target/ppc: Migrate DECR SPR

TCG does not maintain the DEC reigster in the SPR array, so it does get
migrated. TCG also needs to re-start the decrementer timer on the
destination machine.

Load and store the decrementer into the SPR when migrating. This works
for the level-triggered (book3s) decrementer, and should be compatible
with existing KVM machines that do keep the DEC value there.

This fixes lost decrementer interrupt on migration that can cause
hangs, as well as other problems including record-replay bugs.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

target/ppc/machine.c

@@ -209,6 +209,14 @@ static int cpu_pre_save(void *opaque)
     /* Used to retain migration compatibility for pre 6.0 for 601 machines. */
     env->hflags_compat_nmsr = 0;
 
+    if (tcg_enabled()) {
+        /*
+         * TCG does not maintain the DECR spr (unlike KVM) so have to save
+         * it here.
+         */
+        env->spr[SPR_DECR] = cpu_ppc_load_decr(env);
+    }
+
     return 0;
 }
 
@@ -319,6 +327,12 @@ static int cpu_post_load(void *opaque, int version_id)
         ppc_update_ciabr(env);
         ppc_update_daw0(env);
 #endif
+        /*
+         * TCG needs to re-start the decrementer timer and/or raise the
+         * interrupt. This works for level-triggered decrementer. Edge
+         * triggered types (including HDEC) would need to carry more state.
+         */
+        cpu_ppc_store_decr(env, env->spr[SPR_DECR]);
         pmu_mmcr01_updated(env);
     }