Re: Nullifying Recently Introduced Xen Headers Check

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/02/2024 12:05 am, John L. Poole wrote:
> Greetings,
>
> Gentoo Linux maintains two packages for Xen:
>
>    1) app-emulation/xen-tools ("xen-tools")
>    2) app-emulation/xen ("xen")
>
> xen-tools is a prerequisite to xen.  So a failure
> to install xen-tools will preclude any attempt to install xen.
> I acknowledge that building xen-tools before building xen
> is backwards and/or runs contrary to your project's way of
> building, but that is what the Gentoo paradigm has been.
>
> In trying to troubleshoot why a Gentoo 4.18.0 ebuild fails,
> I ended up downloading your archive and successfully building it.
> I then decided to compare the products built to see where
> they varied in processing order.  The Xen Project builds xen first,
> then the tools.
>
> To see a visualization of the to build systems and the object
> files they create matched in chronological order, see:
>
> https://salemdata.us/xen/gentoo-4.18.0/comparison_of_build_orders_Xen_4.18.xhtml
>      or the LibreOffice workbook I created the comparison in:
> https://salemdata.us/xen/gentoo-4.18.0/comparison_of_build_orders_Xen_4.18.ods
>
> In March 2023, this commit added some sort of headers check:
>
> https://xenbits.xen.org/gitweb/?p=qemu-xen.git;a=commit;f=include/hw/xen/xen_native.h;h=e2abfe5ec67b69fb310fbeaacf7e68d61d16609e
>
> Specifically, the lines 4-6 in [qemu-xen.git] / include / hw / xen /
> xen_native.h:
>
>     4 #ifdef __XEN_INTERFACE_VERSION__
>     5 #error In Xen native files, include xen_native.h before other Xen
> headers
>     6 #endif
>
> cause Gentoo's build to error out.  See line 24790:
>
>     5 | #error In Xen native files, include xen_native.h before other
> Xen headers
>
> at https://salemdata.us/xen/xen_tools_20240128_Sun_174740.script.html.
>
> What I have done is create a patch for a draft Gentoo ebuild which nullifies
> lines 4-6 by wrapping them in a comment:
>
>      --- a/tools/qemu-xen/include/hw/xen/xen_native.h
>      +++ b/tools/qemu-xen/include/hw/xen/xen_native.h
>      @@ -1,9 +1,9 @@
>       #ifndef QEMU_HW_XEN_NATIVE_H
>       #define QEMU_HW_XEN_NATIVE_H
>
>      -#ifdef __XEN_INTERFACE_VERSION__
>      +/* #ifdef __XEN_INTERFACE_VERSION__
>       #error In Xen native files, include xen_native.h before other Xen
> headers
>      -#endif
>      +#endif */
>
> This patch allows the Gentoo xen-tools ebuild to successfully build.
>
> Question: is there a risk nullifying the above 3 lines in xen_native.h
> given Gentoo's backward build paradigm?

CC'ing David who authored that Qemu patch.  He'll be best placed to answer.

However, as a word of warning if you're not aware.  The name
__XEN_INTERFACE_VERSION__ is massively dishonest; read it as if it said
"unstable interfaces".

We're working to stabilise the tools->xen interface, but the nice shiny
future is still a while away.

~Andrew