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

Ingo Molnar
2019-11-19T11:10:12Z

* Sean Christopherson <sean.j.christopherson@intel.com> 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.

Thanks,

Ingo