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Date:   Tue, 31 Aug 2021 10:48:06 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Vasily Gorbik <gor@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        qemu-s390x <qemu-s390x@nongnu.org>
Subject: Re: [GIT PULL] s390 updates for 5.15 merge window
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On Tue, Aug 31, 2021 at 05:02:15PM +0200, Marco Elver wrote:
> On Tue, Aug 31, 2021 at 12:46PM +0200, Marco Elver wrote:
> > On Tue, 31 Aug 2021 at 12:13, Heiko Carstens <hca@linux.ibm.com> wrote:
> > [...]
> > > I really don't think this is QEMU related. The test fails are sort of
> > > expected: we've seen KCSAN reports when the kernel boots and wanted to
> > > fix them later.
> > > However I have to admit that I wasn't aware of the KCSAN KUNIT tests,
> > > and wouldn't have sent the s390 KCSAN enablement upstream if I would
> > > have been aware of failing self tests.
> > >
> > > We'll fix them, and I let you know if things are supposed to work.
> > >
> > > Thanks a lot for making aware of this!
> > 
> > Note: Set `CONFIG_KCSAN_REPORT_ONCE_IN_MS=100` (or smaller) instead of
> > the default to make the test complete faster.
> > 
> > The pattern I see from what Nathan reported is that all test cases
> > that expect race reports don't observe them ("not ok" cases), and all
> > those where no races are meant to be reported are fine ("ok" cases).
> > Without actually seeing the log, I'm guessing that no races are
> > reported at all, which is certainly not working as intended.
> 
> I repro'd, and the problem is part QEMU TCG and a minor problem with
> stack_trace_save() on s390:
> 
> 1. QEMU TCG doesn't seem to want to execute threads concurrently,
>    resulting in no "value changes" being observed. This is probably just
>    a limitation of TCG, and if run on a real CPU, shouldn't be a problem.
>    On QEMU, most test cases will pass with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n
>    (There's one left that requires value changes to be observable)

Is this just a limitation of s390's TCG implementation or in general?
Our CI runs on GitHub Actions, which does not support virtualization so
I believe that all of our tests are being done with TCG and x86_64
passes just fine:

https://github.com/ClangBuiltLinux/continuous-integration2/runs/3473222334?check_suite_focus=true

Good to hear that it is working on bare metal now though, we could still
enable build testing of it at a minimum but it would be nice to see the
tests pass even in QEMU :)

Cheers,
Nathan