Date: Thu, 23 Apr 2020 12:18:41 -0400 From: Rich Felker <dalias@...c.org> To: Adhemerval Zanella <adhemerval.zanella@...aro.org> Cc: Nicholas Piggin <npiggin@...il.com>, libc-alpha@...rceware.org, libc-dev@...ts.llvm.org, linuxppc-dev@...ts.ozlabs.org, musl@...ts.openwall.com Subject: Re: Powerpc Linux 'scv' system call ABI proposal take 2 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
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