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

From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Julien Grall <jgrall@amazon.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH] pvcalls: Document explicitly the padding for all arches
Date: Wed, 29 Apr 2020 17:57:59 +0200	[thread overview]
Message-ID: <2bb47891-bf3c-1bb4-8c54-02f6a7ea4676@suse.com> (raw)
In-Reply-To: <92be4ce7-3963-7ee6-7ee2-28a180411c9c@xen.org>

On 29.04.2020 17:30, Julien Grall wrote:
> Hi Jan,
> 
> On 29/04/2020 16:23, Jan Beulich wrote:
>> 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.
> 
> Your example is only valid if we change the padding. In my previous
> e-mail I wrote "without modification" (i.e existing code) and
> marking the implicit padding only for 64-bit x86 and Arm. So there
> is no change in the size of the structure and therefore there is no
> issue to recompile as the size would not change.

Oh, sorry, yes. I was mislead by "I think 32-bit frontend can still
communicate with 64-bit backend (or vice-versa)", because I never
said there would be such an issue.

>>> Is it even possible?
>>
>> In principle yes, I think. I don't think OSes like Linux allow this,
>> though.
> Do we really have to care about this?

I don't think we do, but this is a moot point anyway.

Jan