target/ppc: Fix pending HDEC when entering PM state

HDEC is defined to not wake from PM state. There is a check in the HDEC
timer to avoid setting the interrupt if we are in a PM state, but no
check on PM entry to lower HDEC if it already fired. This can cause a
HDECR wake up and  QEMU abort with unsupported exception in Power Save
mode.

Fixes: 4b236b6 ("ppc: Initial HDEC support")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-ID: <20230726182230.433945-4-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

target/ppc/excp_helper.c

@@ -2685,6 +2685,12 @@ void helper_pminsn(CPUPPCState *env, uint32_t insn)
     env->resume_as_sreset = (insn != PPC_PM_STOP) ||
         (env->spr[SPR_PSSCR] & PSSCR_EC);
 
+    /* HDECR is not to wake from PM state, it may have already fired */
+    if (env->resume_as_sreset) {
+        PowerPCCPU *cpu = env_archcpu(env);
+        ppc_set_irq(cpu, PPC_INTERRUPT_HDECR, 0);
+    }
+
     ppc_maybe_interrupt(env);
 }
 #endif /* defined(TARGET_PPC64) */