[PATCH v1 2/2] migration/multifd: Warn user when zerocopy not working

[Leonardo Bras]

Some errors, like the lack of Scatter-Gather support by the network
interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
zero-copy, which causes it to fall back to the default copying mechanism.

After each full dirty-bitmap scan there should be a zero-copy flush
happening, which checks for errors each of the previous calls to
sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
warn the user about it.

Since it happens once each full dirty-bitmap scan, even in worst case
scenario it should not print a lot of warnings, and will allow tracking
how many dirty-bitmap iterations were not able to use zero-copy send.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
 migration/multifd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/migration/multifd.c b/migration/multifd.c
index 684c014c86..9c62aec84e 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -624,6 +624,9 @@ int multifd_send_sync_main(QEMUFile *f)
             if (ret < 0) {
                 error_report_err(err);
                 return -1;
+            } else if (ret == 1) {
+                warn_report("The network device is not able to use "
+                            "zero-copy-send: copying is being used");
             }
         }
     }
-- 
2.36.1