Re: memop struct packing, 32/64 bits

[Keir Fraser <keir@xen.org>]

On 19/01/2012 20:30, "Andres Lagar-Cavilla" <andres@lagarcavilla.org> 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.

> 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
> 
> 
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