Re: [PATCH V4 for-next 2/4] IB/core: Infra-structure to support verbs extensions through uverbs

[Or Gerlitz <ogerlitz-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>]

On 14/08/2013 12:06, Or Gerlitz wrote:
> On 13/08/2013 22:18, Roland Dreier wrote:
>>> >+struct ib_uverbs_cmd_hdr_ex {
>>> >+       __u32 command;
>>> >+       __u16 in_words;
>>> >+       __u16 out_words;
>>> >+       __u16 provider_in_words;
>>> >+       __u16 provider_out_words;
>>> >+       __u32 cmd_hdr_reserved;
>>> >+};
>>> >+
>> If I understand the vague explanations and the analogy to the
>> presentation about userspace, then cmd_hdr_reserved is supposed to be
>> used as a mask field.  Should a kernel that doesn't understand any
>> mask components make sure that this reserved field is 0, and return an
>> error if it isn't?  I don't see any code to do that, and it seems to
>> risk new userspace silently getting wrong answers on an old kernel.
>>
>> Is there any reason not to name it as a mask field from the start?
>
>
> NO.
>
> The last field in the extended uverbs command header isn't for 
> extending the command for which this instance serves as the header.

Its role is to make sure we are aligned to intergral multiplicative of 
64bit as the non extended header.
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