qcow2: Add incompatibility note between backing files and raw external data files

Backing files and raw external data files are mutually exclusive.
The documentation of the raw external data bit (in autoclear_features)
already indicates that, but we should also mention it on the other
side.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200410121816.8334-1-berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 640e0eca4000587baf91fe25485071fc00ebcb4a..298a0313103e643cd49a201735e6ed9e4a227f84 100644 (file)
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -25,6 +25,9 @@ The first cluster of a qcow2 image contains the file header:
                     is stored (NB: The string is not null terminated). 0 if the
                     image doesn't have a backing file.
 
+                    Note: backing files are incompatible with raw external data
+                    files (auto-clear feature bit 1).
+
          16 - 19:   backing_file_size
                     Length of the backing file name in bytes. Must not be
                     longer than 1023 bytes. Undefined if the image doesn't have