Re: [Xen-devel] [PATCH v8 6/6] tools: enable Cache QoS Monitoring feature for libxl/libxc

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-02-17 at 06:11 +0000, Xu, Dongxiao wrote:
> > -----Original Message-----
> > From: Xu, Dongxiao
> > Sent: Monday, February 10, 2014 7:19 PM
> > To: Ian Campbell
> > Cc: xen-devel@xxxxxxxxxxxxx; keir@xxxxxxx; JBeulich@xxxxxxxx;
> > Ian.Jackson@xxxxxxxxxxxxx; stefano.stabellini@xxxxxxxxxxxxx;
> > andrew.cooper3@xxxxxxxxxx; konrad.wilk@xxxxxxxxxx; dgdegra@xxxxxxxxxxxxx
> > Subject: RE: [PATCH v8 6/6] tools: enable Cache QoS Monitoring feature for
> > libxl/libxc
> > 
> > > > diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
> > > > index 649ce50..43c0f48 100644
> > > > --- a/tools/libxl/libxl_types.idl
> > > > +++ b/tools/libxl/libxl_types.idl
> > > > @@ -596,3 +596,10 @@ libxl_event = Struct("event",[
> > > >                                   ])),
> > > >             ("domain_create_console_available", Struct(None, [])),
> > > >             ]))])
> > > > +
> > > > +libxl_cqminfo = Struct("cqminfo", [
> > >
> > > You need to also patch libxl.h to add a suitable LIBXL_HAVE_FOO define,
> > > see the existing examples in that header.
> > 
> > OK.
> 
> Hi Ian,
> 
> Just had another look at the comments:
> 
> * In the event that a change is required which cannot be made
>  * backwards compatible in this manner a #define of the form
>  * LIBXL_HAVE_<interface> will always be added in order to make it
>  * possible to write applciations which build against any version of
>  * libxl. Such changes are expected to be exceptional and used as a
>  * last resort. The barrier for backporting such a change to a stable
>  * branch will be very high.
> 
> LIBXL_HAVE_<interface> is to address the back compatibility issue, and
> mostly it is used in cases like "adding a new field to the existing
> data structure".
> 
> In this patch, the libxl_cqminfo is a new added data structure, and it
> seems that there is no such compatibility issue. Do we really need to
> add the LIBXL_HAVE_<interface> macro?

I think so -- so that consumers of the libxl API know that it is
available to be used. The fact that it is an entire struct rather than a
new field doesn't make a difference here I think.

Ian.


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