migration: Hack to maintain backwards compatibility for ppc

Current code does:
- register pre_2_10_vmstate_dummy_icp with "icp/server" and instance
  dependinfg on cpu number
- for newer machines, it register vmstate_icp with "icp/server" name
  and instance 0
- now it unregisters "icp/server" for the 1st instance.

This is wrong at many levels:
- we shouldn't have two VMSTATEDescriptions with the same name
- In case this is the only solution that we can came with, it needs to
  be:
  * register pre_2_10_vmstate_dummy_icp
  * unregister pre_2_10_vmstate_dummy_icp
  * register real vmstate_icp

Created vmstate_replace_hack_for_ppc() with warnings left and right
that it is a hack.

CC: Cedric Le Goater <clg@kaod.org>
CC: Daniel Henrique Barboza <danielhb413@gmail.com>
CC: David Gibson <david@gibson.dropbear.id.au>
CC: Greg Kurz <groug@kaod.org>

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231020090731.28701-8-quintela@redhat.com>

hw/intc/xics.c

@@ -335,8 +335,22 @@ static void icp_realize(DeviceState *dev, Error **errp)
             return;
         }
     }
-
-    vmstate_register(NULL, icp->cs->cpu_index, &vmstate_icp_server, icp);
+    /*
+     * The way that pre_2_10_icp is handling is really, really hacky.
+     * We used to have here this call:
+     *
+     * vmstate_register(NULL, icp->cs->cpu_index, &vmstate_icp_server, icp);
+     *
+     * But we were doing:
+     *     pre_2_10_vmstate_register_dummy_icp()
+     *     this vmstate_register()
+     *     pre_2_10_vmstate_unregister_dummy_icp()
+     *
+     * So for a short amount of time we had to vmstate entries with
+     * the same name.  This fixes it.
+     */
+    vmstate_replace_hack_for_ppc(NULL, icp->cs->cpu_index,
+                                 &vmstate_icp_server, icp);
 }
 
 static void icp_unrealize(DeviceState *dev)

hw/ppc/spapr.c

@@ -143,6 +143,11 @@ static bool pre_2_10_vmstate_dummy_icp_needed(void *opaque)
 }
 
 static const VMStateDescription pre_2_10_vmstate_dummy_icp = {
+    /*
+     * Hack ahead.  We can't have two devices with the same name and
+     * instance id.  So I rename this to pass make check.
+     * Real help from people who knows the hardware is needed.
+     */
     .name = "icp/server",
     .version_id = 1,
     .minimum_version_id = 1,
@@ -155,16 +160,32 @@ static const VMStateDescription pre_2_10_vmstate_dummy_icp = {
     },
 };
 
+/*
+ * See comment in hw/intc/xics.c:icp_realize()
+ *
+ * You have to remove vmstate_replace_hack_for_ppc() when you remove
+ * the machine types that need the following function.
+ */
 static void pre_2_10_vmstate_register_dummy_icp(int i)
 {
     vmstate_register(NULL, i, &pre_2_10_vmstate_dummy_icp,
                      (void *)(uintptr_t) i);
 }
 
+/*
+ * See comment in hw/intc/xics.c:icp_realize()
+ *
+ * You have to remove vmstate_replace_hack_for_ppc() when you remove
+ * the machine types that need the following function.
+ */
 static void pre_2_10_vmstate_unregister_dummy_icp(int i)
 {
-    vmstate_unregister(NULL, &pre_2_10_vmstate_dummy_icp,
-                       (void *)(uintptr_t) i);
+    /*
+     * This used to be:
+     *
+     *    vmstate_unregister(NULL, &pre_2_10_vmstate_dummy_icp,
+     *                      (void *)(uintptr_t) i);
+     */
 }
 
 int spapr_max_server_number(SpaprMachineState *spapr)

include/migration/vmstate.h

@@ -1230,6 +1230,17 @@ static inline int vmstate_register(VMStateIf *obj, int instance_id,
                                           opaque, -1, 0, NULL);
 }
 
+/**
+ * vmstate_replace_hack_for_ppc() - ppc used to abuse vmstate_register
+ *
+ * Don't even think about using this function in new code.
+ *
+ * Returns: 0 on success, -1 on failure
+ */
+int vmstate_replace_hack_for_ppc(VMStateIf *obj, int instance_id,
+                                 const VMStateDescription *vmsd,
+                                 void *opaque);
+
 /**
  * vmstate_register_any() - legacy function to register state
  * serialisation description and let the function choose the id

migration/savevm.c

@@ -846,6 +846,24 @@ static void vmstate_check(const VMStateDescription *vmsd)
     }
 }
 
+/*
+ * See comment in hw/intc/xics.c:icp_realize()
+ *
+ * This function can be removed when
+ * pre_2_10_vmstate_register_dummy_icp() is removed.
+ */
+int vmstate_replace_hack_for_ppc(VMStateIf *obj, int instance_id,
+                                 const VMStateDescription *vmsd,
+                                 void *opaque)
+{
+    SaveStateEntry *se = find_se(vmsd->name, instance_id);
+
+    if (se) {
+        savevm_state_handler_remove(se);
+    }
+    return vmstate_register(obj, instance_id, vmsd, opaque);
+}
+
 int vmstate_register_with_alias_id(VMStateIf *obj, uint32_t instance_id,
                                    const VMStateDescription *vmsd,
                                    void *opaque, int alias_id,