target/ppc: fix timebase register reset state

(H)DEC and PURR get reset before icount does, which causes them to be
skewed and not match the init state. This can cause replay to not
match the recorded trace exactly. For DEC and HDEC this is usually not
noticable since they tend to get programmed before affecting the
target machine. PURR has been observed to cause replay bugs when
running Linux.

Fix this by resetting using a time of 0.

Message-ID: <20241219034035.1826173-2-npiggin@gmail.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

hw/ppc/ppc.c

@@ -1123,16 +1123,21 @@ void cpu_ppc_tb_reset(CPUPPCState *env)
         timer_del(tb_env->hdecr_timer);
         ppc_set_irq(cpu, PPC_INTERRUPT_HDECR, 0);
         tb_env->hdecr_next = 0;
+        _cpu_ppc_store_hdecr(cpu, 0, 0, 0, 64);
     }
 
     /*
      * There is a bug in Linux 2.4 kernels:
      * if a decrementer exception is pending when it enables msr_ee at startup,
      * it's not ready to handle it...
+     *
+     * On machine reset, this is called before icount is reset, so for
+     * icount-mode, setting TB registers using now == qemu_clock_get_ns()
+     * results in them being garbage after icount is reset. Use an
+     * explicit now == 0 to get a consistent reset state.
      */
-    cpu_ppc_store_decr(env, -1);
-    cpu_ppc_store_hdecr(env, -1);
-    cpu_ppc_store_purr(env, 0x0000000000000000ULL);
+    _cpu_ppc_store_decr(cpu, 0, 0, -1, 64);
+    _cpu_ppc_store_purr(env, 0, 0);
 }
 
 void cpu_ppc_tb_free(CPUPPCState *env)