Re: glibc 2.35 failures in elf/tst-cpu-features-cpuinfo-static.

[Florian Weimer <fweimer@redhat.com>]

* Carlos O'Donell:

> On 1/14/22 16:11, Florian Weimer wrote:
>> * Carlos O'Donell:
>> 
>>> Can we make our testing detect this and mark the test XFAIL?
>> 
>> If we treat this as our bug, we'd have to run CPUID on *all* CPUs during
>> glibc startup.  The bug is visible to applications as well.  I don't
>> think this is feasible.
>
> I thought we already ran cpuid at startup for all cpus?
>
> In cpu-features.c (init_cpu_features) we call __cpuid() unconditionally, and that
> is called via ARCH_INIT_CPU_FEATURES() in LIBC_START_MAIN (static), and
> DL_PLATFORM_INIT in ld.so (shared).
>
> In fact we might call cpuid five or six times during startup?

It runs on a random CPU.  It could be CPU 0 or another CPU.  We pretend
that it doesn't matter.

>>> Or as HJ say, blacklist the CPU from the test e.g. UNSUPPORTED?
>> 
>> I think the bug isn't CPU-specific.
>
> Could you expand in this a bit more?

Maybe I was mistaken.  Siddhesh has an i7-8665U.  I must have mixed up
my timelines.  That CPU was released in 2019.

> My concern is that testing should be robust and not return false
> positives to the extent that we can prevent that. False positives,
> regardless of who is at fault, call into question the validity of the
> test infrastructure. There is a cost to such prevention of false
> positives, for certain, there is a practical limit to the work we can
> do.

It's a true positive in the sense that the glibc detection (in
<sys/platform/x86.h>) does not work correctly.  Now that might not be
useful information to us as glibc developers because it's realistically
not our problem, but we as Red Hat (or other distributions) should
actually work towards fixing this issue.

Thanks,
Florian