3 files changed, 79 insertions, 9 deletions

diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
index 66e741f649..b5d08d217d 100644
--- a/include/migration/qemu-file.h
+++ b/include/migration/qemu-file.h
@@ -89,6 +89,11 @@ typedef size_t (QEMURamSaveFunc)(QEMUFile *f, void *opaque,
                                uint64_t *bytes_sent);
 
 /*
+ * Return a QEMUFile for comms in the opposite direction
+ */
+typedef QEMUFile *(QEMURetPathFunc)(void *opaque);
+
+/*
  * Stop any read or write (depending on flags) on the underlying
  * transport on the QEMUFile.
  * Existing blocking reads/writes must be woken
@@ -106,6 +111,7 @@ typedef struct QEMUFileOps {
     QEMURamHookFunc *after_ram_iterate;
     QEMURamHookFunc *hook_ram_load;
     QEMURamSaveFunc *save_page;
+    QEMURetPathFunc *get_return_path;
     QEMUFileShutdownFunc *shut_down;
 } QEMUFileOps;
 
@@ -196,6 +202,7 @@ int64_t qemu_file_get_rate_limit(QEMUFile *f);
 int qemu_file_get_error(QEMUFile *f);
 void qemu_file_set_error(QEMUFile *f, int ret);
 int qemu_file_shutdown(QEMUFile *f);
+QEMUFile *qemu_file_get_return_path(QEMUFile *f);
 void qemu_fflush(QEMUFile *f);
 void qemu_file_set_blocking(QEMUFile *f, bool block);
 
diff --git a/migration/qemu-file-unix.c b/migration/qemu-file-unix.c
index 809bf070d7..bcb744bf54 100644
--- a/migration/qemu-file-unix.c
+++ b/migration/qemu-file-unix.c
@@ -97,6 +97,56 @@ static int socket_shutdown(void *opaque, bool rd, bool wr)
     }
 }
 
+static int socket_return_close(void *opaque)
+{
+    QEMUFileSocket *s = opaque;
+    /*
+     * Note: We don't close the socket, that should be done by the forward
+     * path.
+     */
+    g_free(s);
+    return 0;
+}
+
+static const QEMUFileOps socket_return_read_ops = {
+    .get_fd          = socket_get_fd,
+    .get_buffer      = socket_get_buffer,
+    .close           = socket_return_close,
+    .shut_down       = socket_shutdown,
+};
+
+static const QEMUFileOps socket_return_write_ops = {
+    .get_fd          = socket_get_fd,
+    .writev_buffer   = socket_writev_buffer,
+    .close           = socket_return_close,
+    .shut_down       = socket_shutdown,
+};
+
+/*
+ * Give a QEMUFile* off the same socket but data in the opposite
+ * direction.
+ */
+static QEMUFile *socket_get_return_path(void *opaque)
+{
+    QEMUFileSocket *forward = opaque;
+    QEMUFileSocket *reverse;
+
+    if (qemu_file_get_error(forward->file)) {
+        /* If the forward file is in error, don't try and open a return */
+        return NULL;
+    }
+
+    reverse = g_malloc0(sizeof(QEMUFileSocket));
+    reverse->fd = forward->fd;
+    /* I don't think there's a better way to tell which direction 'this' is */
+    if (forward->file->ops->get_buffer != NULL) {
+        /* being called from the read side, so we need to be able to write */
+        return qemu_fopen_ops(reverse, &socket_return_write_ops);
+    } else {
+        return qemu_fopen_ops(reverse, &socket_return_read_ops);
+    }
+}
+
 static ssize_t unix_writev_buffer(void *opaque, struct iovec *iov, int iovcnt,
                                   int64_t pos)
 {
@@ -206,18 +256,19 @@ QEMUFile *qemu_fdopen(int fd, const char *mode)
 }
 
 static const QEMUFileOps socket_read_ops = {
-    .get_fd     = socket_get_fd,
-    .get_buffer = socket_get_buffer,
-    .close      = socket_close,
-    .shut_down  = socket_shutdown
-
+    .get_fd          = socket_get_fd,
+    .get_buffer      = socket_get_buffer,
+    .close           = socket_close,
+    .shut_down       = socket_shutdown,
+    .get_return_path = socket_get_return_path
 };
 
 static const QEMUFileOps socket_write_ops = {
-    .get_fd        = socket_get_fd,
-    .writev_buffer = socket_writev_buffer,
-    .close         = socket_close,
-    .shut_down     = socket_shutdown
+    .get_fd          = socket_get_fd,
+    .writev_buffer   = socket_writev_buffer,
+    .close           = socket_close,
+    .shut_down       = socket_shutdown,
+    .get_return_path = socket_get_return_path
 };
 
 QEMUFile *qemu_fopen_socket(int fd, const char *mode)
diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index 9ec22678a8..0bbd2574a8 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -44,6 +44,18 @@ int qemu_file_shutdown(QEMUFile *f)
     return f->ops->shut_down(f->opaque, true, true);
 }
 
+/*
+ * Result: QEMUFile* for a 'return path' for comms in the opposite direction
+ *         NULL if not available
+ */
+QEMUFile *qemu_file_get_return_path(QEMUFile *f)
+{
+    if (!f->ops->get_return_path) {
+        return NULL;
+    }
+    return f->ops->get_return_path(f->opaque);
+}
+
 bool qemu_file_mode_is_not_valid(const char *mode)
 {
     if (mode == NULL ||