Date: Thu, 23 Apr 2020 09:13:57 -0300 From: Adhemerval Zanella <adhemerval.zanella@...aro.org> To: Rich Felker <dalias@...c.org>, Nicholas Piggin <npiggin@...il.com> Cc: 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 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. On glibc we handle by simulating a function call and analysing the CR0 result: __asm__ __volatile__ ("mtctr %0\n\t" "bctrl\n\t" "mfcr %0\n\t" "0:" : "+r" (r0), "+r" (r3), "+r" (r4), "+r" (r5), "+r" (r6), "+r" (r7), "+r" (r8) : : "r9", "r10", "r11", "r12", "cr0", "ctr", "lr", "memory"); __asm__ __volatile__ ("" : "=r" (rval) : "r" (r3)); On musl you don't have this issue because it does not enable vDSO support on powerpc. And if it eventually does it with the VDSO_* macros the only issue I see is on when vDSO fallbacks to the syscall and it also fails (the return code won't be negated since on musl it uses a default C function pointer issue which does not model the CR0 kernel abi). So I think the extra ctr constraint on glibc powerpc syscall code is not really required. I think I have some patches to optimize this a bit based on previous discussions.
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