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

[Julien Grall <julien.grall@xxxxxxx>]

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.
ARM provides a specification (see AAPCS [1] for ARM32 and AAPCS64 [2]) 
that explain the alignment of the structure and other stuff. Any 
compiler should implement for interoperability. If not, then you are 
already in trouble to issue hypercall to Xen when using standard headers.
x86  has fairly similar requirements as they are all sensible all not 
all specific to the ARM platform.
So it is possible to write header in the way which will accommodate all 
the architecture.
> This is not critical/important ATM, and completely disconnected from the
> purpose of the patch, so I agree it has to be rewritten according to
> current practices.
[1] 
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042f/IHI0042F_aapcs.pdf
[2] 
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf
--
Julien Grall

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