target/ppc: Sign-extend large decrementer to 64-bits

When storing a large decrementer value with the most significant
implemented bit set, it is to be treated as a negative and sign
extended.

This isn't hit for book3s DEC because of another bug, fixing it
in the next patch exposes this one and can cause additional
problems, so fix this first. It can be hit with HDECR and other
edge triggered types.

Fixes: a8dafa5 ("target/ppc: Implement large decrementer support for TCG")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/ppc/ppc.c

@@ -743,7 +743,9 @@ target_ulong cpu_ppc_load_decr(CPUPPCState *env)
      * to 64 bits, otherwise it is a 32 bit value.
      */
     if (env->spr[SPR_LPCR] & LPCR_LD) {
-        return decr;
+        PowerPCCPU *cpu = env_archcpu(env);
+        PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
+        return sextract64(decr, 0, pcc->lrg_decr_bits);
     }
     return (uint32_t) decr;
 }
@@ -762,7 +764,9 @@ target_ulong cpu_ppc_load_hdecr(CPUPPCState *env)
      * extended to 64 bits, otherwise it is 32 bits.
      */
     if (pcc->lrg_decr_bits > 32) {
-        return hdecr;
+        PowerPCCPU *cpu = env_archcpu(env);
+        PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
+        return sextract64(hdecr, 0, pcc->lrg_decr_bits);
     }
     return (uint32_t) hdecr;
 }