----- On Aug 9, 2022, at 8:37 PM, Gavin Shan gshan@xxxxxxxxxx wrote: > 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. In terms of build system, yes, selftests/rseq and selftests/kvm are side-by-side, and I agree it is odd to have a cross-dependency. That's where moving rseq.c to tools/lib/ makes sense. > > - What selftests/kvm needs is rseq-thread-pointer.h, which accounts for ~5% of > functionalities, provided by selftests/rseq/librseq.so. I've never seen this type of argument used to prevent using a library before, except on extremely memory-constrained devices, which is not our target here. Even if you would only use 1% of the features of a library, it does not justify reimplementing that 1% if that code already sits within the same project (kernel selftests). > > - 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. No, it's just moving files around and a bit of Makefile modifications. That's the simple part. > > 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. I strongly favor that we use a two steps approach: 1) immediate fix: include ../rseq/rseq.c into your test code and use the headers, as proposed by Paolo. 2) I'll move librseq code into tools/lib/ and tools/include/rseq/, and adapt the users accordingly. (after the end of my vacation) Thanks, Mathieu > Thanks, > Gavin -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
