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

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Message-ID: <20191119111012.GA109842@gmail.com>
Date:   Tue, 19 Nov 2019 12:10:12 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Nadav Amit <namit@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Cezary Rojewski <cezary.rojewski@...el.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Liam Girdwood <liam.r.girdwood@...ux.intel.com>,
        Jie Yang <yang.jie@...ux.intel.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-acpi@...r.kernel.org, alsa-devel@...a-project.org
Subject: Re: [PATCH 00/12] treewide: break dependencies on x86's RM header


* Sean Christopherson <sean.j.christopherson@...el.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