linux-kernel - Re: [PATCH v7 0/4] kexec: Fix kexec_file

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Date:   Fri, 8 Sep 2023 14:48:29 -0700
From:   Song Liu <song@...nel.org>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Eric Biederman <ebiederm@...ssion.com>,
        Philipp Rudo <prudo@...ux.vnet.ibm.com>,
        Dave Young <dyoung@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Baoquan He <bhe@...hat.com>,
        Philipp Rudo <prudo@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Ross Zwisler <zwisler@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Simon Horman <horms@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, stable@...r.kernel.org,
        Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH v7 0/4] kexec: Fix kexec_file_load for llvm16 with PGO

Hi Ricardo,

Thanks for your kind reply.

On Fri, Sep 8, 2023 at 2:18 PM Ricardo Ribalda <ribalda@...omium.org> wrote:
>
> Hi Song
>
> On Fri, 8 Sept 2023 at 01:08, Song Liu <song@...nel.org> wrote:
> >
> > Hi Ricardo and folks,
> >
> > On Fri, May 19, 2023 at 7:48 AM Ricardo Ribalda <ribalda@...omium.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@...omium.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?

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