Re: [Embedded-pv-devel] [PATCH v9] xen: add para-virtual sound interface header files

From: Artem Mygaiev <artem_mygaiev@epam.com>
To: Julien Grall <julien.grall@arm.com>,
	Jan Beulich <JBeulich@suse.com>,
	Oleksandr Andrushchenko <andr2000@gmail.com>
Cc: Lars Kurth <lars.kurth@citrix.com>,
	Iurii Konovalenko <iurii.konovalenko@globallogic.com>,
	Volodymyr Babchuk <vlad.babchuk@gmail.com>,
	TimDeegan <tim@xen.org>,
	Dario Faggioli <dario.faggioli@citrix.com>,
	ian.jackson@eu.citrix.com,
	Andrii Anisov <andrii.anisov@gmail.com>,
	Oleksandr Tyshchenko <olekstysh@gmail.com>,
	embedded-pv-devel <embedded-pv-devel@lists.xenproject.org>,
	al1img@gmail.com, David Vrabel <david.vrabel@citrix.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Oleksandr Dmytryshyn <oleksandr.dmytryshyn@globallogic.com>,
	Mygaiev Artem <joculator@gmail.com>
Subject: Re: [Embedded-pv-devel] [PATCH v9] xen: add para-virtual sound interface header files
Date: Thu, 24 Nov 2016 18:29:31 +0200	[thread overview]
Message-ID: <c1e3aee1-e9f9-71d5-2431-c1d247ad1209@epam.com> (raw)
In-Reply-To: <95590564-4f6f-6fd7-f9f3-5bf9e3c4ecb2@arm.com>


On 24.11.16 18:08, Julien Grall wrote:
> On 24/11/16 15:50, Artem Mygaiev wrote:
>> On 24.11.16 17:09, Julien Grall wrote:
>>> Hi Artem,
>>>
>>> On 24/11/16 14:58, Artem Mygaiev wrote:
>>>> On 24.11.16 15:31, Jan Beulich wrote:
>>>>>>>> On 24.11.16 at 14:18, <artem_mygaiev@epam.com> wrote:
>>>>>> On 24.11.16 15:04, Jan Beulich wrote:
>>>>>>>> In my case I need it to at least define Linux specific __packed
>>>>>>>> attribute which is not supported by Win AFAIK.
>>>>>>> ... needs to avoid using platform specific constructs (or making
>>>>>>> other assumptions on the platform). I can only re-iterate: Please
>>>>>>> follow the model other PV protocols already present (without
>>>>>>> repeating their mistakes, if at all possible).
>>>>>> But if we don't pack (or enforce some specific alignment) data
>>>>>> structures used between domains they may have different alignment on
>>>>>> different domains, no?
>>>>> Let me repeat what I've said before: Please take a look at existing
>>>>> interface headers. By properly placing fields and adding explicit
>>>>> padding where needed, you can avoid such layout differences.
>>>>
>>>> Don't get me wrong - I have checked the i/f headers before. My concern
>>>> is very generic: C standard defines alignment as an
>>>> *implementation-defined* integer, so current implementation is based
>>>> under two assumptions on the guest platforms: a) guest compiler
>>>> does not
>>>> have "packing" enabled by default and b) alignment follows typical
>>>> values (1 byte for char, 2 bytes for short, ...). While this is
>>>> true in
>>>> most cases, with some embedded RTOS this may become an issue.
>>>
>>> I am not sure why you mentioned embedded RTOS, this is specific to the
>>> compiler and not an OS.
>>>
>> Indeed, my statement is not quite correct. It is just that there are
>> proprietary RTOSes bundled with IDEs with own or customized tools,
>> compilers, etc.
>
> And their own ABI? It sounds crazy to me that someone may decide to
> use padding even when field are naturally aligned.
No, no, that would be overkill... I am just talking about system-wide
alignment configuration for performance/size balance (reduce padding,
not increase).

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