[PATCH v7 0/4] kexec: Fix kexec_file_load for llvm16 with PGO

[Ricardo Ribalda]

Hi Song

On Fri, 8 Sept 2023 at 23:48, Song Liu <song at kernel.org> wrote:
>
> Hi Ricardo,
>
> Thanks for your kind reply.
>
> On Fri, Sep 8, 2023 at 2:18 PM Ricardo Ribalda <ribalda at chromium.org> wrote:
> >
> > Hi Song
> >
> > On Fri, 8 Sept 2023 at 01:08, Song Liu <song at kernel.org> wrote:
> > >
> > > Hi Ricardo and folks,
> > >
> > > On Fri, May 19, 2023 at 7:48 AM Ricardo Ribalda <ribalda at chromium.org> wrote:
> > > >
> > > > When upreving llvm I realised that kexec stopped working on my test
> > > > platform.
> > > >
> > > > The reason seems to be that due to PGO there are multiple .text sections
> > > > on the purgatory, and kexec does not supports that.
> > > >
> > > > Signed-off-by: Ricardo Ribalda <ribalda at chromium.org>
> > >
> > > We are seeing WARNINGs like the following while kexec'ing a PGO and
> > > LTO enabled kernel:
> > >
> > > WARNING: CPU: 26 PID: 110894 at kernel/kexec_file.c:919
> > > kexec_load_purgatory+0x37f/0x390
> > >
> > > AFAICT, the warning was added by this set, and it was triggered when
> > > we have many .text sections
> > > in purgatory.ro. The kexec was actually successful. So I wonder
> > > whether we really need the
> > > WARNING here. If we disable LTO (PGO is still enabled), we don't see
> > > the WARNING any more.
> > >
> > > I also tested an older kernel (5.19 based), where we also see many
> > > .text sections with LTO. It
> > > kexec()'ed fine. (It doesn't have the WARN_ON() in
> > > kexec_purgatory_setup_sechdrs).
> >
> > You have been "lucky" that the code has chosen the correct start
> > address, you need to modify the linker script of your kernel to
> > disable PGO.
> > You need to backport a patch like this:
> > https://lore.kernel.org/lkml/CAPhsuW5_qAvV0N3o+hOiAnb1=buJ1pLzqYW9D+Bwft6hxJvAeQ@mail.gmail.com/T/#md68b7f832216b0c56bbec0c9b07332e180b9ba2b
>
> We already have this commit in our branch. AFAICT, the issue was
> triggered by LTO. So something like the following seems fixes it
> (I haven't finished the end-to-end test yet). Does this change make
> sense to you?

if the end-to-end works, please send it as a patch to the mailing list.

Thanks! :)

>
> Thanks again,
> Song
>
> diff --git i/arch/x86/purgatory/Makefile w/arch/x86/purgatory/Makefile
> index 8f71aaa04cc2..dc306fa7197d 100644
> --- i/arch/x86/purgatory/Makefile
> +++ w/arch/x86/purgatory/Makefile
> @@ -19,6 +19,10 @@ CFLAGS_sha256.o := -D__DISABLE_EXPORTS
>  # optimization flags.
>  KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=%
> -fprofile-use=%,$(KBUILD_CFLAGS))
>
> +# When LTO is enabled, llvm emits many text sections, which is not supported
> +# by kexec. Remove -flto=* flags.
> +KBUILD_CFLAGS := $(filter-out -flto=%,$(KBUILD_CFLAGS))
> +
>  # When linking purgatory.ro with -r unresolved symbols are not checked,
>  # also link a purgatory.chk binary without -r to check for unresolved symbols.
>  PURGATORY_LDFLAGS := -e purgatory_start -z nodefaultlib



-- 
Ricardo Ribalda