[musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Adhemerval Zanella]

On 23/04/2020 13:18, Rich Felker wrote:
> On Thu, Apr 23, 2020 at 09:13:57AM -0300, Adhemerval Zanella wrote:
>>
>>
>> On 22/04/2020 23:36, Rich Felker wrote:
>>> On Wed, Apr 22, 2020 at 04:18:36PM +1000, Nicholas Piggin wrote:
>>>> Yeah I had a bit of a play around with musl (which is very nice code I
>>>> must say). The powerpc64 syscall asm is missing ctr clobber by the way.  
>>>> Fortunately adding it doesn't change code generation for me, but it 
>>>> should be fixed. glibc had the same bug at one point I think (probably 
>>>> due to syscall ABI documentation not existing -- something now lives in 
>>>> linux/Documentation/powerpc/syscall64-abi.rst).
>>>
>>> Do you know anywhere I can read about the ctr issue, possibly the
>>> relevant glibc bug report? I'm not particularly familiar with ppc
>>> register file (at least I have to refamiliarize myself every time I
>>> work on this stuff) so it'd be nice to understand what's
>>> potentially-wrong now.
>>
>> My understanding is the ctr issue only happens for vDSO calls where it
>> fallback to a syscall in case an error (invalid argument, etc. and
>> assuming if vDSO does not fallback to a syscall it always succeed).
>> This makes the vDSO call on powerpc to have same same ABI constraint
>> as a syscall, where it clobbers CR0.
> 
> I think you mean "vsyscall", the old thing glibc used where there are
> in-userspace implementations of some syscalls with call interfaces
> roughly equivalent to a syscall. musl has never used this. It only
> uses the actual exported functions from the vdso which have normal
> external function call ABI.

I wasn't thinking in vsyscall in fact, which afaik it is a x86 thing.
The issue is indeed when calling the powerpc provided functions in 
vDSO, which musl might want to do eventually.