From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from brightrain.aerifal.cx (216-12-86-13.cv.mvl.ntelos.net [216.12.86.13]) by sourceware.org (Postfix) with ESMTPS id 3C1A8385DC1C for ; Sat, 25 Apr 2020 04:52:50 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 3C1A8385DC1C Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=libc.org Authentication-Results: sourceware.org; spf=none smtp.mailfrom=dalias@libc.org Date: Sat, 25 Apr 2020 00:52:48 -0400 From: Rich Felker To: Nicholas Piggin Cc: Adhemerval Zanella , libc-alpha@sourceware.org, libc-dev@lists.llvm.org, linuxppc-dev@lists.ozlabs.org, musl@lists.openwall.com Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2 Message-ID: <20200425045248.GG11469@brightrain.aerifal.cx> References: <20200420172715.GC11469@brightrain.aerifal.cx> <1587531042.1qvc287tsc.astroid@bobo.none> <20200423023642.GP11469@brightrain.aerifal.cx> <20200423161841.GU11469@brightrain.aerifal.cx> <3fe73604-7c92-e073-cbe7-abb4a8ae7c1a@linaro.org> <20200423164314.GX11469@brightrain.aerifal.cx> <64d82a23-1f6e-2e6a-b7a9-0eeab8a53435@linaro.org> <20200423174214.GZ11469@brightrain.aerifal.cx> <1587785455.59207xhucl.astroid@bobo.none> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1587785455.59207xhucl.astroid@bobo.none> User-Agent: Mutt/1.5.21 (2010-09-15) X-Spam-Status: No, score=-4.0 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS, KAM_LAZY_DOMAIN_SECURITY, KAM_NUMSUBJECT, KHOP_HELO_FCRDNS, RDNS_DYNAMIC, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 25 Apr 2020 04:52:52 -0000 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