Re: [Xen-devel] future plans for libxl?

[Vincent Hanquez <vincent.hanquez@xxxxxxxxxxxxx>]

On 15/04/10 08:21, Andre Przywara wrote:
> I stumbled upon missing 'info' support, so I implemented a basic version
> of it. A few questions about it:
> 1.) Is there a fixed interface for libxenlight? I see libxc interfaces
> duplicated (like {xc,libxl_get}_physinfo), is that thought to provide a
> more stable interface than xc does (which changed quite a bit lastly).
> What about extending this interface? For complete xl info I need more
> field of the physinfo sysctl to be provided by libxl_get_physinfo, is it
> OK to extend the struct libxl_physinfo?
yes, i've duplicated them for this exact purpose. it could work 
hand-in-hand with what i'm describing below.
for extending usually, you would just add a new field at the end of the
structure.

it's probably ok to extends the physinfo structure. however we tried
to separate some things that would be not very useful from a toolstack
point of view, compared to actually carrying all sorts of info of
questionable usefulness.

> 2.) I see a simple check of LIBXL_VERSION when doing libxl_ctx_init().
> Is that meant to be that hard or is a compatibility scheme planned
> (like: support apps using on older interface and somehow emulate it?)
it takes lots of resources to do a compat scheme. but eventually it 
could all be done with the LIBXL_VERSION check. if you upgrade
the library and something change, the previously compiled client
would still use a LIBXL_VERSION < compared to the current one in the 
library.
at this point, libxl could provide a mechanism to provide old -> new 
structure mechanism to provide automatic compatibilty with old 
userspace. note that it wouldn't cover function prototype change, but 
only structure change, and this is an extremly painful process
where you need to keep your old structure around, and have a 
(potentially) big fast switch case in function that need this compat layer.
it's much much simpler to just let client and library to be the exact 
same version, which is what the ctx_init function is doing at this point 
compared to the elaborate scheme of compat.
--
Vincent

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