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

[Jan Beulich <jbeulich@suse.com>]

On 29.04.2020 17:06, Julien Grall wrote:
> 
> 
> On 29/04/2020 15:56, Jan Beulich wrote:
>> On 29.04.2020 16:14, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 29/04/2020 15:05, Jan Beulich wrote:
>>>> On 29.04.2020 16:01, Julien Grall wrote:
>>>>> 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.
>>>>
>>>> Can commands be extended at all, i.e. don't extensions require new
>>>> commands? The issue I've described has nothing to do with future
>>>> extending of any of the affected structures.
>>>
>>> I think my point wasn't conveyed correctly. The implicit padding is at
>>> the end of the structure for all the consumers but 32-bit x86. So
>>> without any modification, I think 32-bit frontend can still communicate
>>> with 64-bit backend (or vice-versa).
>>
>> There's no issue communicating afaics, as for communication
>> you wouldn't use the sub-structures, but the single container
>> one. The problem is, as described, with possible uses internal
>> to one side of the communication.
> 
> I am sorry but I can't figure out how this is an issue. The
> problem you described would only happen if you are calling a
> 64-bit library from a 32-bit software.

Why? The example given doesn't require such.

> Is it even possible?

In principle yes, I think. I don't think OSes like Linux allow this,
though.

Jan