migration/multifd: Add mapped-ram support to fd: URI

If we receive a file descriptor that points to a regular file, there's
nothing stopping us from doing multifd migration with mapped-ram to
that file.

Enable the fd: URI to work with multifd + mapped-ram.

Note that the fds passed into multifd are duplicated because we want
to avoid cross-thread effects when doing cleanup (i.e. close(fd)). The
original fd doesn't need to be duplicated because monitor_get_fd()
transfers ownership to the caller.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20240229153017.2221-23-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/fd.c

@@ -15,18 +15,41 @@
  */
 
 #include "qemu/osdep.h"
+#include "qapi/error.h"
 #include "channel.h"
 #include "fd.h"
 #include "migration.h"
 #include "monitor/monitor.h"
+#include "io/channel-file.h"
 #include "io/channel-util.h"
+#include "options.h"
 #include "trace.h"
 
 
+static struct FdOutgoingArgs {
+    int fd;
+} outgoing_args;
+
+int fd_args_get_fd(void)
+{
+    return outgoing_args.fd;
+}
+
+void fd_cleanup_outgoing_migration(void)
+{
+    if (outgoing_args.fd > 0) {
+        close(outgoing_args.fd);
+        outgoing_args.fd = -1;
+    }
+}
+
 void fd_start_outgoing_migration(MigrationState *s, const char *fdname, Error **errp)
 {
     QIOChannel *ioc;
     int fd = monitor_get_fd(monitor_cur(), fdname, errp);
+
+    outgoing_args.fd = -1;
+
     if (fd == -1) {
         return;
     }
@@ -38,6 +61,8 @@ void fd_start_outgoing_migration(MigrationState *s, const char *fdname, Error **
         return;
     }
 
+    outgoing_args.fd = fd;
+
     qio_channel_set_name(ioc, "migration-fd-outgoing");
     migration_channel_connect(s, ioc, NULL, NULL);
     object_unref(OBJECT(ioc));
@@ -73,4 +98,23 @@ void fd_start_incoming_migration(const char *fdname, Error **errp)
                                fd_accept_incoming_migration,
                                NULL, NULL,
                                g_main_context_get_thread_default());
+
+    if (migrate_multifd()) {
+        int channels = migrate_multifd_channels();
+
+        while (channels--) {
+            ioc = QIO_CHANNEL(qio_channel_file_new_fd(dup(fd)));
+
+            if (QIO_CHANNEL_FILE(ioc)->fd == -1) {
+                error_setg(errp, "Failed to duplicate fd %d", fd);
+                return;
+            }
+
+            qio_channel_set_name(ioc, "migration-fd-incoming");
+            qio_channel_add_watch_full(ioc, G_IO_IN,
+                                       fd_accept_incoming_migration,
+                                       NULL, NULL,
+                                       g_main_context_get_thread_default());
+        }
+    }
 }

migration/fd.h

@@ -20,4 +20,6 @@ void fd_start_incoming_migration(const char *fdname, Error **errp);
 
 void fd_start_outgoing_migration(MigrationState *s, const char *fdname,
                                  Error **errp);
+void fd_cleanup_outgoing_migration(void);
+int fd_args_get_fd(void);
 #endif

migration/file.c

@@ -11,6 +11,7 @@
 #include "qemu/error-report.h"
 #include "qapi/error.h"
 #include "channel.h"
+#include "fd.h"
 #include "file.h"
 #include "migration.h"
 #include "io/channel-file.h"
@@ -53,15 +54,20 @@ bool file_send_channel_create(gpointer opaque, Error **errp)
 {
     QIOChannelFile *ioc;
     int flags = O_WRONLY;
-    bool ret = true;
+    bool ret = false;
+    int fd = fd_args_get_fd();
 
-    ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, errp);
-    if (!ioc) {
-        ret = false;
-        goto out;
+    if (fd && fd != -1) {
+        ioc = qio_channel_file_new_fd(dup(fd));
+    } else {
+        ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, errp);
+        if (!ioc) {
+            goto out;
+        }
     }
 
     multifd_channel_connect(opaque, QIO_CHANNEL(ioc));
+    ret = true;
 
 out:
     /*

migration/migration.c

@@ -140,6 +140,10 @@ static bool transport_supports_multi_channels(MigrationAddress *addr)
     if (addr->transport == MIGRATION_ADDRESS_TYPE_SOCKET) {
         SocketAddress *saddr = &addr->u.socket;
 
+        if (saddr->type == SOCKET_ADDRESS_TYPE_FD) {
+            return migrate_mapped_ram();
+        }
+
         return (saddr->type == SOCKET_ADDRESS_TYPE_INET ||
                 saddr->type == SOCKET_ADDRESS_TYPE_UNIX ||
                 saddr->type == SOCKET_ADDRESS_TYPE_VSOCK);

migration/multifd.c

@@ -17,6 +17,7 @@
 #include "exec/ramblock.h"
 #include "qemu/error-report.h"
 #include "qapi/error.h"
+#include "fd.h"
 #include "file.h"
 #include "migration.h"
 #include "migration-stats.h"
@@ -731,6 +732,7 @@ static bool multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
 static void multifd_send_cleanup_state(void)
 {
     file_cleanup_outgoing_migration();
+    fd_cleanup_outgoing_migration();
     socket_cleanup_outgoing_migration();
     qemu_sem_destroy(&multifd_send_state->channels_created);
     qemu_sem_destroy(&multifd_send_state->channels_ready);