[PULL 29/29] migration: Avoid false-positive on non-supported scenarios for zero-copy-send

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Leonardo Bras <leobras@redhat.com>

Migration with zero-copy-send currently has it's limitations, as it can't
be used with TLS nor any kind of compression. In such scenarios, it should
output errors during parameter / capability setting.

But currently there are some ways of setting this not-supported scenarios
without printing the error message:

!) For 'compression' capability, it works by enabling it together with
zero-copy-send. This happens because the validity test for zero-copy uses
the helper unction migrate_use_compression(), which check for compression
presence in s->enabled_capabilities[MIGRATION_CAPABILITY_COMPRESS].

The point here is: the validity test happens before the capability gets
enabled. If all of them get enabled together, this test will not return
error.

In order to fix that, replace migrate_use_compression() by directly testing
the cap_list parameter migrate_caps_check().

2) For features enabled by parameters such as TLS & 'multifd_compression',
there was also a possibility of setting non-supported scenarios: setting
zero-copy-send first, then setting the unsupported parameter.

In order to fix that, also add a check for parameters conflicting with
zero-copy-send on migrate_params_check().

3) XBZRLE is also a compression capability, so it makes sense to also add
it to the list of capabilities which are not supported with zero-copy-send.

Fixes: 1abaec9a1b2c ("migration: Change zero_copy_send from migration parameter to migration capability")
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Message-Id: <20220719122345.253713-1-leobras@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 migration/migration.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/migration/migration.c b/migration/migration.c
index 15ae48b209..e03f698a3c 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1306,7 +1306,9 @@ static bool migrate_caps_check(bool *cap_list,
 #ifdef CONFIG_LINUX
     if (cap_list[MIGRATION_CAPABILITY_ZERO_COPY_SEND] &&
         (!cap_list[MIGRATION_CAPABILITY_MULTIFD] ||
-         migrate_use_compression() ||
+         cap_list[MIGRATION_CAPABILITY_COMPRESS] ||
+         cap_list[MIGRATION_CAPABILITY_XBZRLE] ||
+         migrate_multifd_compression() ||
          migrate_use_tls())) {
         error_setg(errp,
                    "Zero copy only available for non-compressed non-TLS multifd migration");
@@ -1550,6 +1552,17 @@ static bool migrate_params_check(MigrationParameters *params, Error **errp)
         error_prepend(errp, "Invalid mapping given for block-bitmap-mapping: ");
         return false;
     }
+
+#ifdef CONFIG_LINUX
+    if (migrate_use_zero_copy_send() &&
+        ((params->has_multifd_compression && params->multifd_compression) ||
+         (params->has_tls_creds && params->tls_creds && *params->tls_creds))) {
+        error_setg(errp,
+                   "Zero copy only available for non-compressed non-TLS multifd migration");
+        return false;
+    }
+#endif
+
     return true;
 }
 
-- 
2.36.1