Re: [Xen-devel] [PATCH] libxl: add option for discard support to xl disk configuration

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-01-29 at 17:07 +0100, Olaf Hering wrote:
> On Wed, Jan 29, Ian Campbell wrote:
> 
> > On Wed, 2014-01-29 at 16:06 +0100, Olaf Hering wrote:
> > > This new field changes the API, _libxl_types.h:struct libxl_device_disk
> > > gets a new member. How should code using this new flag recognize if its
> > > present? If it is supposed to be part of a new libxl-4.5 API then
> > > out-of-tree code could put the code into #ifdef LIBXL_API_VERSION >= X.
> > > If not, how should it be done?
> > You should add a #define LIBXL_HAVE_FOO to libxl.h, there are a few
> > examples in there already.
> 
> I will add such a define.

THanks.

> > There is no need to make the actual field conditional -- that would
> > actually be wrong since it would modify the ABI depending on what the
> > application asked for, meaning it would differ from how libxl was
> > actually built. An application which us using an ABI before 4.5 simply
> > won't think to touch this field.
> 
> I meant the access of the field in libvirt, like "p->discard_enable = val;".
> Putting such code into #ifdef LIBXL_HAVE_FOO is fine.

Yes, I misunderstood what you meant.

The applications choices are to #define LIBXL_API_VERSION to a big
enough number or to make things conditional on the appropriate
LIBXL_HAVE_FOO. AIUI libvirt has chosen to use the LIBXL_HAVE option.

Ian.


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