Re: [Xen-devel] memop struct packing, 32/64 bits

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> On 19/01/2012 20:30, "Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>
> wrote:
>
>> Hi,
>> I had the following painful experience. I declared
>>
>> struct xen_mem_event_op {
>>     uint8_t       op;           /* XENMEM_*_op_* */
>>     domid_t       domain;
>>     uint64_t buffer;
>>     uint64_t gfn;          /* IN:  gfn of page being operated on */
>> };
>> typedef struct xen_mem_event_op xen_mem_event_op_t;
>>
>> to be passed as the argument of a memory op called form the toolstack.
>> The
>> hypervisor is 64 bits and the toolstack is 32 bits. My toolstack code
>> simply:
>>
>>     xen_mem_event_op_t meo;
>> ... set fields ...
>>     return do_memory_op(xch, mode, &meo, sizeof(meo));
>>
>> No joy because 32 bits was packing the struct differently than 64 bits.
>> Namely, both were adding a 1 byte pad between 'op' and 'domain', but
>> when
>> compiled in 64 bits mode for the hypervisor, an additional 4 byte pad
>> was
>> thrown between 'domain' and 'buffer'.
>>
>> The first question is, what is the preferred way around this. Declare
>> pads
>> inside the struct?
>
> Yes.

Ok. And as Konrad says, I'll add __attribute__((packed)). A first for
memory.h

Thanks!
Andres

>
>> Exploring the include/public/memory.h declarations and toolstack code, I
>> see that no current declare includes __attribute__((aligned)) or
>> __attribute__((packed)), or explicit pads.
>>
>> So how come things don't break more often for 32 bit toolstacks? pure
>> luck? Am I missing something?
>
> Where older structs were not 32/64-bit invariant, compat shims were
> implemented. See common/compat/memory.c, for example. Well worth avoiding
> that!
>
>  -- Keir
>
>> Thanks!
>> Andres
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
>
>
>



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