Re: [PATCH v10 1/2] docs: improve qcow2 spec about extending image header

[Eric Blake <eblake@redhat.com>]

On 1/20/20 11:13 AM, Vladimir Sementsov-Ogievskiy wrote:
> 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:

s/so for/so that for/

> 
> 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. 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 | 44 +++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index af5711e533..355925c35e 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, the header is exactly 72 bytes in length, and finishes here.
> +For version 3 or higher, the header length is at least 104 bytes, including
> +the next fields through header_length.
>   
>            72 -  79:  incompatible_features
>                       Bitmask of incompatible features. An implementation must
> @@ -164,6 +164,44 @@ 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 and must be a multiple
> +                    of 8.
> +
> +
> +=== Additional fields (version 3 and higher) ===
> +
> +In general, these fields are optional and may be safely ignored by the software,
> +as well as filled by zeros (which is equal to field absence), if software needs

We're inconsistent on 'zeros' (git grep has 201 hits) vs. 'zeroes' (688 
hits); I prefer the latter, but won't object if you don't tweak it since 
this is the first use of either spelling in qcow2.txt.

> +to set field B, but does not care about field A, which precedes B. More

s/A, which/A which/

> +formally, additional fields have the following compatibility rules:
> +
> +1. If the value of the additional field must not be ignored for correct
> +handling of the file, it will be accompanied by a corresponding incompatible
> +feature bit.
> +
> +2. If there are no unrecognized incompatible feature bits set, an unknown
> +additional field may be safely ignored other than preserving its value when
> +rewriting the image header.
> +
> +3. An explicit value of 0 will have the same behavior as when the field is not
> +present*, if not altered by a specific incompatible bit.
> +
> +*. A field is considered not present when header_length is less or equal to the

s/less/less than/

> +field's offset. Also, all additional fields are not present for version 2.
> +
> +        < ... No additional fields in the header currently ... >
> +
> +
> +=== Header padding ===
> +
> +@header_length must be a multiple of 8, which means that if the end of the last
> +additional field is not aligned, some padding is needed. This padding must be
> +zeroed, so that, if some existing (or future) additional field will fall into

s/that, if/that if/

> +the padding, it will be interpreted accordingly to point [3.] of the previous
> +paragraph, i.e.  in the same manner as when this field is not present.
> +
> +
> +=== Header extensions ===
>   
>   Directly after the image header, optional sections called header extensions can
>   be stored. Each extension has a structure like the following:
> 

We're down to few enough grammar nits that I'm happy with:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org