Re: [Xen-API] [PATCH 2 of 4] xc: split xc non-upstream bindings into xcext module

[Vincent Hanquez <vincent.hanquez@eu.citrix.com>]

On 18/11/10 15:35, Ian Campbell wrote:
> Absolutely, I think the XIU stuff is really very useful indeed. I think
> it even has the potential for wider usefulness than just XCP.
>
> Addressing this is next on my list so my plan is only half baked (if
> that!) but I think the hooks necessary to support an injection
> infrastructure of this type could be integrated into libxc without too
> much pain. Doing this would allow xl, libvirt etc to also use the XIU
> functionality for testing which I think would be really cool.
>
> I've several possible approaches in mind:
>
>        * Just whack the hooks and injection layer into libxc itself,
>          there aren't really that many hook points or that much code in
>          the injection layer...
>        * Turn xc_injection_lib.c into a LD_PRELOAD'able library. Requires
>          moving various, do_{domctl,ioctl,etc} stuff out of line in the
>          library -- which I think is a good idea anyway.
>        * Add functionality to libxc to allow it to dlopen a backend (e.g.
>          pointed to by an envvar) containing the hook implementation with
>          explicit calls to the layer as necessary.
>
> Probably the second two are pretty much equivalent modulo the name of
> the environment variable being either LD_PRELOAD or something else.
>
> Initially I think I prefer the dynamic loading approaches to dropping
> the injection stuff directly into libxc. In particular the dynamic
> options allow the injection layer and XIU backend to live together
> whereas the first option effectively has the injection layer in
> xen-unstable.hg and the XIU backend possibly somewhere else which
> doesn't seem helpful.
>
> The dynamic solution also allows for other injection layers and backends
> but I'm not sure how useful that actually is. Perhaps a valgrind
> friendly backend or something like that, dunno
I pretty much agree with you, however just a bit of comments.

option 1 is more dynamic than option 2. you can mix and match backend 
just setting/unsetting
your XIU environment before any XC call technically. this isn't possible 
with LD_PRELOAD, since
it's one shot during the start of your program.

3 is pretty much equivalent to 1 and best technically IMO. it's just a 
bit more efforts to do, hence why option 1 is the one available now.

-- 
Vincent