Re: [Qemu-devel] [Xen-devel] [PATCH RFC v2 4/4] xen/MSI: re-expose masking capability

[Ian Campbell <ian.campbell@citrix.com>]

On Mon, 2015-12-14 at 11:19 +0000, Stefano Stabellini wrote:
> On Fri, 11 Dec 2015, Ian Campbell wrote:
> > On Fri, 2015-12-11 at 16:44 +0000, Stefano Stabellini wrote:
> > >  
> > > It is not possible to do this at runtime. I think we should do this
> > > at
> > > compile time because in any case it is not supported to run a QEMU
> > > built
> > > for a given Xen version on a different Xen version.
> > 
> > I am currently working pretty hard to make this possible in the future,
> > it
> > would be a shame to add another reason it wasn't possible at this
> > stage.
> > 
> > I proposed (in <1445442435.9563.184.camel@citrix.com>) that as well as
> > the
> > various stable libraries extracted from libxenctrl we will probably
> > also
> > want to have a libxendevicemodel.so at some point, to provide a stable
> > way
> > to interface with all the stuff which being a DM involves.
> 
> I understand the direction we are heading toward, but unfortunately we
> are still pretty far from it. I don't think we want to block this patch
> until we have a stable libxendevicemodel ABI?

No, but I would appreciate if such things were explicitly considered on a
case by case by case basis rather than just bundled under a generic "it's
not possible yet", since there may be cases where we want to hold off, or
more likely where doing something a particular way now will ease things for
the transition in the future.

>  Also this particular
> change regards PCI passthrough, which is not convered by the proposed
> ABI yet.
> 
> 
> > Maybe that library could contain a way to get this information? (In
> > which
> > case it could be hardcoded at compile time now and I'll see what I can
> > do
> > when I get to producing the library).
> 
> Given the choice, I would rather have only compile time or only run time
> Xen version checks in QEMU and not both to avoid complexity. Especially
> as long as the underlying libraries don't make any stability guarantees.

"that library" obviously will make such guarantees as a matter of design.

Ian.