When this capability is enabled, QEMU will use the return path even for
precopy migration. This is helpful at least in one case when destination
failed to load the image while source quited without confirmation. With
return path, source will wait for the last response from destination,
and if destination fails, it'll fail the migration on source, then the
guest can be run again on the source (rather than assuming to be good,
then the guest will be lost after source quits).

It needs to be enabled explicitly on source, otherwise disabled.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.c | 16 +++++++++++++---
 migration/migration.h |  1 +
 qapi-schema.json      |  5 ++++-
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index f588329..66fd81e 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1304,6 +1304,15 @@ bool migrate_use_block(void)
     return s->enabled_capabilities[MIGRATION_CAPABILITY_BLOCK];
 }
 
+bool migrate_use_return_path(void)
+{
+    MigrationState *s;
+
+    s = migrate_get_current();
+
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_RETURN_PATH];
+}
+
 bool migrate_use_block_incremental(void)
 {
     MigrationState *s;
@@ -1968,10 +1977,11 @@ void migrate_fd_connect(MigrationState *s)
     notifier_list_notify(&migration_state_notifiers, s);
 
     /*
-     * Open the return path; currently for postcopy but other things might
-     * also want it.
+     * Open the return path. For postcopy, it is used exclusively. For
+     * precopy, only if user specified "return-path" capability would
+     * QEMU uses the return path.
      */
-    if (migrate_postcopy_ram()) {
+    if (migrate_postcopy_ram() || migrate_use_return_path()) {
         if (open_return_path_on_source(s)) {
             error_report("Unable to open return-path for postcopy");
             migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
diff --git a/migration/migration.h b/migration/migration.h
index d9a268a..e41fb68 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -144,6 +144,7 @@ bool migrate_colo_enabled(void);
 
 bool migrate_use_block(void);
 bool migrate_use_block_incremental(void);
+bool migrate_use_return_path(void);
 
 bool migrate_use_compression(void);
 int migrate_compress_level(void);
diff --git a/qapi-schema.json b/qapi-schema.json
index 4b50b65..9e34cf7 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -900,12 +900,15 @@
 #          offers more flexibility.
 #          (Since 2.10)
 #
+# @return-path: If enabled, migration will use the return path even
+#               for precopy. (since 2.10)
+#
 # Since: 1.2
 ##
 { 'enum': 'MigrationCapability',
   'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
            'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram',
-           'block' ] }
+           'block', 'return-path' ] }
 
 ##
 # @MigrationCapabilityStatus:
-- 
2.7.4

Peter Xu <peterx@redhat.com> wrote:
> When this capability is enabled, QEMU will use the return path even for
> precopy migration. This is helpful at least in one case when destination
> failed to load the image while source quited without confirmation. With
> return path, source will wait for the last response from destination,
> and if destination fails, it'll fail the migration on source, then the
> guest can be run again on the source (rather than assuming to be good,
> then the guest will be lost after source quits).
>
> It needs to be enabled explicitly on source, otherwise disabled.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

* Peter Xu (peterx@redhat.com) wrote:
> When this capability is enabled, QEMU will use the return path even for
> precopy migration. This is helpful at least in one case when destination
> failed to load the image while source quited without confirmation. With
> return path, source will wait for the last response from destination,
> and if destination fails, it'll fail the migration on source, then the
> guest can be run again on the source (rather than assuming to be good,
> then the guest will be lost after source quits).
> 
> It needs to be enabled explicitly on source, otherwise disabled.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

OK, so this looks fine, lets see what else we have.
The migration thread already calls savevm_send_open_return_path
if it's got a from_dst_file.

We already call 'await_return_path_close_on_source' in
migration_completion if we have a from_dst_file  so that should
already work.

and on the incoming side, migration_incoming_state_destroy calls
migrate_send_rp_shut  if it has a to_src_file.  So that should close it
as well.

So that looks like a full set.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.c | 16 +++++++++++++---
>  migration/migration.h |  1 +
>  qapi-schema.json      |  5 ++++-
>  3 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index f588329..66fd81e 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1304,6 +1304,15 @@ bool migrate_use_block(void)
>      return s->enabled_capabilities[MIGRATION_CAPABILITY_BLOCK];
>  }
>  
> +bool migrate_use_return_path(void)
> +{
> +    MigrationState *s;
> +
> +    s = migrate_get_current();
> +
> +    return s->enabled_capabilities[MIGRATION_CAPABILITY_RETURN_PATH];
> +}
> +
>  bool migrate_use_block_incremental(void)
>  {
>      MigrationState *s;
> @@ -1968,10 +1977,11 @@ void migrate_fd_connect(MigrationState *s)
>      notifier_list_notify(&migration_state_notifiers, s);
>  
>      /*
> -     * Open the return path; currently for postcopy but other things might
> -     * also want it.
> +     * Open the return path. For postcopy, it is used exclusively. For
> +     * precopy, only if user specified "return-path" capability would
> +     * QEMU uses the return path.
>       */
> -    if (migrate_postcopy_ram()) {
> +    if (migrate_postcopy_ram() || migrate_use_return_path()) {
>          if (open_return_path_on_source(s)) {
>              error_report("Unable to open return-path for postcopy");
>              migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> diff --git a/migration/migration.h b/migration/migration.h
> index d9a268a..e41fb68 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -144,6 +144,7 @@ bool migrate_colo_enabled(void);
>  
>  bool migrate_use_block(void);
>  bool migrate_use_block_incremental(void);
> +bool migrate_use_return_path(void);
>  
>  bool migrate_use_compression(void);
>  int migrate_compress_level(void);
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 4b50b65..9e34cf7 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -900,12 +900,15 @@
>  #          offers more flexibility.
>  #          (Since 2.10)
>  #
> +# @return-path: If enabled, migration will use the return path even
> +#               for precopy. (since 2.10)
> +#
>  # Since: 1.2
>  ##
>  { 'enum': 'MigrationCapability',
>    'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
>             'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram',
> -           'block' ] }
> +           'block', 'return-path' ] }
>  
>  ##
>  # @MigrationCapabilityStatus:
> -- 
> 2.7.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK