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Date:   Tue, 9 Aug 2022 21:38:29 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Gavin Shan <gshan@redhat.com>, shuah <shuah@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        kvmarm@lists.cs.columbia.edu, KVM list <kvm@vger.kernel.org>,
        linux-kselftest <linux-kselftest@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, maz <maz@kernel.org>,
        oliver upton <oliver.upton@linux.dev>,
        andrew jones <andrew.jones@linux.dev>, yihyu@redhat.com,
        shan gavin <shan.gavin@gmail.com>
Subject: Re: [PATCH 1/2] KVM: selftests: Make rseq compatible with glibc-2.35
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On Tue, Aug 09, 2022, Mathieu Desnoyers wrote:
> ----- On Aug 9, 2022, at 8:21 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
> 
> > ----- Gavin Shan <gshan@redhat.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?