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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 64618C47255 for ; Mon, 11 May 2020 16:05:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3E9A9206E6 for ; Mon, 11 May 2020 16:05:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730589AbgEKQFt (ORCPT ); Mon, 11 May 2020 12:05:49 -0400 Received: from mga17.intel.com ([192.55.52.151]:4979 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729556AbgEKQFt (ORCPT ); Mon, 11 May 2020 12:05:49 -0400 IronPort-SDR: WyOoBFdLgQZkD3SGqrwvEQD9YVgA35ycz63XCKo4Jb4HX2FJ4JfeGyKLXOy1D0MHtyiwlgdXgf NFdiZ4LTt7og== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 May 2020 09:05:37 -0700 IronPort-SDR: CuMDmvp+guQCKnW5QRpcjkWNuNQFRokn/Nj3L8qOqtPmgy06fw4MKiTsfMC/lEktCXYOblITrY UgO4wK6iXg2Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,380,1583222400"; d="scan'208";a="463220514" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.152]) by fmsmga005.fm.intel.com with ESMTP; 11 May 2020 09:05:37 -0700 Date: Mon, 11 May 2020 09:05:37 -0700 From: Sean Christopherson To: Peter Xu , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Oliver Upton , Tony Cook , zoran.davidovac@gmail.com, euloanty@live.com Subject: Re: [PATCH] KVM: Fix a warning in __kvm_gfn_to_hva_cache_init() Message-ID: <20200511160537.GC24052@linux.intel.com> References: <20200504190526.84456-1-peterx@redhat.com> <20200505013929.GA17225@linux.intel.com> <20200505141245.GH6299@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200505141245.GH6299@xz-x1> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +cc a few other people that have reported this at one time or another. On Tue, May 05, 2020 at 10:12:45AM -0400, Peter Xu wrote: > On Mon, May 04, 2020 at 06:39:29PM -0700, Sean Christopherson wrote: > > On Mon, May 04, 2020 at 03:05:26PM -0400, Peter Xu wrote: > > > GCC 10.0.1 gives me this warning when building KVM: > > > > > > warning: ‘nr_pages_avail’ may be used uninitialized in this function [-Wmaybe-uninitialized] > > > 2442 | for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) { > > > > > > It should not happen, but silent it. > > > > Heh, third times a charm? This has been reported and proposed twice > > before[1][2]. Are you using any custom compiler flags? E.g. -O3 is known > > to cause false positives with -Wmaybe-uninitialized. > > No, what I did was only upgrading to Fedora 32 (which will auto-upgrade GCC), > so it should be using the default params of whatever provided. > > > > > If we do end up killing this warning, I'd still prefer to use > > uninitialized_var() over zero-initializing the variable. > > > > [1] https://lkml.kernel.org/r/20200218184756.242904-1-oupton@google.com > > [2] https://bugzilla.kernel.org/show_bug.cgi?id=207173 > > OK, I didn't know this is a known problem and discussions going on. But I > guess it would be good to address this sooner because it could become a common > warning very soon after people upgrades gcc. Ya, others are hitting this as well. It's especially painful with the existence of KVM_WERROR. Paolo, any preference on how to resolve this? It would appear GCC 10 got "smarter".