kvm: Persistent per kvmslot dirty bitmap

When synchronizing dirty bitmap from kernel KVM we do it in a
per-kvmslot fashion and we allocate the userspace bitmap for each of
the ioctl.  This patch instead make the bitmap cache be persistent
then we don't need to g_malloc0() every time.

More importantly, the cached per-kvmslot dirty bitmap will be further
used when we want to add support for the KVM_CLEAR_DIRTY_LOG and this
cached bitmap will be used to guarantee we won't clear any unknown
dirty bits otherwise that can be a severe data loss issue for
migration code.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190603065056.25211-9-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

accel/kvm/kvm-all.c

@@ -516,17 +516,19 @@ static int kvm_physical_sync_dirty_bitmap(KVMMemoryListener *kml,
          */
         size = ALIGN(((mem->memory_size) >> TARGET_PAGE_BITS),
                      /*HOST_LONG_BITS*/ 64) / 8;
-        d.dirty_bitmap = g_malloc0(size);
+        if (!mem->dirty_bmap) {
+            /* Allocate on the first log_sync, once and for all */
+            mem->dirty_bmap = g_malloc0(size);
+        }
 
+        d.dirty_bitmap = mem->dirty_bmap;
         d.slot = mem->slot | (kml->as_id << 16);
         if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) {
             DPRINTF("ioctl failed %d\n", errno);
-            g_free(d.dirty_bitmap);
             return -1;
         }
 
         kvm_get_dirty_pages_log_range(section, d.dirty_bitmap);
-        g_free(d.dirty_bitmap);
     }
 
     return 0;
@@ -801,6 +803,8 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
         }
 
         /* unregister the slot */
+        g_free(mem->dirty_bmap);
+        mem->dirty_bmap = NULL;
         mem->memory_size = 0;
         mem->flags = 0;
         err = kvm_set_user_memory_region(kml, mem, false);

include/sysemu/kvm_int.h

@@ -21,6 +21,8 @@ typedef struct KVMSlot
     int slot;
     int flags;
     int old_flags;
+    /* Dirty bitmap cache for the slot */
+    unsigned long *dirty_bmap;
 } KVMSlot;
 
 typedef struct KVMMemoryListener {