Re: [PATCH 00/12] treewide: break dependencies on x86's RM header

[Ingo Molnar]

* Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:

> On Tue, 19 Nov 2019 at 12:10, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> >
> > * Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > > x86's asm/realmode.h, which defines low level structures, variables and
> > > helpers used to bring up APs during SMP boot, ends up getting included in
> > > practically every nook and cranny of the kernel because the address used
> > > by ACPI for resuming from S3 also happens to be stored in the real mode
> > > header, and ACPI bleeds the dependency into its widely included headers.
> > >
> > > As a result, modifying realmode.h for even the most trivial change to the
> > > boot code triggers a full kernel rebuild, which is frustrating to say the
> > > least as it some of the most difficult code to get exactly right *and* is
> > > also some of the most functionally isolated code in the kernel.
> > >
> > > To break the kernel's widespread dependency on realmode.h, add a wrapper
> > > in the aforementioned ACPI S3 code to access the real mode header instead
> > > of derefencing the header directly in asm/acpi.h and thereby exposing it
> > > to the world via linux/acpi.h.
> > >
> > > Build tested on x86 with allyesconfig and allmodconfig, so hopefully there
> > > aren't more build issues lurking, but at this point it wouldn't surprise
> > > me in the least if this somehow manages to break the build.
> > >
> > > Based on tip/master, commit ceceaf1f12ba ("Merge branch 'WIP.x86/cleanups'").
> > >
> > > Patch Synopsis:
> > >   - Patches 01-09 fix a variety of build errors that arise when patch 12
> > >     drops realmode.h from asm/acpi.h.  Most of the errors are quite absurb
> > >     as they have no relation whatsoever to x86's RM boot code, but occur
> > >     because realmode.h happens to include asm/io.h.
> >
> > Yeah, these kind of parasitic header dependencies are the main driving
> > force behind kernel header spaghetti hell: it's super easy to add a new
> > header, but very hard to remove them...
> >
> > Hence they practically only accumulate.
> >
> > As a result header removal patches get priority, from me at least. :-)
> >
> > >   - Patch 10 removes a spurious include of realmode.h from an ACPI header.
> > >
> > >   - Patches 11 and 12 implement the wrapper and move it out of acpi.h.
> >
> > So if the ACPI maintainers are fine with -tip carrying patches #11 and #12
> > then I'd be glad to route these patches upstream.
> >
> > I've applied them to tip:WIP.core/headers as a work-in-progress tree, and
> > I'm testing them on randconfigs to make sure there's no broken
> > dependencies. I'll wait for the ACPI acks.
> >
> > I edited the title of patch 12 slightly, to:
> >
> >    c8bceb321209: x86/ACPI/sleep: Move acpi_wakeup_address() definition into sleep.c, remove <asm/realmode.h> from <asm/acpi.h>
> >
> > to make sure the big header dependency change is obvious at first sight.
> >
> 
> I'm fine with the patches but can we drop the fixes headers please?
> This doesn't actually fix anything, and touching early boot stuff for
> no good reason should be avoided imo.

Agreed and done.

Thanks,

	Ingo