memop struct packing, 32/64 bits

["Andres Lagar-Cavilla" <andres@lagarcavilla.org>]

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?

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?

Thanks!
Andres