2 files changed, 24 insertions, 6 deletions
diff --git a/docs/devel/migration/main.rst b/docs/devel/migration/main.rst
index 495cdcb112..784c899dca 100644
--- a/docs/devel/migration/main.rst
+++ b/docs/devel/migration/main.rst
@@ -47,11 +47,25 @@ over any transport.
   QEMU interference. Note that QEMU does not flush cached file
   data/metadata at the end of migration.
 
-In addition, support is included for migration using RDMA, which
-transports the page data using ``RDMA``, where the hardware takes care of
-transporting the pages, and the load on the CPU is much lower.  While the
-internals of RDMA migration are a bit different, this isn't really visible
-outside the RAM migration code.
+  The file migration also supports using a file that has already been
+  opened. A set of file descriptors is passed to QEMU via an "fdset"
+  (see add-fd QMP command documentation). This method allows a
+  management application to have control over the migration file
+  opening operation. There are, however, strict requirements to this
+  interface if the multifd capability is enabled:
+
+    - the fdset must contain two file descriptors that are not
+      duplicates between themselves;
+    - if the direct-io capability is to be used, exactly one of the
+      file descriptors must have the O_DIRECT flag set;
+    - the file must be opened with WRONLY on the migration source side
+      and RDONLY on the migration destination side.
+
+- rdma migration: support is included for migration using RDMA, which
+  transports the page data using ``RDMA``, where the hardware takes
+  care of transporting the pages, and the load on the CPU is much
+  lower.  While the internals of RDMA migration are a bit different,
+  this isn't really visible outside the RAM migration code.
 
 All these migration protocols use the same infrastructure to
 save/restore state devices.  This infrastructure is shared with the
diff --git a/docs/devel/migration/mapped-ram.rst b/docs/devel/migration/mapped-ram.rst
index fa4cefd9fc..d352b546e9 100644
--- a/docs/devel/migration/mapped-ram.rst
+++ b/docs/devel/migration/mapped-ram.rst
@@ -16,7 +16,7 @@ location in the file, rather than constantly being added to a
 sequential stream. Having the pages at fixed offsets also allows the
 usage of O_DIRECT for save/restore of the migration stream as the
 pages are ensured to be written respecting O_DIRECT alignment
-restrictions (direct-io support not yet implemented).
+restrictions.
 
 Usage
 -----
@@ -35,6 +35,10 @@ Use a ``file:`` URL for migration:
 Mapped-ram migration is best done non-live, i.e. by stopping the VM on
 the source side before migrating.
 
+For best performance enable the ``direct-io`` parameter as well:
+
+    ``migrate_set_parameter direct-io on``
+
 Use-cases
 ---------