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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 D745DC432BE for ; Wed, 1 Sep 2021 07:18:24 +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 3685460F3A for ; Wed, 1 Sep 2021 07:18:24 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 3685460F3A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ellerman.id.au 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 4GzwQ64qqRz2yn7 for ; Wed, 1 Sep 2021 17:18:22 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=ellerman.id.au header.i=@ellerman.id.au header.a=rsa-sha256 header.s=201909 header.b=OBrNPLMS; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=ellerman.id.au (client-ip=2401:3900:2:1::2; helo=ozlabs.org; envelope-from=mpe@ellerman.id.au; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=ellerman.id.au header.i=@ellerman.id.au header.a=rsa-sha256 header.s=201909 header.b=OBrNPLMS; dkim-atps=neutral Received: from ozlabs.org (bilbo.ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4GzwPN5p5wz2xXd for ; Wed, 1 Sep 2021 17:17:44 +1000 (AEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ellerman.id.au; s=201909; t=1630480657; bh=/dQxymwNiKkIDn/Vw5+hgsGkv4ijl+N7qYeYs+68+bs=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=OBrNPLMSdTKuUlyMWWWiY6PSXxYtZ1lD6Qq4zBKnsEbu8YjVIVGPoZ8KjjAx46xgg xrD6qRSvcO5zzLKWJtxUXwLx4sRnaS9H2JSKVEClF1ZQyWGx4GJvdLbDNMD7UhzqKN xA+P0XVF17jPzeRNN0K/3hEiYNfr40J+A7851mDm5TGPCRKQ+cttiNPBaFY4BHL9Dw NyxFGGtFBp/rYbogLYll/b3weNjfV7/BWtH4vRp9YEhis1fPIP2K+yCYprazB8Gk3n ZmZa8iyb034dIg/zPsRwHZCwn90JF6w+jU77OzBWt6EgjwZWa9Xgj7BUBCgdPT/z/G smE7Ki7AHsJ2g== Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 4GzwPF5tmVz9sCD; Wed, 1 Sep 2021 17:17:37 +1000 (AEST) From: Michael Ellerman To: Segher Boessenkool Subject: Re: [PATCH] powerpc/bug: Cast to unsigned long before passing to inline asm In-Reply-To: <20210831213432.GF1583@gate.crashing.org> References: <20210831132720.881643-1-mpe@ellerman.id.au> <20210831213432.GF1583@gate.crashing.org> Date: Wed, 01 Sep 2021 17:17:26 +1000 Message-ID: <87tuj43gu1.fsf@mpe.ellerman.id.au> 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: nathan@kernel.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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 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: asm goto( "1: " "tdnei" " " " % 4, 0 " "\n " ".section __ex_table,\"a\";" " " ".balign 4;" " " ".long (1b) - . ;" " " ".long (%l[__label_warn_on]) - . ;" " " ".previous" " " ".section __bug_table,\"aw\"\n" "2:\t.4byte 1b - 2b, %0 - 2b\n" "\t.short %1, %2\n" ".org 2b+%3\n" ".previous\n" : : "i"("lib/klist.c"), "i"(62), "i"((1 << 0) | ((9) << 8)), "i"(sizeof(struct bug_entry)), "r"(knode_dead(knode)) ^^^^^^^^^^^^^^^^^^^^^ : : __label_warn_on); 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. > But td wants a 64-bit register, not a 32-bit one (which are the only two > possibilities for the "r" constraint on PowerPC). The cast to "long" is > fine for powerpc64, but it has nothing to do with "narrower than a > register". If it's 32-bit vs 64-bit, and the clang explanation is correct, then we'd expect the low 32-bits of the value passed to the asm to have the correct value, ie. have been masked with KNODE_DEAD. > 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 :) cheeers