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

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Apr 11, 2023 at 2:46 PM Ricardo Ribalda <ribalda@chromium.org> wrote:
>
> Hi Nick
>
> On Sat, 8 Apr 2023 at 01:22, Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > Hi Ricardo,
> > Thanks for the patch!  Please make sure to cc our mailing list
> > <llvm@lists.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@alien8.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@alien8.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")
>
> It was indeed due to the AutoFDO, adding
>
> KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,
> $(KBUILD_CFLAGS))
>
> to arch/x86/purgatory/Makefile
>
> It is definitely simpler than adding a linker script, but I am not
> sure if it is the correct way to fix this... Seems like splitting
> .text in multiple sections is an implementation detail of the compiler
> and the only way to force it is with a linker script... Or am I
> missing something?

I think with the use of `unlikely` GCC will put code in .text.cold, so
it is possible to trigger this using simpler means, but...

>
> Shall I send a new version with the KBUILD_CFLAGS ?

I still think the cflags approach is way simpler.  If someone tries to
use unlikely in purgatory: "don't do that."  Same for PGO.

>
> Thanks!
>
> >
> > > > >
> > > > > 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.
>
> The simplest test is to run:
>
> $readelf -S arch/x86/purgatory/purgatory.ro | grep "] \.text"
> [ 3] .text             PROGBITS         0000000000000000  000002a0
>
> If there is only one .text section then that kernel will be load
> properly via kexec_file().

Got it, profile data will be required to reproduce then. If you can share.
-- 
Thanks,
~Nick Desaulniers