migration/multifd: Add a compat property for TLS termination

We're currently changing the way the source multifd migration handles
the shutdown of the multifd channels when TLS is in use to perform a
clean termination by calling gnutls_bye().

Older src QEMUs will always close the channel without terminating the
TLS session. New dst QEMUs treat an unclean termination as an error.

Add multifd_clean_tls_termination (default true) that can be switched
on the destination whenever a src QEMU <= 9.2 is in use.

(Note that the compat property is only strictly necessary for src
QEMUs older than 9.1. Due to synchronization coincidences, src QEMUs
9.1 and 9.2 can put the destination in a condition where it doesn't
see the unclean termination. Still, make the property more inclusive
to facilitate potential backports.)

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

hw/core/machine.c

@@ -42,6 +42,7 @@ GlobalProperty hw_compat_9_2[] = {
     { "virtio-balloon-pci-transitional", "vectors", "0" },
     { "virtio-balloon-pci-non-transitional", "vectors", "0" },
     { "virtio-mem-pci", "vectors", "0" },
+    { "migration", "multifd-clean-tls-termination", "false" },
 };
 const size_t hw_compat_9_2_len = G_N_ELEMENTS(hw_compat_9_2);
 

migration/migration.h

@@ -443,6 +443,39 @@ struct MigrationState {
      * Default value is false. (since 8.1)
      */
     bool multifd_flush_after_each_section;
+
+    /*
+     * This variable only makes sense when set on the machine that is
+     * the destination of a multifd migration with TLS enabled. It
+     * affects the behavior of the last send->recv iteration with
+     * regards to termination of the TLS session.
+     *
+     * When set:
+     *
+     * - the destination QEMU instance can expect to never get a
+     *   GNUTLS_E_PREMATURE_TERMINATION error. Manifested as the error
+     *   message: "The TLS connection was non-properly terminated".
+     *
+     * When clear:
+     *
+     * - the destination QEMU instance can expect to see a
+     *   GNUTLS_E_PREMATURE_TERMINATION error in any multifd channel
+     *   whenever the last recv() call of that channel happens after
+     *   the source QEMU instance has already issued shutdown() on the
+     *   channel.
+     *
+     *   Commit 637280aeb2 (since 9.1) introduced a side effect that
+     *   causes the destination instance to not be affected by the
+     *   premature termination, while commit 1d457daf86 (since 10.0)
+     *   causes the premature termination condition to be once again
+     *   reachable.
+     *
+     * NOTE: Regardless of the state of this option, a premature
+     * termination of the TLS connection might happen due to error at
+     * any moment prior to the last send->recv iteration.
+     */
+    bool multifd_clean_tls_termination;
+
     /*
      * This decides the size of guest memory chunk that will be used
      * to track dirty bitmap clearing.  The size of memory chunk will

migration/multifd.c

@@ -1151,6 +1151,7 @@ void multifd_recv_sync_main(void)
 
 static void *multifd_recv_thread(void *opaque)
 {
+    MigrationState *s = migrate_get_current();
     MultiFDRecvParams *p = opaque;
     Error *local_err = NULL;
     bool use_packets = multifd_use_packets();
@@ -1159,18 +1160,27 @@ static void *multifd_recv_thread(void *opaque)
     trace_multifd_recv_thread_start(p->id);
     rcu_register_thread();
 
+    if (!s->multifd_clean_tls_termination) {
+        p->read_flags = QIO_CHANNEL_READ_FLAG_RELAXED_EOF;
+    }
+
     while (true) {
         uint32_t flags = 0;
         bool has_data = false;
         p->normal_num = 0;
 
         if (use_packets) {
+            struct iovec iov = {
+                .iov_base = (void *)p->packet,
+                .iov_len = p->packet_len
+            };
+
             if (multifd_recv_should_exit()) {
                 break;
             }
 
-            ret = qio_channel_read_all_eof(p->c, (void *)p->packet,
-                                           p->packet_len, &local_err);
+            ret = qio_channel_readv_full_all_eof(p->c, &iov, 1, NULL, NULL,
+                                                 p->read_flags, &local_err);
             if (!ret) {
                 /* EOF */
                 assert(!local_err);

migration/multifd.h

@@ -244,6 +244,8 @@ typedef struct {
     uint32_t zero_num;
     /* used for de-compression methods */
     void *compress_data;
+    /* Flags for the QIOChannel */
+    int read_flags;
 } MultiFDRecvParams;
 
 typedef struct {

migration/options.c

@@ -99,6 +99,8 @@ const Property migration_properties[] = {
                       clear_bitmap_shift, CLEAR_BITMAP_SHIFT_DEFAULT),
     DEFINE_PROP_BOOL("x-preempt-pre-7-2", MigrationState,
                      preempt_pre_7_2, false),
+    DEFINE_PROP_BOOL("multifd-clean-tls-termination", MigrationState,
+                     multifd_clean_tls_termination, true),
 
     /* Migration parameters */
     DEFINE_PROP_UINT8("x-throttle-trigger-threshold", MigrationState,