The capability is used for background snapshot enabling.
The background snapshot logic is going to be added in the following
patch.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 qapi/migration.json   |  7 ++++++-
 migration/migration.h |  1 +
 migration/migration.c | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/qapi/migration.json b/qapi/migration.json
index d5000558c6..46681a5c3c 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -424,6 +424,11 @@
 # @validate-uuid: Send the UUID of the source to allow the destination
 #                 to ensure it is the same. (since 4.2)
 #
+# @background-snapshot: If enabled, the migration stream will be a snapshot
+#                       of the VM exactly at the point when the migration
+#                       procedure starts. The VM RAM is saved with running VM.
+#                       (since 5.2)
+#
 # Since: 1.2
 ##
 { 'enum': 'MigrationCapability',
@@ -431,7 +436,7 @@
            'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram',
            'block', 'return-path', 'pause-before-switchover', 'multifd',
            'dirty-bitmaps', 'postcopy-blocktime', 'late-block-activate',
-           'x-ignore-shared', 'validate-uuid' ] }
+           'x-ignore-shared', 'validate-uuid', 'background-snapshot' ] }
 
 ##
 # @MigrationCapabilityStatus:
diff --git a/migration/migration.h b/migration/migration.h
index f617960522..63f2fde9a3 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -322,6 +322,7 @@ int migrate_compress_wait_thread(void);
 int migrate_decompress_threads(void);
 bool migrate_use_events(void);
 bool migrate_postcopy_blocktime(void);
+bool migrate_background_snapshot(void);
 
 /* Sending on the return path - generic and then for each message type */
 void migrate_send_rp_shut(MigrationIncomingState *mis,
diff --git a/migration/migration.c b/migration/migration.c
index 2ed9923227..2ec0451abe 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1086,6 +1086,32 @@ static bool migrate_caps_check(bool *cap_list,
             error_setg(errp, "Postcopy is not compatible with ignore-shared");
             return false;
         }
+
+        if (cap_list[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
+            error_setg(errp, "Postcopy is not compatible "
+                        "with background snapshot");
+            return false;
+        }
+    }
+
+    if (cap_list[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
+        if (cap_list[MIGRATION_CAPABILITY_RELEASE_RAM]) {
+            error_setg(errp, "Background snapshot is not compatible "
+                        "with release ram capability");
+            return false;
+        }
+
+        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
+            error_setg(errp, "Background snapshot is not "
+                        "currently compatible with compression");
+            return false;
+        }
+
+        if (cap_list[MIGRATION_CAPABILITY_XBZRLE]) {
+            error_setg(errp, "Background snapshot is not "
+                        "currently compatible with XBZLRE");
+            return false;
+        }
     }
 
     return true;
@@ -2390,6 +2416,15 @@ bool migrate_use_block_incremental(void)
     return s->parameters.block_incremental;
 }
 
+bool migrate_background_snapshot(void)
+{
+    MigrationState *s;
+
+    s = migrate_get_current();
+
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT];
+}
+
 /* migration thread support */
 /*
  * Something bad happened to the RP stream, mark an error
-- 
2.17.0

On Wed, Jul 22, 2020 at 11:11:31AM +0300, Denis Plotnikov wrote:
> diff --git a/migration/migration.c b/migration/migration.c
> index 2ed9923227..2ec0451abe 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1086,6 +1086,32 @@ static bool migrate_caps_check(bool *cap_list,
>              error_setg(errp, "Postcopy is not compatible with ignore-shared");
>              return false;
>          }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
> +            error_setg(errp, "Postcopy is not compatible "
> +                        "with background snapshot");
> +            return false;
> +        }
> +    }
> +
> +    if (cap_list[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
> +        if (cap_list[MIGRATION_CAPABILITY_RELEASE_RAM]) {
> +            error_setg(errp, "Background snapshot is not compatible "
> +                        "with release ram capability");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
> +            error_setg(errp, "Background snapshot is not "
> +                        "currently compatible with compression");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_XBZRLE]) {
> +            error_setg(errp, "Background snapshot is not "
> +                        "currently compatible with XBZLRE");
> +            return false;
> +        }

Are these four the only ones that is not compatible with background snapshot?
I'm looking at:

typedef enum MigrationCapability {
    MIGRATION_CAPABILITY_XBZRLE,
    MIGRATION_CAPABILITY_RDMA_PIN_ALL,
    MIGRATION_CAPABILITY_AUTO_CONVERGE,
    MIGRATION_CAPABILITY_ZERO_BLOCKS,
    MIGRATION_CAPABILITY_COMPRESS,
    MIGRATION_CAPABILITY_EVENTS,
    MIGRATION_CAPABILITY_POSTCOPY_RAM,
    MIGRATION_CAPABILITY_X_COLO,
    MIGRATION_CAPABILITY_RELEASE_RAM,
    MIGRATION_CAPABILITY_BLOCK,
    MIGRATION_CAPABILITY_RETURN_PATH,
    MIGRATION_CAPABILITY_PAUSE_BEFORE_SWITCHOVER,
    MIGRATION_CAPABILITY_MULTIFD,
    MIGRATION_CAPABILITY_DIRTY_BITMAPS,
    MIGRATION_CAPABILITY_POSTCOPY_BLOCKTIME,
    MIGRATION_CAPABILITY_LATE_BLOCK_ACTIVATE,
    MIGRATION_CAPABILITY_X_IGNORE_SHARED,
    MIGRATION_CAPABILITY_VALIDATE_UUID,
    MIGRATION_CAPABILITY__MAX,
} MigrationCapability;

My gut feeling is that most of them is not compatible with it... If background
snapshot is majorly used on its own, not sure whether it's worth it to create a
new qmp command, rather than reusing the "migrate" command.  The thing is it
could be confusing when people noticed when all the parameters won't work again
with snapshots.

Btw, it does not mean we need to duplicate the code.  We should still be able
to leverage most of the codes in qmp_migrate(), maybe even call qmp_migrate()
inside a new qmp_snapshot().

Thoughts?..

-- 
Peter Xu