Re: [Xen-devel] [PATCH RFC v2 1/7] xen/vNUMA: vNUMA support for PV guests.

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On mar, 2013-09-17 at 03:19 -0400, Elena Ufimtseva wrote:
> On Tue, Sep 17, 2013 at 3:11 AM, Dario Faggioli
> <dario.faggioli@xxxxxxxxxx> wrote:
> > On mar, 2013-09-17 at 08:05 +0100, Jan Beulich wrote:
> >> But - all this is only for the internal representations. Anything in
> >> the public interface should be wide enough to allow future
> >> extension.
> >>
> > And, in fact, 'node_to_node_distance' in xen/include/public/sysctl.h
> > (http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/sysctl.h)
> > is uint32.
> >
> 
> Linux has u8 for distance. Ok, thank you for pointing that out.
> 
EhEh... So, very hard to be consistent with  every actor in the
play! :-)

Anyway, I think the point here is, as Jan was saying, to distinguish
internal representation from interface. Within Xen, we should store
everything in the smallest and nicest possible way (which, BTW, is what
Linux does by using u8 for distances).

OTOH, when it comes to exported interfaces, we should be much more
cautious, since changing the way Xen stores distances internally is
trivial, changing the interface (either API or ABI) could be a
nightmare!

In this case, what we (Xen) tell Linux is the interface, it is up to him
(Linux) to convert that in a way that suits its own internals. In fact,
despite Linux being the only one OS using this interface for now, it's
not wise to design the interface itself specifically for Linux, since
many OSes may want to support it in the future.

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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