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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 D6804C432BE for ; Wed, 1 Sep 2021 14:46:41 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (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 F3E9261056 for ; Wed, 1 Sep 2021 14:46:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org F3E9261056 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=lists.ozlabs.org Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4H06MM48Lpz306d for ; Thu, 2 Sep 2021 00:46:39 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=permerror (SPF Permanent Error: Unknown mechanism found: ip:192.40.192.88/32) smtp.mailfrom=kernel.crashing.org (client-ip=63.228.1.57; helo=gate.crashing.org; envelope-from=segher@kernel.crashing.org; receiver=) Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by lists.ozlabs.org (Postfix) with ESMTP id 4H06Ls1dQbz2xXd for ; Thu, 2 Sep 2021 00:46:12 +1000 (AEST) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 181Ei1DX012275; Wed, 1 Sep 2021 09:44:01 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 181Ei1Gd012274; Wed, 1 Sep 2021 09:44:01 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 1 Sep 2021 09:44:01 -0500 From: Segher Boessenkool To: Michael Ellerman Subject: Re: [PATCH] powerpc/bug: Cast to unsigned long before passing to inline asm Message-ID: <20210901144401.GI1583@gate.crashing.org> References: <20210831132720.881643-1-mpe@ellerman.id.au> <20210831213432.GF1583@gate.crashing.org> <87tuj43gu1.fsf@mpe.ellerman.id.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87tuj43gu1.fsf@mpe.ellerman.id.au> User-Agent: Mutt/1.4.2.3i 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: nathan@kernel.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Wed, Sep 01, 2021 at 05:17:26PM +1000, Michael Ellerman wrote: > Segher Boessenkool writes: > > On Tue, Aug 31, 2021 at 11:27:20PM +1000, Michael Ellerman wrote: > >> Nathan filed an LLVM bug [2], in which Eli Friedman explained that "if > >> you pass a value of a type that's narrower than a register to an inline > >> asm, the high bits are undefined". In this case we are passing a bool > >> to the inline asm, which is a single bit value, and so the compiler > >> thinks it can leave the high bits of r30 unmasked, because only the > >> value of bit 0 matters. > >> > >> Because the inline asm compares the full width of the register (32 or > >> 64-bit) we need to ensure the value passed to the inline asm has all > >> bits defined. So fix it by casting to long. > >> > >> We also already cast to long for the similar case in BUG_ENTRY(), which > >> it turns out was added to fix a similar bug in 2005 in commit > >> 32818c2eb6b8 ("[PATCH] ppc64: Fix issue with gcc 4.0 compiled kernels"). > > > > That points to , which shows the correct > > explanation. > > That's a pity because I don't understand that explanation ^_^ > > Why does sign extension matter when we're comparing against zero? The "td" insn wants a 64-bit quantity. You have to provide one, the compiler will not do an extend itself, it does not try to understand the asm template in any way. > > The code as it was did **not** pass a bool. It perhaps passed an int > > (so many macros in play, it is hard to tell for sure, but it is int or > > long int, perhaps unsigned (which does not change anything here). > > I don't understand that. It definitely is passing a bool at the source > level. Are you saying it's getting promoted somehow? > > It expands to: > And knode_dead() returns bool: > > static bool knode_dead(struct klist_node *knode) > { > return (unsigned long)knode->n_klist & KNODE_DEAD; > } > > So in my book that means the type there is bool. But I'm not a compiler > guy so I guessing I'm missing something. I was confused by all the macros and stuff. And "bool" in the kernel means "_Bool" now (so it is a character type, with GCC). > > If this is not the correct explanation for LLVM, it needs to solve some > > other bug. > > OK. I just need to get this fixed in the kernel, so I'll do a new > version with a changelog that is ~= "shrug not sure what's going on" and > merge that. Then we can argue about what is really going on later :) One thing you should probably do is never pass expressions as asm operands that are "r". Instead, make a temporary var and assign to that, so it will have the type you want, without being able to forget to add a cast :-) Segher