Re: [PATCH] pvcalls: Document explicitly the padding for all arches

[Julien Grall <julien@xen.org>]

Hi,

On 22/04/2020 10:20, Jan Beulich wrote:
>> Even if it was possible to use the sub-structs defined in the header
>> that way, keep in mind that we also wrote:
>>
>>          /* dummy member to force sizeof(struct xen_pvcalls_request)
>>           * to match across archs */
>>          struct xen_pvcalls_dummy {
>>              uint8_t dummy[56];
>>          } dummy;
> 
> This has nothing to do with how a consumer may use the structs.
> 
>> And the spec also clarifies that the size of each specific request is
>> always 56 bytes.
> 
> Sure, and I didn't mean to imply that a consumer would be allowed
> to break this requirement. Still something like this
> 
> int pvcall_new_socket(struct xen_pvcalls_socket *s) {
>      struct xen_pvcalls_request req = {
>          .req_id = REQ_ID,
>          .cmd = PVCALLS_SOCKET,
>          .u.socket = *s,
>      };
> 
>      return pvcall(&req);
> }
> 
> may break.

I think I understand your concern now. So yes I agree this would break 
32-bit consumer.

As the padding is at the end of the structure, I think a 32-bit frontend 
and 64-bit backend (or vice-versa) should currently work without any 
trouble. The problem would come later if we decide to extend a command.

I will document the padding only for non 32-bit x86 guest and rework the 
documentation.

Cheers,

-- 
Julien Grall