Make it more obvious how to add new fields to the version 3 header and how to interpret them.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@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..3f2855593f 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 @@ -165,6 +165,26 @@ in the description of a field. Length of the header structure in bytes. For version 2 images, the length is always assumed to be 72 bytes. +Additional fields (version 3 and higher) + +The following fields of the header are optional: if software don't know how to +interpret the field, it may safely ignore it. Still the field must be kept as is +when rewriting the image. @header_length must be bound to the end of one of +these fields (or to @header_length field end itself, to be 104 bytes). +This definition implies the following: +1. Software may support some of these optional fields and ignore the others, + which means that features may be backported to downstream Qemu independently. +2. Software may check @header_length, if it knows optional fields specification + enough (knows about the field which exceeds @header_length). +3. If @header_length is higher than the highest field end that software knows, + it should assume that additional fields are correct, @header_length is + correct and keep @header_length and 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. + + < ... 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