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

[Rich Felker <dalias@libc.org>]

On Sat, Apr 25, 2020 at 01:40:24PM +1000, Nicholas Piggin wrote:
> Excerpts from Rich Felker's message of April 24, 2020 3:42 am:
> > On Thu, Apr 23, 2020 at 02:15:58PM -0300, Adhemerval Zanella wrote:
> >> 
> >> 
> >> On 23/04/2020 13:43, Rich Felker wrote:
> >> > On Thu, Apr 23, 2020 at 01:35:01PM -0300, Adhemerval Zanella wrote:
> >> >>
> >> >>
> >> >> 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.
> >> > 
> >> > AIUI (at least this is true for all other archs) the functions have
> >> > normal external function call ABI and calling them has nothing to do
> >> > with syscall mechanisms.
> >> 
> >> My point is powerpc specifically does not follow it, since it issues a
> >> syscall in fallback and its semantic follow kernel syscalls (error
> >> signalled in cr0, r3 being always a positive value):
> > 
> > Oh, then I think we'll just ignore these unless the kernel can make
> > ones with a reasonable ABI. It's not worth having ppc-specific code
> > for this... It would be really nice if ones that actually behave like
> > functions could be added though.
> 
> Yeah this is an annoyance for me after making the scv ABI return -ve in 
> r3 for error and other things that more closely follow function calls, 
> we still have the vdso functions using the old style.
> 
> Maybe we should add function call style vdso too.

Please do.

Rich