linux-kernel - Re: [PATCH 1/2] KVM: selftests: Make rseq compatible with glibc-2.35

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Date:   Wed, 10 Aug 2022 08:13:46 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Gavin Shan <gshan@...hat.com>, shuah <shuah@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        KVM list <kvm@...r.kernel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, maz <maz@...nel.org>,
        oliver upton <oliver.upton@...ux.dev>,
        andrew jones <andrew.jones@...ux.dev>,
        yihyu <yihyu@...hat.com>, shan gavin <shan.gavin@...il.com>
Subject: Re: [PATCH 1/2] KVM: selftests: Make rseq compatible with
 glibc-2.35

----- On Aug 9, 2022, at 5:38 PM, Sean Christopherson seanjc@...gle.com wrote:

> On Tue, Aug 09, 2022, Mathieu Desnoyers wrote:
>> ----- On Aug 9, 2022, at 8:21 AM, Mathieu Desnoyers
>> mathieu.desnoyers@...icios.com wrote:
>> 
>> > ----- Gavin Shan <gshan@...hat.com> wrote:
>> >> Hi Florian,
>> >> 
>> >> On 8/9/22 5:16 PM, Florian Weimer wrote:
>> >> >>> __builtin_thread_pointer doesn't work on all architectures/GCC
>> >> >>> versions.
>> >> >>> Is this a problem for selftests?
>> >> >>>
>> >> >>
>> >> >> It's a problem as the test case is running on all architectures. I think I
>> >> >> need introduce our own __builtin_thread_pointer() for where it's not
>> >> >> supported: (1) PowerPC  (2) x86 without GCC 11
>> >> >>
>> >> >> Please let me know if I still have missed cases where
>> >> >> __buitin_thread_pointer() isn't supported?
>> >> > 
>> >> > As far as I know, these are the two outliers that also have rseq
>> >> > support.  The list is a bit longer if we also consider non-rseq
>> >> > architectures (csky, hppa, ia64, m68k, microblaze, sparc, don't know
>> >> > about the Linux architectures without glibc support).
>> >> > 
>> >> 
>> >> For kvm/selftests, there are 3 architectures involved actually. So we
>> >> just need consider 4 cases: aarch64, x86, s390 and other. For other
>> >> case, we just use __builtin_thread_pointer() to maintain code's
>> >> integrity, but it's not called at all.
>> >> 
>> >> I think kvm/selftest is always relying on glibc if I'm correct.
>> > 
>> > All those are handled in the rseq selftests and in librseq. Why duplicate all
>> > that logic again?
>> 
>> More to the point, considering that we have all the relevant rseq registration
>> code in tools/testing/selftests/rseq/rseq.c already, and the relevant thread
>> pointer getter code in tools/testing/selftests/rseq/rseq-*thread-pointer.h,
>> is there an easy way to get test applications in tools/testing/selftests/kvm
>> and in tools/testing/selftests/rseq to share that common code ?
>> 
>> Keeping duplicated compatibility code is bad for long-term maintainability.
> 
> Any reason not to simply add tools/lib/rseq.c and then expose a helper to get
> the
> registered rseq struct?

Indeed, moving rseq.c to tools/lib/ would allow building a .so from any selftest
which needs to use it.

And we could move the relevant rseq helper header files to tools/include/rseq/*
as well.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com