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

From: Gavin Shan <gshan@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: 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
Date: Wed, 10 Aug 2022 10:37:40 +1000	[thread overview]
Message-ID: <8c1f33b4-a5a1-fcfa-4521-36253ffa22c8@redhat.com> (raw)
In-Reply-To: <YvLT1fd8ddybF5Uw@google.com>

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