Re: [Xen-devel] Should we revert "mm: New XENMEM space, XENMAPSPACE_gmfn_range"?

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.08.12 at 11:45, Attilio Rao <attilio.rao@xxxxxxxxxx> wrote:
> On 02/08/12 10:23, Jan Beulich wrote:
>>>>> On 01.08.12 at 19:55, Stefano 
>>>>> Stabellini<stefano.stabellini@xxxxxxxxxxxxx>  
> wrote:
>>>>>          
>>> I was reading more about this commit because this patch breaks the ABI
>>> on ARM, when I realized that on x86 there is no standard that specifies
>>> the alignment of fields in a struct.
>>>      
>> There is - the psABI supplements to the SVR4 ABI.
>>
>>    
> 
> This is a completely different issue.
> The problem here gcc/whatever compiler padding added to the struct in 
> order to have alignment of the members to the word boundry. The 
> difference is that this is not enforced in the ARM case (apparently, 
> from Stefano's report) while it happens in the x86 case.
> 
> This is why it is a good rule to organize member of a struct from the 
> bigger to the smaller when compiling with gcc and this is not the case 
> of the struct in question.
> 
> In the end it is a compiler decisional thing, not something decided by 
> the ABI.

No, definitely not. Otherwise inter-operation between code
compiled with different compilers would be impossible. To
allow this is what the various ABI specifications exist for (and
their absence had, e.g. on DOS, lead to a complete mess).

As to the ARM issue - mind pointing out where mis-aligned
structure fields are specified as being the standard?

Jan


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