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_PASS autolearn=ham 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 BBC79C433EF for ; Tue, 19 Jun 2018 19:11:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 75F1E20693 for ; Tue, 19 Jun 2018 19:11:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 75F1E20693 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=perches.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030591AbeFSTLj (ORCPT ); Tue, 19 Jun 2018 15:11:39 -0400 Received: from smtprelay0030.hostedemail.com ([216.40.44.30]:35663 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1030259AbeFSTLf (ORCPT ); Tue, 19 Jun 2018 15:11:35 -0400 Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay03.hostedemail.com (Postfix) with ESMTP id BF630837F242; Tue, 19 Jun 2018 19:11:34 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: size48_4751e7b48fd3c X-Filterd-Recvd-Size: 4114 Received: from XPS-9350 (unknown [172.58.43.195]) (Authenticated sender: joe@perches.com) by omf02.hostedemail.com (Postfix) with ESMTPA; Tue, 19 Jun 2018 19:11:29 +0000 (UTC) Message-ID: <7547dfd4045a5acf23505887065bc228e65f2ff9.camel@perches.com> Subject: Re: [PATCH] kvm: x86: mmu: Add cast to negated bitmasks in update_permission_bitmask() From: Joe Perches To: Matthias Kaehlcke Cc: Paolo Bonzini , Nick Desaulniers , rkrcmar@redhat.com, Thomas Gleixner , hpa@zytor.com, x86@kernel.org, kvm@vger.kernel.org, LKML Date: Tue, 19 Jun 2018 12:11:27 -0700 In-Reply-To: <20180619183622.GB169030@google.com> References: <20180615182945.GN88063@google.com> <66b5e5c3ed82a5e1559419deb841c57bf0751fd3.camel@perches.com> <20180619183622.GB169030@google.com> Content-Type: text/plain; charset="ISO-8859-1" X-Mailer: Evolution 3.28.1-2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2018-06-19 at 11:36 -0700, Matthias Kaehlcke wrote: > On Tue, Jun 19, 2018 at 11:07:47AM -0700, Joe Perches wrote: > > On Tue, 2018-06-19 at 19:35 +0200, Paolo Bonzini wrote: > > > On 19/06/2018 19:23, Joe Perches wrote: > > > > On Tue, 2018-06-19 at 10:08 -0700, Nick Desaulniers wrote: > > > > > On Tue, Jun 19, 2018 at 8:19 AM Paolo Bonzini wrote: > > > > > > > > > > > > On 15/06/2018 20:45, Nick Desaulniers wrote: > > > > > > > > > > > > > > > > > In any case I think it it preferable to fix the code over disabling > > > > > > > > > the warning, unless the warning is bogus or there are just too many > > > > > > > > > occurrences. > > > > > > > > > > > > > > > > Maybe. > > > > > > > > > > > > > > Spurious warning today, actual bug tomorrow? I prefer to not to > > > > > > > disable warnings wholesale. They don't need to find actual bugs to be > > > > > > > useful. Flagging code that can be further specified does not hurt. > > > > > > > Part of the effort to compile the kernel with different compilers is > > > > > > > to add warning coverage, not remove it. That said, there may be > > > > > > > warnings that are never useful (or at least due to some invariant that > > > > > > > only affects the kernel). I cant think of any off the top of my head, > > > > > > > but I'm also not sure this is one. > > > > > > > > > > > > This one really makes the code uglier though, so I'm not really inclined > > > > > > to applying the patch. > > > > > > > > > > Note that of the three variables (w, u, x), only u is used later on. > > > > > What about declaring them as negated with the cast, that way there's > > > > > no cast in a ternary? > > > > > > > > It'd be simpler to cast in the BYTE_MASK macro itself > > > > > > I don't think that would work, as the ~ would be done on a zero-extended > > > signed int. > > > > True, but the whole concept is dubious. > > The implicit casts are all over the place, > > not just in the file. > > Would that have been any different with the solution you proposed (if > it worked)? > > Apparently both gcc and clang limit the warning to the potentially > more problematic case where a value with sign bit is promoted. I think the warning is exactly equivalent to -Wsign-conversion and we don't normally compile the kernel with that either. Trying to allow a "make W=3" to be compiler warning message free is also silly. I think it's better to make the warning emitted only at a W=3 level instead.