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Thu, 23 Apr 2020 05:14:01 -0700 (PDT) Received: from [192.168.1.4] ([177.194.48.209]) by smtp.googlemail.com with ESMTPSA id h25sm1509603qto.87.2020.04.23.05.13.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Apr 2020 05:14:01 -0700 (PDT) To: Rich Felker , Nicholas Piggin Cc: libc-alpha@sourceware.org, libc-dev@lists.llvm.org, linuxppc-dev@lists.ozlabs.org, musl@lists.openwall.com References: <20200416175932.GZ11469@brightrain.aerifal.cx> <4f824a37-e660-8912-25aa-fde88d4b79f3@linaro.org> <20200416183151.GA11469@brightrain.aerifal.cx> <1587344003.daumxvs1kh.astroid@bobo.none> <20200420013412.GZ11469@brightrain.aerifal.cx> <1587348538.l1ioqml73m.astroid@bobo.none> <20200420040926.GA11469@brightrain.aerifal.cx> <1587356128.aslvdnmtbw.astroid@bobo.none> <20200420172715.GC11469@brightrain.aerifal.cx> <1587531042.1qvc287tsc.astroid@bobo.none> <20200423023642.GP11469@brightrain.aerifal.cx> From: Adhemerval Zanella Autocrypt: addr=adhemerval.zanella@linaro.org; prefer-encrypt=mutual; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200423023642.GP11469@brightrain.aerifal.cx> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-12.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, KAM_NUMSUBJECT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, 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: Thu, 23 Apr 2020 12:14:03 -0000 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.