Am 25.07.2017 um 16:41 hat Peter Lieven geschrieben: > Signed-off-by: Peter Lieven <p...@kamp.de> > --- > docs/interop/qcow2.txt | 51 > +++++++++++++++++++++++++++++++++++++++++++++++++- > roms/ipxe | 2 +- > 2 files changed, 51 insertions(+), 2 deletions(-) > > diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt > index d7fdb1f..d0d2a8f 100644 > --- a/docs/interop/qcow2.txt > +++ b/docs/interop/qcow2.txt > @@ -86,7 +86,12 @@ in the description of a field. > be written to (unless for regaining > consistency). > > - Bits 2-63: Reserved (set to 0) > + Bit 2: Compress format bit. If and only if this bit > + is set then the compress format extension > + MUST be present and MUST be parsed and > checked > + for compatibility. > + > + Bits 3-63: Reserved (set to 0) > > 80 - 87: compatible_features > Bitmask of compatible features. An implementation can > @@ -137,6 +142,7 @@ be stored. Each extension has a structure like the > following: > 0x6803f857 - Feature name table > 0x23852875 - Bitmaps extension > 0x0537be77 - Full disk encryption header pointer > + 0xC03183A3 - Compress format extension > other - Unknown header extension, can be safely > ignored > > @@ -311,6 +317,49 @@ The algorithms used for encryption vary depending on the > method > in the LUKS header, with the physical disk sector as the > input tweak. > > + > +== Compress format extension == > + > +The compress format extension is an optional header extension. It provides > +the ability to specify the compress algorithm and compress parameters > +that are used for compressed clusters. This new header MUST be present if > +the incompatible-feature bit "compress format bit" is set and MUST be absent > +otherwise. > + > +The fields of the compress format extension are: > + > + Byte 0 - 13: compress_format_name (padded with zeros, but not > + necessarily null terminated if it has full length). > + Valid compression format names currently are: > + > + deflate: Standard zlib deflate compression without > + compression header > + > + 14: compress_level (uint8_t) > + > + 0 = default compress level (valid for all formats, > default) > + > + Additional valid compression levels for deflate > compression: > + > + All values between 1 and 9. 1 gives best speed, 9 gives > best > + compression. The default compression level is defined by > zlib > + and currently defaults to 6. > + > + 15: compress_window_size (uint8_t) > + > + Window or dictionary size used by the compression format. > + Currently only used by the deflate compression algorithm. > + > + Valid window sizes for deflate compression range from 8 to > + 15 inclusively.
So what is the plan with respect to adding new compression algorithms? If I understand correctly, we'll use the same extension type (0xC03183A3) and a different compress_format_name. However, the other algorithm will likely have different options and also a different number of options. Will the meaning of bytes 14-end then depend on the specific algorithm? Part of why I'm asking this is because I wonder why compress_format_name is 14 characters. It looks to me as if that is intended to make the header a round 16 bytes for the deflate algorithm. But unless we say "16 bits ought to be enough for every algorithm", this is just optimising a special case. (Or actually not optimising, but just moving the padding from the end of the header extension to padding inside the compress_format_name field.) Wouldn't 8 bytes still be plenty of space for a format name, and look more natural? Then any format that requires options would have a little more space without immediately going to a 24 byte header extension. Kevin