[PATCH 4/9] vfio/migration: Skip pre-copy if dirty page tracking is not supported

[Avihai Horon]

Currently, if IOMMU of a VFIO container doesn't support dirty page
tracking, migration is blocked completely. This is because a DMA-able
VFIO device can dirty RAM pages without updating QEMU about it, thus
breaking the migration.

However, this doesn't mean that migration can't be done at all. If
migration pre-copy phase is skipped, the VFIO device doesn't have a
chance to dirty RAM pages that have been migrated already, thus
eliminating the problem previously mentioned.

Hence, in such case allow migration but skip pre-copy phase.

Signed-off-by: Avihai Horon <avih...@nvidia.com>
---
 hw/vfio/migration.c   | 9 ++++++++-
 migration/migration.c | 5 +++++
 migration/migration.h | 3 +++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
index 21e8f9d4d4..d4b6653026 100644
--- a/hw/vfio/migration.c
+++ b/hw/vfio/migration.c
@@ -863,10 +863,17 @@ int vfio_migration_probe(VFIODevice *vbasedev, Error 
**errp)
     struct vfio_region_info *info = NULL;
     int ret = -ENOTSUP;
 
-    if (!vbasedev->enable_migration || !container->dirty_pages_supported) {
+    if (!vbasedev->enable_migration) {
         goto add_blocker;
     }
 
+    if (!container->dirty_pages_supported) {
+        warn_report(
+            "%s: IOMMU of the device's VFIO container doesn't support dirty 
page tracking, migration pre-copy phase will be skipped",
+            vbasedev->name);
+        migrate_get_current()->skip_precopy = true;
+    }
+
     ret = vfio_get_dev_region_info(vbasedev,
                                    VFIO_REGION_TYPE_MIGRATION_DEPRECATED,
                                    VFIO_REGION_SUBTYPE_MIGRATION_DEPRECATED,
diff --git a/migration/migration.c b/migration/migration.c
index 5a31b23bd6..668343508d 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3593,6 +3593,11 @@ static MigIterateState 
migration_iteration_run(MigrationState *s)
     uint64_t pending_size, pend_pre, pend_compat, pend_post;
     bool in_postcopy = s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE;
 
+    if (s->skip_precopy) {
+        migration_completion(s);
+        return MIG_ITERATE_BREAK;
+    }
+
     qemu_savevm_state_pending(s->to_dst_file, s->threshold_size, &pend_pre,
                               &pend_compat, &pend_post);
     pending_size = pend_pre + pend_compat + pend_post;
diff --git a/migration/migration.h b/migration/migration.h
index a863032b71..876713e7e1 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -332,6 +332,9 @@ struct MigrationState {
      * This save hostname when out-going migration starts
      */
     char *hostname;
+
+    /* Whether to skip pre-copy phase of migration or not */
+    bool skip_precopy;
 };
 
 void migrate_set_state(int *state, int old_state, int new_state);
-- 
2.21.3