[PULL 27/29] migration/multifd: Report to user when zerocopy not working

[Dr. David Alan Gilbert (git)]

From: Leonardo Bras <leobras@redhat.com>

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
increment dirty_sync_missed_zero_copy migration stat to let the user know
about it.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220711211112.18951-4-leobras@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 migration/multifd.c | 2 ++
 migration/ram.c     | 5 +++++
 migration/ram.h     | 2 ++
 3 files changed, 9 insertions(+)

diff --git a/migration/multifd.c b/migration/multifd.c
index 1e49594b02..586ddc9d65 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -624,6 +624,8 @@ int multifd_send_sync_main(QEMUFile *f)
             if (ret < 0) {
                 error_report_err(err);
                 return -1;
+            } else if (ret == 1) {
+                dirty_sync_missed_zero_copy();
             }
         }
     }
diff --git a/migration/ram.c b/migration/ram.c
index 4fbad74c6c..b94669ba5d 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -434,6 +434,11 @@ static void ram_transferred_add(uint64_t bytes)
     ram_counters.transferred += bytes;
 }
 
+void dirty_sync_missed_zero_copy(void)
+{
+    ram_counters.dirty_sync_missed_zero_copy++;
+}
+
 /* used by the search for pages to send */
 struct PageSearchStatus {
     /* Current block being searched */
diff --git a/migration/ram.h b/migration/ram.h
index 5d90945a6e..c7af65ac74 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -89,4 +89,6 @@ void ram_write_tracking_prepare(void);
 int ram_write_tracking_start(void);
 void ram_write_tracking_stop(void);
 
+void dirty_sync_missed_zero_copy(void);
+
 #endif
-- 
2.36.1