[PATCH 2/5] migration/postcopy: magic value for postcopy channel

[Prasad Pandit <ppandit@redhat.com>]

From: Prasad Pandit <pjp@fedoraproject.org>

During migration, the precopy and the multifd channels
send magic value (4-bytes) to the destination side,
for it to identify the channel and properly establish
connection. But Postcopy channel did not send such value.

Introduce a magic value to be sent on the postcopy channel.
It helps to identify channels when both multifd and postcopy
migration are enabled together. An explicitly defined magic
value makes code easier to follow, because it is consistent
with the other channels.

Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
---
 migration/postcopy-ram.c | 7 +++++++
 migration/postcopy-ram.h | 3 +++
 2 files changed, 10 insertions(+)

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 83f6160a36..5bc19b541c 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -1580,6 +1580,9 @@ void postcopy_unregister_shared_ufd(struct PostCopyFD *pcfd)
 
 void postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file)
 {
+    if (mis->postcopy_qemufile_dst) {
+        return;
+    }
     /*
      * The new loading channel has its own threads, so it needs to be
      * blocked too.  It's by default true, just be explicit.
@@ -1612,6 +1615,10 @@ postcopy_preempt_send_channel_done(MigrationState *s,
      * postcopy_qemufile_src to know whether it failed or not.
      */
     qemu_sem_post(&s->postcopy_qemufile_src_sem);
+
+    /* Send magic value to identify postcopy channel on the destination */
+    uint32_t magic = cpu_to_be32(POSTCOPY_MAGIC);
+    qio_channel_write_all(ioc, (char *)&magic, sizeof(magic), NULL);
 }
 
 static void
diff --git a/migration/postcopy-ram.h b/migration/postcopy-ram.h
index a6df1b2811..49e2982558 100644
--- a/migration/postcopy-ram.h
+++ b/migration/postcopy-ram.h
@@ -15,6 +15,9 @@
 
 #include "qapi/qapi-types-migration.h"
 
+/* Magic value to identify postcopy channel on the destination */
+#define POSTCOPY_MAGIC  0x55667788U
+
 /* Return true if the host supports everything we need to do postcopy-ram */
 bool postcopy_ram_supported_by_host(MigrationIncomingState *mis,
                                     Error **errp);
-- 
2.47.0