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Date: Thu, 23 Apr 2020 12:18:41 -0400
From: Rich Felker <dalias@libc.org>
To: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
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Cc: libc-dev@lists.llvm.org, libc-alpha@sourceware.org,
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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.

Rich