Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)

[Lynn Boger]

Fixes will be in Go 1.16.7 and 1.15.15. Backports are no longer being 
done for Go 1.14.

On 8/2/21 1:18 AM, Michael Ellerman wrote:
> "Paul Murphy" <murp at ibm.com> writes:
>>   
>> (My apologies for however IBM's email client munges this)
>>
>>> I heard it is going to be in Go 1.16.7, but I do not know much about Go.
>>> Maybe the folks in Cc can chime in.
>>   
>>   
>> We have backports primed and ready for the next point release. They
>> are waiting on the release manager to cherrypick them.
> OK good, that is still the correct fix in the long run.
>
>> I think we were aware that our VDSO usage may have exploited some
>> peculiarities in how the ppc64 version was constructed (i.e hand
>> written assembly which just didn't happen to clobber R30).
> Yeah I was "somewhat surprised" that Go thought it could use r30 like
> that across a VDSO call :D
>
> But to be fair the ABI of the VDSO has always been a little fishy,
> because the entry points pretend to be a transparent wrapper around
> system calls, but then in a case like this are not.
>
>> Go up to this point has only used the vdso function __kernel_clock_gettime; it
>> is the only entry point which would need to explicitly avoid R30 for
>> Go's sake.
> I thought about limiting the workaround to just that code, but it seemed
> silly and likely to come back to bite us. Once the compilers decides to
> spill a non-volatile there are plenty of other ones to choose from.
>
> cheers