Re: Strange symbols_lookup() behaviour in test-symbols on arm64 CI

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

On Thu, Dec 11, 2025 at 1:15 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 11.12.2025 11:29, Mykola Kvach wrote:
> > While working on an arm64 s2ram series for Xen I have hit what looks
> > like very strange behaviour in symbols_lookup() as exercised by 
> > test-symbols.
> >
> > The series is in the branch referenced at [1]. All patches there except
> > the last one build and pass CI; adding only the last patch makes the CI
> > job referenced at [2] start failing.
> >
> > Note that the tests in that job are built without CONFIG_SYSTEM_SUSPEND
> > enabled, so most of the code introduced by the s2ram branch is not
> > compiled at all for that configuration. That is why I initially did not
> > expect my series to affect this job.
> >
> > To investigate, I tried to reproduce the issue locally. I downloaded the
> > xen-config artifact from the failing job [3] and used it to build Xen
> > with my local aarch64 cross compiler. With this local toolchain
> > I could not reproduce the failure, and the resulting .config changed 
> > slightly
> > compared to the job's config. The relevant part of the diff looks like this:
> >
> >     diff --git a/xen/.config b/xen-config
> >     index 057553f510..44dcf6bacc 100644
> >     --- a/xen/.config
> >     +++ b/xen-config
> >     @@ -3,11 +3,11 @@
> >      # Xen/arm 4.22-unstable Configuration
> >      #
> >      CONFIG_CC_IS_GCC=y
> >     -CONFIG_GCC_VERSION=130300
> >     +CONFIG_GCC_VERSION=120201
> >      CONFIG_CLANG_VERSION=0
> >      CONFIG_LD_IS_GNU=y
> >      CONFIG_CC_HAS_ASM_INLINE=y
> >     -CONFIG_CC_HAS_ASM_GOTO_OUTPUT=y
> >     +CONFIG_GCC_ASM_GOTO_OUTPUT_BROKEN=y
> >      CONFIG_FUNCTION_ALIGNMENT_4B=y
> >      CONFIG_FUNCTION_ALIGNMENT=4
> >      CONFIG_ARM_64=y
> >
> > So there is at least a difference in GCC version and asm-goto related
> > Kconfig options between the CI environment and my local one.
> >
> > After that I tried rebuilding inside the same Docker image that GitLab
> > CI uses:
> >
> >     registry.gitlab.com/xen-project/xen/alpine:3.18-arm64v8
> >
> > When I build Xen in that container, using the same branch, the problem
> > reproduces in the same way as in the CI job.
> >
> > Even more confusingly, adding extra prints in test_symbols just before
> > the calls to test_lookup() makes the problem disappear. This made me
> > suspect some undefined behaviour or logic issue that is very sensitive
> > to optimisation or layout changes.
>
> All symptoms described make me suspect you're hitting a problem we're
> already in the process of hunting down. Can you please take [1], make
> the small adjustment necessary to Arm's linking rule, and see whether
> you get a build failure in the case where right now you get a boot time
> crash? Of course no other changes to code or data layout should be done,
> or else you may observe false negatives.

I tested the issue with the provided patch, and it is still reproducible.

This is my working branch:

e8d5baab50 (HEAD -> reg) symbols: check table sizes don't change
between linking passes 2 and 3
e53439fdfc (xen_gitlab/reg) xen/arm: Add support for system suspend
triggered by hardware domain
eaa461f3b5 xen/arm: Implement PSCI SYSTEM_SUSPEND call (host interface)
4236fff9a4 xen/arm: Save/restore context on suspend/resume
a150f3d4bb xen/arm: Resume memory management on Xen resume

You can find the following line in the attached Xen boot log:

(XEN) [ 0.010785] Latest ChangeSet: Tue Dec 9 11:11:40 2025 +0100 git:e8d5baab50

>
> Jan
>
> [1] https://lists.xen.org/archives/html/xen-devel/2025-12/msg00390.html

Best regards,
Mykola

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