Re: [PATCH v14] This is the ABI for the two halves of a para-virtualized sound driver to communicate with each to other.

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 11/30/2016 10:45 AM, Jan Beulich wrote:
>>>> On 29.11.16 at 19:44, <andr2000@gmail.com> wrote:
>> On 11/29/2016 08:30 PM, Dario Faggioli wrote:
>>> On Tue, 2016-11-29 at 19:27 +0200, Oleksandr Andrushchenko wrote:
>>>> On 11/29/2016 07:05 PM, Jan Beulich wrote:
>>>>> If you document it as padding, you can't easily use it later on for
>>>>> some extension.
>>>> Why not? I would be more careful about reserved, rather than padding.
>>>> Reserved means that it might be used for something, but padding at
>>>> the
>>>> end of the structure (clearly?) says it was added just to align the
>>>> size of
>>>> this structure and most probably is not used
>>>>
>>> I think that's exactly the point. Padding must be zeroed and can
>>> (should?) be checked to be zero.
>>>
>>> That means that if, say in 2 years time, we want to support a new fancy
>>> feature being introduced in sound cards, and that requires adding a new
>>> field in the struct, we can't use these 27 bytes, because we can't set
>>> them to anything else than a bunch of 0s.
>>>
>>> In fact, if you use them in frontend, and happen to speak with a
>>> backend that does not support the extension and enforces the padding to
>>> be 0, you're doomed. :-/
>>>
>>> OTOH, if you say reserved, neither of the endpoints is authorized to
>>> assume anything about the content of that area. Therefore:
>>> 1) you can (with some care) use it for extensions
>>> 2) if you do that in a frontend, even when speaking with a backend that
>>> does not support the extension, it will just ignore the new content
>>> (which is still just reserved space for him), and won't crash the
>>> communication
>>>
>>> Hope this is both correct and clear. :-)
>> Indeed, it does sound reasonable
>> Then, should I turn all paddings in all structures into reserved?
> Yes, I think so.
ok
>> Also, should I remove this:
>> "All reserved and padding fields in the structures below must be 0."
> Definitely not.
ok
> Jan
>


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