From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 16EC9C2BB55 for ; Thu, 16 Apr 2020 18:16:15 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C29DA206D6 for ; Thu, 16 Apr 2020 18:16:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C29DA206D6 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=deneb.enyo.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4936qH2y2BzDrCm for ; Fri, 17 Apr 2020 04:16:11 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=deneb.enyo.de (client-ip=37.24.231.21; helo=albireo.enyo.de; envelope-from=fw@deneb.enyo.de; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=deneb.enyo.de Received: from albireo.enyo.de (albireo.enyo.de [37.24.231.21]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4936l40WTGzDqP3 for ; Fri, 17 Apr 2020 04:12:30 +1000 (AEST) Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1jP8zr-00055z-Uu; Thu, 16 Apr 2020 18:12:19 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.92) (envelope-from ) id 1jP8zr-0003ZV-Qv; Thu, 16 Apr 2020 20:12:19 +0200 From: Florian Weimer To: Rich Felker Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2 References: <1586931450.ub4c8cq8dj.astroid@bobo.none> <20200415225539.GL11469@brightrain.aerifal.cx> <87k12gf32r.fsf@mid.deneb.enyo.de> <20200416153509.GT11469@brightrain.aerifal.cx> <87sgh3e613.fsf@mid.deneb.enyo.de> <20200416165257.GY11469@brightrain.aerifal.cx> Date: Thu, 16 Apr 2020 20:12:19 +0200 In-Reply-To: <20200416165257.GY11469@brightrain.aerifal.cx> (Rich Felker's message of "Thu, 16 Apr 2020 12:52:57 -0400") Message-ID: <87ftd3e1vg.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: musl@lists.openwall.com, libc-alpha@sourceware.org, linuxppc-dev@lists.ozlabs.org, Nicholas Piggin , libc-dev@lists.llvm.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" * Rich Felker: > On Thu, Apr 16, 2020 at 06:42:32PM +0200, Florian Weimer wrote: >> * Rich Felker: >> >> > On Thu, Apr 16, 2020 at 06:48:44AM +0200, Florian Weimer wrote: >> >> * Rich Felker: >> >> >> >> > My preference would be that it work just like the i386 AT_SYSINFO >> >> > where you just replace "int $128" with "call *%%gs:16" and the kernel >> >> > provides a stub in the vdso that performs either scv or the old >> >> > mechanism with the same calling convention. >> >> >> >> The i386 mechanism has received some criticism because it provides an >> >> effective means to redirect execution flow to anyone who can write to >> >> the TCB. I am not sure if it makes sense to copy it. >> > >> > Indeed that's a good point. Do you have ideas for making it equally >> > efficient without use of a function pointer in the TCB? >> >> We could add a shared non-writable mapping at a 64K offset from the >> thread pointer and store the function pointer or the code there. Then >> it would be safe. >> >> However, since this is apparently tied to POWER9 and we already have a >> POWER9 multilib, and assuming that we are going to backport the kernel >> change, I would tweak the selection criterion for that multilib to >> include the new HWCAP2 flag. If a user runs this glibc on a kernel >> which does not have support, they will get set baseline (POWER8) >> multilib, which still works. This way, outside the dynamic loader, no >> run-time dispatch is needed at all. I guess this is not at all the >> answer you were looking for. 8-) > > How does this work with -static? :-) -static is not supported. 8-) (If you use the unsupported static libraries, you get POWER8 code.) (Just to be clear, in case someone doesn't get the joke: This is about a potential approach for a heavily constrained, vertically integrated environment. It does not reflect general glibc recommendations.) >> If a single binary is needed, I would perhaps follow what Arm did for >> -moutline-atomics: lay out the code so that its easy to execute for >> the non-POWER9 case, assuming that POWER9 machines will be better at >> predicting things than their predecessors. >> >> Or you could also put the function pointer into a RELRO segment. Then >> there's overlap with the __libc_single_threaded discussion, where >> people objected to this kind of optimization (although I did not >> propose to change the TCB ABI, that would be required for >> __libc_single_threaded because it's an external interface). > > Of course you can use a normal global, but now every call point needs > to setup a TOC pointer (= two entry points and more icache lines for > otherwise trivial functions). > > I think my choice would be just making the inline syscall be a single > call insn to an asm source file that out-of-lines the loading of TOC > pointer and call through it or branch based on hwcap so that it's not > repeated all over the place. I don't know how problematic control flow out of an inline asm is on POWER. But this is basically the -moutline-atomics approach. > Alternatively, it would perhaps work to just put hwcap in the TCB and > branch on it rather than making an indirect call to a function pointer > in the TCB, so that the worst you could do by clobbering it is execute > the wrong syscall insn and thereby get SIGILL. The HWCAP is already in the TCB. I expect this is what generic glibc builds are going to use (perhaps with a bit of tweaking favorable to POWER8 implementations, but we'll see).