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

[Stefano Stabellini]

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 <address@hidden>) 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? 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.


> For the original issue here, could the flag be exposed as a
> XEN_SYSCTL_PHYSCAP_????

Feature flags are welcome and the best course of action in my opinion.