Re: [PATCH v1 1/1] include/hw/xen/xen_native.h: downgrade include-order assertion to warning

On Mon, 24 Aug 2026 at 14:31, Frager, Neal <[email protected]> wrote:
>
> AMD General
>
> Hi Peter,
>
> >
> > Hi Stewart,
> >
> > > The -I$(XEN_ROOT)/tools/include added to QEMU's extra-cflags causes
> > > __XEN_INTERFACE_VERSION__ to be defined before xen_native.h is included,
> > > triggering an include-order assertion. Downgrade to a warning since the
> > > version is consistent in cross-compile.
> > > Ref: https://github.com/qemu/qemu/commit/e2abfe5ec6
>
> > > This is a buildroot issue, so I don't believe it's necessary to fix from the
> > > qemu side.
> >
> > I am not sure I fully agree here. While this is a buildroot identified issue,
> > there could be other use cases for __XEN_INTERFACE_VERSION__ to be defined
> > before xen_native.h is included.
>
> > But what, though?
>
> > And what we have found is that if
> > __XEN_INTERFACE_VERSION__ to be defined before xen_native.h is included, it
> > is not a hard error.  For buildroot, the qemu works just fine in spite of
> > this.
>
>
> > I think that just means you got lucky. Either there is a hard requirement
> > for one header to be included before the other (in which case it must
> > be a #error, and whatever is causing the mis-ordering to happen must be
> > fixed), or it's fine for the ordering to be either way (in which case it
> > doesn't even need to be a #warning).
>
> From my view, the order the header files are included does not matter, and
> this should not be an error.  I agree with removing the warning as well, if
> that is what we all agree on in the end.

The rationale for the header ordering is in the comment in include/hw/xen/xen.h:

/*
 * C files using Xen toolstack libraries will have included those headers
 * already via xen_native.h, and having __XEM_TOOLS__ defined will have
 * automatically set __XEN_INTERFACE_VERSION__ to the latest supported
 * by the *system* Xen headers which were transitively included.
 *
 * C files which are part of the internal emulation, and which did not
 * include xen_native.h, may need this defined so that the Xen headers
 * imported to include/hw/xen/interface/ will expose the appropriate API
 * version.
 *
 * This is why there's a rule that xen_native.h must be included first.
 */

...basically, if something doesn't include xen_native.h before xen.h
then __XEN_INTERFACE_VERSION__ can end up defined to the wrong thing.
(Disclaimer: I'm not a Xen expert, I'm just applying Chesterton's Fence.)

thanks
-- PMM