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

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

> On Thu, 2012-01-19 at 20:57 +0000, Andres Lagar-Cavilla wrote:
>> > 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
>
> I don't think gcc extensions such as this are allowed in
> xen/include/public. You should explicitly pack the struct instead.

Ok, I'll back off on 'packed'. It makes me uneasy though not being able to
control the layout exactly.

Thanks,
Andres
>
> Ian.
>
>>
>> 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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