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    SubjectRe: [PATCH 1/2] KVM: selftests: Make rseq compatible with glibc-2.35
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    Hi Mathieu and Sean,

    On 8/10/22 7:38 AM, Sean Christopherson wrote:
    > 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:
    >>>> 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?
    >

    There are couple of reasons, not to share tools/testing/selftests/rseq/librseq.so
    or add tools/lib/librseq.so. Please let me know if the arguments making sense
    to you?

    - By design, selftests/rseq and selftests/kvm are parallel. It's going to introduce
    unnecessary dependency for selftests/kvm to use selftests/rseq/librseq.so. To me,
    it makes the maintainability even harder.

    - What selftests/kvm needs is rseq-thread-pointer.h, which accounts for ~5% of
    functionalities, provided by selftests/rseq/librseq.so.

    - I'm not too much familiar with selftests/rseq, but it seems it need heavy
    rework before it can become tools/lib/librseq.so. However, I'm not sure if
    the effort is worthwhile. The newly added library is fully used by
    testtests/rseq. ~5% of that is going to be used by selftests/kvm.
    In this case, we still have cross-dependency issue.

    I personally prefer not to use selftests/rseq/librseq.so or add tools/lib/librseq.so,
    but I need your feedback. Please share your thoughts.
    Thanks,
    Gavin

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