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Subject: Re: [GIT PULL] s390 updates for 5.15 merge window
To:     Nathan Chancellor <nathan@kernel.org>,
        Marco Elver <elver@google.com>,
        Richard Henderson <rth@twiddle.net>
Cc:     Heiko Carstens <hca@linux.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>
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From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <4ae5cf63-4f86-5a1c-90b5-73912cfe42b9@de.ibm.com>
Date:   Tue, 31 Aug 2021 19:49:41 +0200
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On 31.08.21 19:48, Nathan Chancellor wrote:
> 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:

Maybe Richard knows?

> 
> 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
>