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=-18.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,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 D0311C433DB for ; Thu, 11 Mar 2021 20:09:39 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 43D5564F80 for ; Thu, 11 Mar 2021 20:09:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 43D5564F80 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:42392 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lKRco-0002L4-3A for qemu-devel@archiver.kernel.org; Thu, 11 Mar 2021 15:09:38 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:56216) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lKRaZ-0000WD-Vk for qemu-devel@nongnu.org; Thu, 11 Mar 2021 15:07:20 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:38477) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lKRaT-0000RV-Ij for qemu-devel@nongnu.org; Thu, 11 Mar 2021 15:07:18 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615493231; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=05azV5Bd1mGh9WjLTiND9BuiQ7RvEdhPVXzl2I02Kzs=; b=ELrdQfL/pVPe1ZiDYI0CHypQCZ5gA1ZwGbrzD+6YFH25pYAfSywLtz1SQXuq15OJJ7f4pQ 6/bTJEhHMLgsF9BSqyF74dISW1/bNCZs/nRnWqX+CPhqAKV1jHQhTxy5iFE0qplWS5/Gx8 FMawV4X0w4qWFyKUjBPyio1OB5HKmx8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-7-mK17ONFaMKmJKrtTMTF3Sw-1; Thu, 11 Mar 2021 15:07:09 -0500 X-MC-Unique: mK17ONFaMKmJKrtTMTF3Sw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C645418460E0; Thu, 11 Mar 2021 20:07:08 +0000 (UTC) Received: from blue.redhat.com (ovpn-113-66.phx2.redhat.com [10.3.113.66]) by smtp.corp.redhat.com (Postfix) with ESMTP id 72BA960853; Thu, 11 Mar 2021 20:07:08 +0000 (UTC) From: Eric Blake To: qemu-devel@nongnu.org Subject: [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz. Date: Thu, 11 Mar 2021 14:07:02 -0600 Message-Id: <20210311200702.1302855-1-eblake@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="US-ASCII" Received-SPF: pass client-ip=216.205.24.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , berrange@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Not all floating point fractions are precise. For example, the two nearest 32-bit IEEE float values for 0.345 are 0.344999998808 and 0.34500002861, with the lower one being closer. When our scaling unit is 1000, that in turn can produce instances of double rounding (our first when truncating to get the floating point fraction compared to what the user typed, the second in converting the result of the multiplication back to an integer), resulting in a final result 1 byte smaller than the intuitive integer. For the actual test failure encountered on gitlab cross-i386-user, we can clean things up by adding in DBL_EPSILON (with IEEE double, that is 2^-53) for all values on a scale smaller than Petabytes (that is 2^50), where our introduced error is not enough to add a full byte, but will be enough to cause the subsequent multiplication to overshoot rather than undershoot the nearest integer. And ultimately, we've already documented that fractional values are for human convenience: if a user is worried about byte-level precision when specifying more than 50 bits of sizing, they should already be specifying things in bytes rather than fractions. Fixes: cf923b783efd5 (utils: Improve qemu_strtosz() to have 64 bits of precision) Reported-by: Peter Maydell Signed-off-by: Eric Blake --- I'm not actually sure how to kick off a gitlab CI run of this to see if it fixes the failure originally reported at https://gitlab.com/qemu-project/qemu/-/jobs/1090685134 Pointers welcome! An alternative patch might be writing (uint64_t)(fraction * mul + 0.5) (that is, introduce the fudge factor after the multiplication instead of before). Preferences? util/cutils.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/util/cutils.c b/util/cutils.c index d89a40a8c325..c124d8165f57 100644 --- a/util/cutils.c +++ b/util/cutils.c @@ -25,6 +25,7 @@ #include "qemu/osdep.h" #include "qemu/host-utils.h" #include +#include #include "qemu-common.h" #include "qemu/sockets.h" @@ -329,6 +330,15 @@ static int do_strtosz(const char *nptr, const char **end, "is deprecated: %s", nptr); } endptr++; + /* + * Add in a fudge-factor (2^53 when double is IEEE format) for + * all scales less than P (2^50), so that things like + * 12.345M with unit 1000 produce 12345000 instead of + * 12344999. + */ + if (mul > 1e49) { + fraction += DBL_EPSILON; + } } else { mul = suffix_mul(default_suffix, unit); assert(mul > 0); -- 2.30.1