[Qemu-devel] [RFC PATCH v0 2/5] cpu: Optionally use arch_id instead of cpu_index in cpu vmstate_register()

[Bharata B Rao]

Introduce CPUState.prefer_arch_id_over_cpu_index and
MachineClass.prefer_arch_id_over_cpu_index that allow target
machines to optionally switch to using arch_id instead of cpu_index
as instance_id in vmstate_register(). This will help allow successful
migration in cases where holes are introduced in cpu_index range
after CPU hot removals.

Whether to use arch_id or cpu_index is based on machine type version
and hence added MachineClass.prefer_arch_id_over_cpu_index. However the
enforcement is via and during CPU creation and hence added
CPUState.prefer_arch_id_over_cpu_index. So it becomes a two step
process for the target to enable the use of arch_id:

1. Set MachineClass.prefer_arch_id_over_cpu_index.
2. Ensure CPUState.prefer_arch_id_over_cpu_index is set for all CPUs
   based on 1. above.

Suggested-by: Igor Mammedov <address@hidden>
Signed-off-by: Bharata B Rao <address@hidden>
---
 exec.c              | 10 ++++++++--
 include/hw/boards.h |  1 +
 include/qom/cpu.h   |  4 ++++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/exec.c b/exec.c
index 8ce8e90..7cc1d06 100644
--- a/exec.c
+++ b/exec.c
@@ -616,15 +616,21 @@ static void cpu_release_index(CPUState *cpu)
     bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
 }
 
+/*
+ * TODO: cpu_index and instance_id are of type int while .get_arch_id()is
+ * of type int64_t. What is the consequence of changing instance_id to int64_t 
?
+ */
 static void cpu_vmstate_register(CPUState *cpu)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);
+    int instance_id = cpu->prefer_arch_id_over_cpu_index ?
+                      cc->get_arch_id(cpu) : cpu->cpu_index;
 
     if (qdev_get_vmsd(DEVICE(cpu)) == NULL) {
-        vmstate_register(NULL, cpu->cpu_index, &vmstate_cpu_common, cpu);
+        vmstate_register(NULL, instance_id, &vmstate_cpu_common, cpu);
     }
     if (cc->vmsd != NULL) {
-        vmstate_register(NULL, cpu->cpu_index, cc->vmsd, cpu);
+        vmstate_register(NULL, instance_id, cc->vmsd, cpu);
     }
 }
 
diff --git a/include/hw/boards.h b/include/hw/boards.h
index 3ed6155..decabba 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -123,6 +123,7 @@ struct MachineClass {
     ram_addr_t default_ram_size;
     bool option_rom_has_mr;
     bool rom_file_has_mr;
+    bool prefer_arch_id_over_cpu_index;
 
     HotplugHandler *(*get_hotplug_handler)(MachineState *machine,
                                            DeviceState *dev);
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index 32f3af3..1f1706e 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -273,6 +273,9 @@ 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.
+ * @prefer_arch_id_over_cpu_index: Set to enforce the use of
+ *         CPUClass.get_arch_id() over cpu_index during vmstate registration
+ *         and any other uses by target machine where arch_id is preferred.
  *
  * State of one CPU core or thread.
  */
@@ -360,6 +363,7 @@ 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;
+    bool prefer_arch_id_over_cpu_index;
 };
 
 QTAILQ_HEAD(CPUTailQ, CPUState);
-- 
2.7.4