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





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@xxxxxxxx> 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.

Regards,

--
Julien Grall

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