From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-x443.google.com (mail-pf1-x443.google.com [IPv6:2607:f8b0:4864:20::443]) by sourceware.org (Postfix) with ESMTPS id 1BC3C385DC1C; Sat, 25 Apr 2020 11:00:23 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 1BC3C385DC1C Received: by mail-pf1-x443.google.com with SMTP id x2so2404469pfx.7; Sat, 25 Apr 2020 04:00:23 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:subject:to:cc:references:in-reply-to :mime-version:message-id:content-transfer-encoding; bh=WJ1ftYIEA71lZ9W+K+8CT7Do3y8++eTLYCbmeD3KoUI=; b=HcA09ab8BuXIq18c1Lou6BhHn7J1vUu97cp87R7TItWqFjcOHkYGwqFdTF4jVJhtLM wJneYyoP1pmd7IU9IubqCO+fJzdtLKHuF3FiH38Anp7taF6ny9NHXsfbaZNztsN/bNY1 0CPs+eWQX4x0E4hh+lcRXPJYjfoItEut3+SKivT6DSg/q4hsyraXXdJYFepYGpDMCGNS Xqu9mNOiR947eF0WvujNHydegGLlYuiMhSZlAQVXt+7d0QN3SfcA8j2ZshN41EW46sqz HjubFa8595eBuc2wwje6FgWffpc4f5t7vB6LKLqR4aLME/gvEk81M0gzc9XvCMQSKcbL xGTA== X-Gm-Message-State: AGi0PuZp1fyj1QRwVOr33aXf9rsZOwUVecSnbOJRge5ITPyTaiLKSOOr MuCisecrz7e3hl7hZSe55dwHesSY X-Google-Smtp-Source: APiQypItDEkctIh/qxACRlcEm3tb75XxC/s0wL9AAIHQLpsPTBcEow031P839mfNKFW4unJOZbVOFA== X-Received: by 2002:a63:7f1a:: with SMTP id a26mr1174761pgd.252.1587812421787; Sat, 25 Apr 2020 04:00:21 -0700 (PDT) Received: from localhost ([203.220.177.17]) by smtp.gmail.com with ESMTPSA id 1sm7110011pjc.32.2020.04.25.04.00.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 25 Apr 2020 04:00:21 -0700 (PDT) Date: Sat, 25 Apr 2020 20:56:54 +1000 From: Nicholas Piggin Subject: Re: New powerpc vdso calling convention To: binutils@sourceware.org, Christophe Leroy , linuxppc-dev@lists.ozlabs.org Cc: Adhemerval Zanella , Rich Felker , libc-alpha@sourceware.org, libc-dev@lists.llvm.org, Andy Lutomirski , musl@lists.openwall.com, Thomas Gleixner , Vincenzo Frascino References: <1587790194.w180xsw5be.astroid@bobo.none> <9371cac5-20bb-0552-2609-0d537f41fecd@c-s.fr> In-Reply-To: <9371cac5-20bb-0552-2609-0d537f41fecd@c-s.fr> MIME-Version: 1.0 Message-Id: <1587810370.tg8ym9yjpc.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-1.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: binutils@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Binutils mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 25 Apr 2020 11:00:24 -0000 Excerpts from Christophe Leroy's message of April 25, 2020 5:47 pm: >=20 >=20 > Le 25/04/2020 =C3=A0 07:22, Nicholas Piggin a =C3=A9crit=C2=A0: >> As noted in the 'scv' thread, powerpc's vdso calling convention does not >> match the C ELF ABI calling convention (or the proposed scv convention). >> I think we could implement a new ABI by basically duplicating function >> entry points with different names. >=20 > I think doing this is a real good idea. >=20 > I've been working at porting powerpc VDSO to the GENERIC C VDSO, and the=20 > main pitfall has been that our vdso calling convention is not compatible=20 > with C calling convention, so we have go through an ASM entry/exit. >=20 > See https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=3D1714= 69 >=20 > We should kill this error flag return through CR[SO] and get it the=20 > "modern" way like other architectectures implementing the C VDSO: return=20 > 0 when successfull, return -err when failed. Agreed. >> The ELF v2 ABI convention would suit it well, because the caller already >> requires the function address for ctr, so having it in r12 will >> eliminate the need for address calculation, which suits the vdso data >> page access. >>=20 >> Is there a need for ELF v1 specific calls as well, or could those just b= e >> deprecated and remain on existing functions or required to use the ELF >> v2 calls using asm wrappers? >=20 > What's ELF v1 and ELF v2 ? Is ELF v1 what PPC32 uses ? If so, I'd say=20 > yes, it would be good to have it to avoid going through ASM in the middle= . I'm not sure about PPC32. On PPC64, ELFv2 functions must be called with=20 their address in r12 if called at their global entry point. ELFv1 have a=20 function descriptor with call address and TOC in it, caller has to load=20 the TOC if it's global. The vdso doesn't have TOC, it has one global address (the vdso data=20 page) which it loads by calculating its own address. The kernel doesn't change the vdso based on whether it's called by a v1=20 or v2 userspace (it doesn't really know itself and would have to export=20 different functions). glibc has a hack to create something: # define VDSO_IFUNC_RET(value) \ ({ \ static Elf64_FuncDesc vdso_opd =3D { .fd_toc =3D ~0x0 }; \ vdso_opd.fd_func =3D (Elf64_Addr)value; \ &vdso_opd; \ }) If we could make something which links more like any other dso with ELFv1, that would be good. Otherwise I think v2 is preferable so it=20 doesn't have to calculate its own address. >> Is there a good reason for the system call fallback to go in the vdso >> function rather than have the caller handle it? >=20 > I've seen at least one while porting powerpc to the C VDSO: arguments=20 > toward VDSO functions are in volatile registers. If the caller has to=20 > call the fallback by itself, it has to save them before calling the=20 > VDSO, allthought in 99% of cases it won't use them again. With the=20 > fallback called by the VDSO itself, the arguments are still hot in=20 > volatile registers and ready for calling the fallback. That make it very=20 > easy to call them, see patch 5 in the series=20 > (https://patchwork.ozlabs.org/project/linuxppc-dev/patch/59bea35725ab4cef= c67a678577da8b3ab7771af5.1587401492.git.christophe.leroy@c-s.fr/) I see. Well the kernel can probably patch in sc or scv depending on=20 which is supported, so we could keep the automatic fallback. Thanks, Nick