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

[Gavin Shan]

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@xxxxxxxxxxxx wrote:
> > > ----- Gavin Shan <gshan@xxxxxxxxxx> 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