linux-kernel - Re: [PATCH v5 2/2] x86/purgatory: Add linker script

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Message-ID: <CAKwvOdkULKxoifiVO4H4yomt1DHQxw0ntdO6QgMXk2BpM-urRg@mail.gmail.com>
Date:   Fri, 7 Apr 2023 16:22:36 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Baoquan He <bhe@...hat.com>, Philipp Rudo <prudo@...hat.com>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ross Zwisler <zwisler@...gle.com>,
        Simon Horman <horms@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        linux-toolchains@...r.kernel.org,
        clang-built-linux <llvm@...ts.linux.dev>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v5 2/2] x86/purgatory: Add linker script

Hi Ricardo,
Thanks for the patch!  Please make sure to cc our mailing list
<llvm@...ts.linux.dev> for llvm specific issues.
scripts/get_maintainer.pl should recommend it, or you can find it from
clangbuiltlinux.github.io.  You can also ping me internally for
toolchain related issues.

Start of thread.
https://lore.kernel.org/lkml/20230321-kexec_clang16-v5-0-5563bf7c4173@chromium.org/

On Thu, Mar 30, 2023 at 9:00 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Mar 30, 2023 at 11:31:27AM -0400, Steven Rostedt wrote:
> > On Thu, 30 Mar 2023 17:18:26 +0200
> > Borislav Petkov <bp@...en8.de> wrote:
> >
> > > On Thu, Mar 30, 2023 at 11:15:23AM -0400, Steven Rostedt wrote:
> > > > > Make sure that the .text section is not divided in multiple overlapping
> > > > > sections. This is not supported by kexec_file.

Perhaps this is related to CrOS' use of AutoFDO creating .text.hot?
If so, it's probably more straightforward to straight up disable PGO
for kexec. See also:

commit bde971a83bbf ("KVM: arm64: nvhe: Fix build with profile optimization")

> > >
> > > And?
> > >
> > > What is the failure scenario? Why are you fixing it? Why do we care?
> > >
> > > This is way too laconic.
> > >
> >
> > Yeah, I think the change log in patch 1 needs to be in this patch too,
> > which gives better context.
>
> Just read it.
>
> Why did it work with clang version < 16?

I'll bet if we bisect llvm, we can spot what might have changed, which
may give us a clue on how to get the old behavior back; maybe without
the need for a linker script.

Ricardo, how did you verify that your fix was correct? Surely we can
check using command line utilities without needing a full blown kexec
setup? If you can share more info, I can bisect llvm quickly.  If it
requires profile data, you'll need to share it, since CrOS engineers
still have not posted public documentation on AutoFDO as I have
repeatedly asked for.

>
> + toolchains ML.
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette

-- 
Thanks,
~Nick Desaulniers