multifd: Send header packet without flags if zero-copy-send is enabled

Since d48c3a0445 ("multifd: Use a single writev on the send side"),
sending the header packet and the memory pages happens in the same
writev, which can potentially make the migration faster.

Using channel-socket as example, this works well with the default copying
mechanism of sendmsg(), but with zero-copy-send=true, it will cause
the migration to often break.

This happens because the header packet buffer gets reused quite often,
and there is a high chance that by the time the MSG_ZEROCOPY mechanism get
to send the buffer, it has already changed, sending the wrong data and
causing the migration to abort.

It means that, as it is, the buffer for the header packet is not suitable
for sending with MSG_ZEROCOPY.

In order to enable zero copy for multifd, send the header packet on an
individual write(), without any flags, and the remanining pages with a
writev(), as it was happening before. This only changes how a migration
with zero-copy-send=true works, not changing any current behavior for
migrations with zero-copy-send=false.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220513062836.965425-8-leobras@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

diff --git a/migration/multifd.c b/migration/multifd.c
index 15fb668e64d0eccd7648dae1af794910315638d0..2541cd232298c0b29c90a50d0fb3a957f4dbb9ce 100644 (file)
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -617,6 +617,7 @@ static void *multifd_send_thread(void *opaque)
     MultiFDSendParams *p = opaque;
     Error *local_err = NULL;
     int ret = 0;
+    bool use_zero_copy_send = migrate_use_zero_copy_send();
 
     trace_multifd_send_thread_start(p->id);
     rcu_register_thread();
@@ -639,9 +640,14 @@ static void *multifd_send_thread(void *opaque)
         if (p->pending_job) {
             uint64_t packet_num = p->packet_num;
             uint32_t flags = p->flags;
-            p->iovs_num = 1;
             p->normal_num = 0;
 
+            if (use_zero_copy_send) {
+                p->iovs_num = 0;
+            } else {
+                p->iovs_num = 1;
+            }
+
             for (int i = 0; i < p->pages->num; i++) {
                 p->normal[p->normal_num] = p->pages->offset[i];
                 p->normal_num++;
@@ -665,8 +671,18 @@ static void *multifd_send_thread(void *opaque)
             trace_multifd_send(p->id, packet_num, p->normal_num, flags,
                                p->next_packet_size);
 
-            p->iov[0].iov_len = p->packet_len;
-            p->iov[0].iov_base = p->packet;
+            if (use_zero_copy_send) {
+                /* Send header first, without zerocopy */
+                ret = qio_channel_write_all(p->c, (void *)p->packet,
+                                            p->packet_len, &local_err);
+                if (ret != 0) {
+                    break;
+                }
+            } else {
+                /* Send header using the same writev call */
+                p->iov[0].iov_len = p->packet_len;
+                p->iov[0].iov_base = p->packet;
+            }
 
             ret = qio_channel_writev_all(p->c, p->iov, p->iovs_num,
                                          &local_err);