Re: [PATCH v8 2/3] docs: define padding for qcow2 header

[Vladimir Sementsov-Ogievskiy]

18.10.2019 17:02, Eric Blake wrote:
> On 10/18/19 4:47 AM, Vladimir Sementsov-Ogievskiy wrote:
>> Header extensions ends are already defined to be multiply of 8. Let's
>> gently ask for header length to be a multiply of 8 too, when we have
>> some additional fields. Requiring this may be considered as an
>> incompatible change, so the padding is optional. Actually, padding is
>> allowed before this patch (due to definition of additional fields),
>> the only actual change is "SHOULD" word.
> 
> Too weak. I've already argued that this should be mandatory, and that we are 
> not breaking backwards compatibility, but merely clarifying what has already 
> been implicit by the fact that header extensions are required to be 8-byte 
> size multiple (which makes no sense unless they are also 8-byte aligned).

OK

> 
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> ---
>>   docs/interop/qcow2.txt | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
>> index 4709f3bb30..b971e59b1a 100644
>> --- a/docs/interop/qcow2.txt
>> +++ b/docs/interop/qcow2.txt
>> @@ -185,6 +185,11 @@ which is covered by @header_length must be zeroed.
>>           < ... No additional fields in the header currently ... >
>> +Header padding
>> +        If @header_length is larger than 104, software SHOULD make it a
>> +        multiply of 8, adding zero-padding after additional fields. Still 
>> the
>> +        padding is optional and may be absent in the image.
>> +
>>   Directly after the image header, optional sections called header 
>> extensions can
>>   be stored. Each extension has a structure like the following:
>>
> 


-- 
Best regards,
Vladimir