Re: [PATCH 02/10] xen: make GUEST_HANDLE_64() and uint64_aligned_t available everywhere

[David Vrabel <david.vrabel@citrix.com>]

On 25/06/13 12:36, Jan Beulich wrote:
>>>> On 25.06.13 at 11:42, David Vrabel <david.vrabel@citrix.com> wrote:
>> On 25/06/13 08:42, Jan Beulich wrote:
>>>>>> On 24.06.13 at 19:42, David Vrabel <david.vrabel@citrix.com> wrote:
>>>>  #define uint64_aligned_t uint64_t __attribute__((aligned(8)))
>>>
>>> This line is the reason why such a change is not acceptable: We
>>> require the headers to not use gcc extensions outside of regions
>>> guarded by dependencies on __XEN__ and/or __XEN_TOOLS__ (which
>>> we know/require will always be built by gcc compatible tool chains).
>>
>> I did this because this is identical to what ARM is doing.
>>
>> I think we do what a guest handle type that is always 64 bits long.  For
>> x86, perhaps something like (but with a better name):
>>
>> #define ___DEFINE_XEN_GUEST_HANDLE(name, type)                  \
>>     typedef struct { type *p; }                                 \
>>         __guest_handle_ ## name;                                \
>> #if defined(__XEN__) || (__XEN_TOOLS__)
>>     typedef struct { union { type *p; uint64_aligned_t q; }; }  \
>>         __guest_handle_64_ ## name                              \
>> #endif
>>     typedef struct { union { type *p; uint64_t q; }; }          \
>>         __guest_handle_new_ ## name
> 
> The uint64_t here ...
> 
>> #undef set_xen_guest_handle_raw
>> #define set_xen_guest_handle_raw(hnd, val)                  \
>>     do { if ( sizeof(hnd) == 8 ) *(uint64_t *)&(hnd) = 0;   \
>>          (hnd).p = val;                                     \
>>     } while ( 0 )
>>
>> #if defined(__XEN__) || (__XEN_TOOLS__)
>> #define uint64_aligned_t uint64_t __attribute__((aligned(8)))
>> #define __XEN_GUEST_HANDLE_64(name) __guest_handle_64_ ## name
>> #define XEN_GUEST_HANDLE_64(name) __XEN_GUEST_HANDLE_64(name)
>> #endif
>>
>> #define __XEN_GUEST_HANDLE_NEW(name) __guest_handle_new_ ## name
>> /* This must be aligned to 8 bytes with padding if necessary. */
>> #define XEN_GUEST_HANDLE_NEW(name) __XEN_GUEST_HANDLE_NEW(name)
> 
> ... does in no way satisfy the comment here, so what's the point?

The comment is unclear, sorry.

/* A structure containing this type of guest handle must align the
   field to 8 bytes, using padding fields as necessary. */

>>> I'm afraid you'll need to find a way to do what you want in the
>>> kexec interface with the traditional manual padding approach.
>>
>> This is fine.  The kexec interface has the necessary padding and doesn't
>> need the the aligned attribute.
> 
> Not afaict, unless you meant if substituting XEN_GUEST_HANDLE_NEW()
> (rather than XEN_GUEST_HANDLE()) for XEN_GUEST_HANDLE_64().

Yes.

David