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 16:05:24 +0200	[thread overview]
Message-ID: <423c0369-9c90-dbfe-2f90-d49a2ce5b283@suse.com> (raw)
In-Reply-To: <9eb39857-2e33-4a6b-1825-f9dc537a6515@xen.org>

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.

Jan