powerpc Linux scv support and scv system call ABI proposal

[Tulio Magno Quites Machado Filho]

Nicholas Piggin <npiggin at gmail.com> writes:

> Adhemerval Zanella's on January 29, 2020 3:26 am:
>> 
>> We already had to push a similar hack where glibc used to abort transactions
>> prior syscalls to avoid some side-effects on kernel (commit 56cf2763819d2f).
>> It was eventually removed from syscall handling by f0458cf4f9ff3d870, where
>> we only enable TLE if kernel suppors PPC_FEATURE2_HTM_NOSC.
>> 
>> The transaction syscall abort used to read a variable directly from TCB,
>> so this could be an option. I would expect that we could optimize it where
>> if glibc is building against a recent kernel and compiler is building
>> for a ISA 3.0+ cpu we could remove the 'sc' code.
>> 
>
> We would just have to be careful of running on ISA 3.0 CPUs on older
> kernels which do not support scv.

Can we assume that, if a syscall is available through sc it's also available
in scv 0?

Because if that's true, I believe your suggestion to interpret PPC_FEATURE2_SCV
as scv 0 support would be helpful to provide this support via IFUNC even
when glibc is built using --with-cpu=power8, which is the most common scenario
in ppc64le.

In that scenario, it seems new HWCAP bits for new vectors wouldn't be too
frequent, which was the only downside of this proposal.

-- 
Tulio Magno