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Re: [Xen-devel] Empty struct in public headers Was: Re: [PATCH for Xen 4.5] xen/arm: Add support for GICv3 for domU



On 11/18/2014 04:21 PM, Ian Campbell wrote:
> On Tue, 2014-11-18 at 15:49 +0000, Julien Grall wrote:
>> (Rename the mail and strip the cc list)
>>
>> On 11/18/2014 03:35 PM, Ian Jackson wrote:
>>> Julien Grall writes ("Re: [PATCH for Xen 4.5] xen/arm: Add support for 
>>> GICv3 for domU"):
>>>> On 11/18/2014 03:10 PM, Ian Jackson wrote:
>>>>> Empty structs are a gcc extension (`(gcc-4.4) Empty Structures').  I
>>>>> would be very surprised if clang didn't support them too.
>>>>
>>>> AFAIK, clang doesn't complain about empty structures.
>>>
>>> Right.
>>>
>>>>> AIUI our policy, gcc extensions are fine except in the Xen public
>>>>> headers.
>>>>
>>>> We have at least 2 "empty" structure on the ARM public header.
>>>
>>> That ought to be fixed, in case anyone ever wants to build ARM guests
>>> with Norcroft C or something.
>>>
>>> Does the size of these structs matter ?
>>
>> The 2 structures are arch_vcpu_info and arch_shared_info.
>>
>> They are used only at the end of the structure vcpu_info (resp.
>> shared_info). So I guess we could fix it?
> 
> arch_vcpu_info isn't at the end of vcpu_info (vcpu_time_info follows it)
> and also vcpu_info is part of an array at the start of shared_info (an
> array of 1 on ARM, but things still follow it). I'm also not sure of the
> impact on the vcpu placement hypercall or the uses of it.

Oops, right. I looked too quickly to the code, sorry.

> So it looks like changing vcpu_info at least will be hard/impossible. If
> we want rid of these empty structs then I think an ifdef at the point of
> use is the only option :-(

I will send a patch to ifdef arch_vcpu and arch_shared_info when Xen 4.6
windows will be opened.

Regards,

-- 
Julien Grall

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