[PATCH v8 1/3] docs: improve qcow2 spec about extending image header

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Make it more obvious how to add new fields to the version 3 header and
how to interpret them.

The specification is adjusted so for new defined optional fields:

1. Software may support some of these optional fields and ignore the
   others, which means that features may be backported to downstream
   Qemu independently.
3. If @header_length is higher than the highest field end that software
   knows, it should assume that topmost unknown additional fields are
   correct, and keep additional unknown fields as is on rewriting the
   image.
3. If we want to add incompatible field (or a field, for which some its
   values would be incompatible), it must be accompanied by
   incompatible feature bit.

Also the concept of "default is zero" is clarified, as it's strange to
say that the value of the field is assumed to be zero for the software
version which don't know about the field at all and don't know how to
treat it be it zero or not.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 docs/interop/qcow2.txt | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index af5711e533..4709f3bb30 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -79,9 +79,9 @@ The first cluster of a qcow2 image contains the file header:
                     Offset into the image file at which the snapshot table
                     starts. Must be aligned to a cluster boundary.
 
-If the version is 3 or higher, the header has the following additional fields.
-For version 2, the values are assumed to be zero, unless specified otherwise
-in the description of a field.
+For version 2, header is always 72 bytes length and finishes here.
+For version 3 or higher the header length is at least 104 bytes and has at
+least next five fields, up to the @header_length field.
 
          72 -  79:  incompatible_features
                     Bitmask of incompatible features. An implementation must
@@ -164,6 +164,26 @@ in the description of a field.
         100 - 103:  header_length
                     Length of the header structure in bytes. For version 2
                     images, the length is always assumed to be 72 bytes.
+                    For version 3 it's at least 104 bytes.
+
+Additional fields (version 3 and higher)
+
+The following fields of the header are optional: if software doesn't know how
+to interpret the field, it may be safely ignored, other than preserving the
+field unchanged when rewriting the image header.
+
+For all additional fields zero value equals to absence of field (absence is
+when field.offset + field.size > @header_length). This implies
+that if software want's to set fields up to some field not aligned to multiply
+of 8 it must align header up by zeroes. And on the other hand, if software
+need some optional field which is absent it should assume that it's value is
+zero.
+
+It's allowed for the header end to cut some field in the middle (in this case
+the field is considered as absent), but in this case the part of the field
+which is covered by @header_length must be zeroed.
+
+        < ... No additional fields in the header currently ... >
 
 Directly after the image header, optional sections called header extensions can
 be stored. Each extension has a structure like the following:
-- 
2.21.0