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=-20.1 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, USER_AGENT_SANE_1 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 4057BC433DB for ; Mon, 15 Mar 2021 11:30:14 +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 BA71F64E20 for ; Mon, 15 Mar 2021 11:30:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BA71F64E20 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]:43512 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lLlQK-0008R7-NN for qemu-devel@archiver.kernel.org; Mon, 15 Mar 2021 07:30:12 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37844) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lLlP6-0007tb-4G for qemu-devel@nongnu.org; Mon, 15 Mar 2021 07:28:56 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:41348) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lLlP3-00046e-Bb for qemu-devel@nongnu.org; Mon, 15 Mar 2021 07:28:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615807731; h=from:from:reply-to: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: in-reply-to:in-reply-to:references:references; bh=BF/kPbGmyPXKU5KMTfMDPVa76hTc325kiYTd/fAt1Zs=; b=aCPeno2QttayhKgq+zcUyaXFgolsz+aIHAl10rK6JAIg/Iw+Pn5lAVx00rpy15s1le6z0g ZXgq0LPB9nnI/ctq53gMUdNNhyVTg3kfJRXdnim5ed8hxdUH0H5nY8akVusACgebzH9PVc PzmAcOHj7SgeoH4IsFTifI17egeh0Jg= 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-254-nk3bRxAWOTCFFHf5Jdq6Kw-1; Mon, 15 Mar 2021 07:28:47 -0400 X-MC-Unique: nk3bRxAWOTCFFHf5Jdq6Kw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A1A4D101F000; Mon, 15 Mar 2021 11:28:46 +0000 (UTC) Received: from redhat.com (ovpn-115-81.ams2.redhat.com [10.36.115.81]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 89C0A60C5F; Mon, 15 Mar 2021 11:28:45 +0000 (UTC) Date: Mon, 15 Mar 2021 11:28:42 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Subject: Re: [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz. Message-ID: References: <20210311200702.1302855-1-eblake@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210311200702.1302855-1-eblake@redhat.com> User-Agent: Mutt/2.0.5 (2021-01-21) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=berrange@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_H4=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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Peter Maydell , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Mar 11, 2021 at 02:07:02PM -0600, Eric Blake wrote: > 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! Create a gitlab.com account, and fork the QEMU repo. Then simply push your branch to your QEMU fork. Pipeline will run automagically and be visible in https://gitlab.com//qemu/-/pipelines > 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? No pref from me if both achieve the same end result. > 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; > + } Reviewed-by: Daniel P. Berrangé Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|