[Qemu-devel] [PATCH v2 1/2] cpu: add migration_id to allow board to provide migration instance_id

[Igor Mammedov <imammedo@redhat.com>]

Migration breaks if not the last added CPU is removed due to fact
that when target QEMU starts it uses cpu_index - 1 as migration
instance_id for all CPUs that where added after the removed CPU.

Introduce compat property use-migration-id and CPUState::migration_id
field to allow board provide an id that doesn't change regardless
of the order CPUs are created so that instance_ids on source and
target QEMU will match.

By default use-migration-id is enabled however CPUState::migration_id
isn't set (-1) which forces cpu_exec_init() to use legacy cpu_index
so that we don't have to fix all the targets that doesn't support
device_add/device_del for CPUs and/or device compat logic.

And only targets that whish to support device_add/del for CPUs
will set CPUState::migration_id explicitly allwoing them to use
stable migration id.

If boards sets CPUState::migration_id explictly then for compatibility
with old machine types use-migration-id property should be turned off
to force usage of cpu_index as instance_id for machine types
that used cpu_index for instance_id.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 exec.c            |  7 +++++--
 include/qom/cpu.h |  8 ++++++++
 qom/cpu.c         | 13 +++++++++++++
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/exec.c b/exec.c
index 0122ef7..0eda24b 100644
--- a/exec.c
+++ b/exec.c
@@ -670,6 +670,7 @@ void cpu_exec_init(CPUState *cpu, Error **errp)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);
     Error *local_err = NULL;
+    int migration_id;
 
     cpu->as = NULL;
     cpu->num_ases = 0;
@@ -708,11 +709,13 @@ void cpu_exec_init(CPUState *cpu, Error **errp)
     (void) cc;
     cpu_list_unlock();
 #else
+    migration_id = cc->use_migration_id  && cpu->migration_id != -1 ?
+        cpu->migration_id : cpu->cpu_index;
     if (qdev_get_vmsd(DEVICE(cpu)) == NULL) {
-        vmstate_register(NULL, cpu->cpu_index, &vmstate_cpu_common, cpu);
+        vmstate_register(NULL, migration_id, &vmstate_cpu_common, cpu);
     }
     if (cc->vmsd != NULL) {
-        vmstate_register(NULL, cpu->cpu_index, cc->vmsd, cpu);
+        vmstate_register(NULL, migration_id, cc->vmsd, cpu);
     }
 #endif
 }
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index cacb100..7058fc4 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -115,6 +115,8 @@ struct TranslationBlock;
  * @write_elf32_qemunote: Callback for writing a CPU- and QEMU-specific ELF
  * note to a 32-bit VM coredump.
  * @vmsd: State description for migration.
+ * @use_migration_id: Set to use @migration_id if has been set instead of
+ *      cpu_index during vmstate registration.
  * @gdb_num_core_regs: Number of core registers accessible to GDB.
  * @gdb_core_xml_file: File name for core registers GDB XML description.
  * @gdb_stop_before_watchpoint: Indicates whether GDB expects the CPU to stop
@@ -177,6 +179,8 @@ typedef struct CPUClass {
                                 void *opaque);
 
     const struct VMStateDescription *vmsd;
+    bool use_migration_id;
+
     int gdb_num_core_regs;
     const char *gdb_core_xml_file;
     gchar * (*gdb_arch_name)(CPUState *cpu);
@@ -273,6 +277,9 @@ struct qemu_work_item {
  * @kvm_fd: vCPU file descriptor for KVM.
  * @work_mutex: Lock to prevent multiple access to queued_work_*.
  * @queued_work_first: First asynchronous work pending.
+ * @migration_id: Use as instance_id argument in cpu vmstate_register calls
+ *      by default -1, which forces fallback to using legacy cpu_index even if
+ *      use_migration_id is true
  *
  * State of one CPU core or thread.
  */
@@ -360,6 +367,7 @@ struct CPUState {
        (absolute value) offset as small as possible.  This reduces code
        size, especially for hosts without large memory offsets.  */
     uint32_t tcg_exit_req;
+    int migration_id;
 };
 
 QTAILQ_HEAD(CPUTailQ, CPUState);
diff --git a/qom/cpu.c b/qom/cpu.c
index a9727a1..480e0c6 100644
--- a/qom/cpu.c
+++ b/qom/cpu.c
@@ -357,11 +357,24 @@ static int64_t cpu_common_get_arch_id(CPUState *cpu)
     return cpu->cpu_index;
 }
 
+static void cpu_common_set_use_migration_id(Object *obj, bool value,
+                                            Error **err)
+{
+    CPUClass *cc = CPU_GET_CLASS(obj);
+    cc->use_migration_id = value;
+}
+
 static void cpu_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
     CPUClass *k = CPU_CLASS(klass);
 
+    k->use_migration_id = true;
+    object_class_property_add_bool(klass, "use-migration-id",
+                                   NULL,
+                                   cpu_common_set_use_migration_id,
+                                   &error_abort);
+
     k->class_by_name = cpu_common_class_by_name;
     k->parse_features = cpu_common_parse_features;
     k->reset = cpu_common_reset;
-- 
1.8.3.1