vmstate: add subsections code

This commit adds subsections for each device section.
Subsections is the way to handle information that don't need to be sent
to de destination of a migration because its values are not needed.  It is
the way to handle optional information.  Notice that only the source can
decide if the information is optional or not.  The destination needs to
understand all subsections that it receives to have a sucessful load.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/hw.h

@@ -313,6 +313,11 @@ typedef struct {
     bool (*field_exists)(void *opaque, int version_id);
 } VMStateField;
 
+typedef struct VMStateSubsection {
+    const VMStateDescription *vmsd;
+    bool (*needed)(void *opaque);
+} VMStateSubsection;
+
 struct VMStateDescription {
     const char *name;
     int version_id;
@@ -323,6 +328,7 @@ struct VMStateDescription {
     int (*post_load)(void *opaque, int version_id);
     void (*pre_save)(void *opaque);
     VMStateField *fields;
+    const VMStateSubsection *subsections;
 };
 
 extern const VMStateInfo vmstate_info_int8;

savevm.c

@@ -551,6 +551,19 @@ int qemu_get_buffer(QEMUFile *f, uint8_t *buf, int size1)
     return size1 - size;
 }
 
+static int qemu_peek_byte(QEMUFile *f)
+{
+    if (f->is_write)
+        abort();
+
+    if (f->buf_index >= f->buf_size) {
+        qemu_fill_buffer(f);
+        if (f->buf_index >= f->buf_size)
+            return 0;
+    }
+    return f->buf[f->buf_index];
+}
+
 int qemu_get_byte(QEMUFile *f)
 {
     if (f->is_write)
@@ -1198,10 +1211,16 @@ void vmstate_unregister(DeviceState *dev, const VMStateDescription *vmsd,
     }
 }
 
+static void vmstate_subsection_save(QEMUFile *f, const VMStateDescription *vmsd,
+                                    void *opaque);
+static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription *vmsd,
+                                   void *opaque);
+
 int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
                        void *opaque, int version_id)
 {
     VMStateField *field = vmsd->fields;
+    int ret;
 
     if (version_id > vmsd->version_id) {
         return -EINVAL;
@@ -1223,7 +1242,7 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
             (!field->field_exists &&
              field->version_id <= version_id)) {
             void *base_addr = opaque + field->offset;
-            int ret, i, n_elems = 1;
+            int i, n_elems = 1;
             int size = field->size;
 
             if (field->flags & VMS_VBUFFER) {
@@ -1261,6 +1280,10 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
         }
         field++;
     }
+    ret = vmstate_subsection_load(f, vmsd, opaque);
+    if (ret != 0) {
+        return ret;
+    }
     if (vmsd->post_load) {
         return vmsd->post_load(opaque, version_id);
     }
@@ -1313,6 +1336,7 @@ void vmstate_save_state(QEMUFile *f, const VMStateDescription *vmsd,
         }
         field++;
     }
+    vmstate_subsection_save(f, vmsd, opaque);
 }
 
 static int vmstate_load(QEMUFile *f, SaveStateEntry *se, int version_id)
@@ -1341,6 +1365,7 @@ static void vmstate_save(QEMUFile *f, SaveStateEntry *se)
 #define QEMU_VM_SECTION_PART         0x02
 #define QEMU_VM_SECTION_END          0x03
 #define QEMU_VM_SECTION_FULL         0x04
+#define QEMU_VM_SUBSECTION           0x05
 
 int qemu_savevm_state_begin(Monitor *mon, QEMUFile *f, int blk_enable,
                             int shared)
@@ -1529,6 +1554,65 @@ static SaveStateEntry *find_se(const char *idstr, int instance_id)
     return NULL;
 }
 
+static const VMStateDescription *vmstate_get_subsection(const VMStateSubsection *sub, char *idstr)
+{
+    while(sub && sub->needed) {
+        if (strcmp(idstr, sub->vmsd->name) == 0) {
+            return sub->vmsd;
+        }
+        sub++;
+    }
+    return NULL;
+}
+
+static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription *vmsd,
+                                   void *opaque)
+{
+    while (qemu_peek_byte(f) == QEMU_VM_SUBSECTION) {
+        char idstr[256];
+        int ret;
+        uint8_t version_id, subsection, len;
+        const VMStateDescription *sub_vmsd;
+
+        subsection = qemu_get_byte(f);
+        len = qemu_get_byte(f);
+        qemu_get_buffer(f, (uint8_t *)idstr, len);
+        idstr[len] = 0;
+        version_id = qemu_get_be32(f);
+
+        sub_vmsd = vmstate_get_subsection(vmsd->subsections, idstr);
+        if (sub_vmsd == NULL) {
+            return -ENOENT;
+        }
+        ret = vmstate_load_state(f, sub_vmsd, opaque, version_id);
+        if (ret) {
+            return ret;
+        }
+    }
+    return 0;
+}
+
+static void vmstate_subsection_save(QEMUFile *f, const VMStateDescription *vmsd,
+                                    void *opaque)
+{
+    const VMStateSubsection *sub = vmsd->subsections;
+
+    while (sub && sub->needed) {
+        if (sub->needed(opaque)) {
+            const VMStateDescription *vmsd = sub->vmsd;
+            uint8_t len;
+
+            qemu_put_byte(f, QEMU_VM_SUBSECTION);
+            len = strlen(vmsd->name);
+            qemu_put_byte(f, len);
+            qemu_put_buffer(f, (uint8_t *)vmsd->name, len);
+            qemu_put_be32(f, vmsd->version_id);
+            vmstate_save_state(f, vmsd, opaque);
+        }
+        sub++;
+    }
+}
+
 typedef struct LoadStateEntry {
     QLIST_ENTRY(LoadStateEntry) entry;
     SaveStateEntry *se;