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=-2.3 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 03EB9C4BA12 for ; Wed, 26 Feb 2020 13:08:18 +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 CA1052467F for ; Wed, 26 Feb 2020 13:08:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CA1052467F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=eik.bme.hu Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:44170 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j6wQC-0003zp-TZ for qemu-devel@archiver.kernel.org; Wed, 26 Feb 2020 08:08:16 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:51357) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j6wPY-0003O8-3l for qemu-devel@nongnu.org; Wed, 26 Feb 2020 08:07:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1j6wPW-0008Ov-CM for qemu-devel@nongnu.org; Wed, 26 Feb 2020 08:07:35 -0500 Received: from zero.eik.bme.hu ([152.66.115.2]:40256) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1j6wPV-00085c-MW; Wed, 26 Feb 2020 08:07:34 -0500 Received: from zero.eik.bme.hu (blah.eik.bme.hu [152.66.115.182]) by localhost (Postfix) with SMTP id AFD84747E03; Wed, 26 Feb 2020 14:07:23 +0100 (CET) Received: by zero.eik.bme.hu (Postfix, from userid 432) id 8BABB747E02; Wed, 26 Feb 2020 14:07:23 +0100 (CET) Received: from localhost (localhost [127.0.0.1]) by zero.eik.bme.hu (Postfix) with ESMTP id 8A0E9747E00; Wed, 26 Feb 2020 14:07:23 +0100 (CET) Date: Wed, 26 Feb 2020 14:07:23 +0100 (CET) From: BALATON Zoltan To: =?ISO-8859-15?Q?Alex_Benn=E9e?= Subject: Re: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC In-Reply-To: <87mu95y1le.fsf@linaro.org> Message-ID: References: <20200218171702.979F074637D@zero.eik.bme.hu> <87mu95y1le.fsf@linaro.org> User-Agent: Alpine 2.22 (BSF 395 2020-01-19) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="3866299591-712543236-1582722443=:81576" X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x [fuzzy] X-Received-From: 152.66.115.2 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 , Richard Henderson , qemu-devel@nongnu.org, John Arbuckle , qemu-ppc , Paul Clarke , Howard Spoelstra , David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --3866299591-712543236-1582722443=:81576 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On Wed, 26 Feb 2020, Alex Benn=C3=A9e wrote: > BALATON Zoltan writes: >> OK, then how about keeping the default accurate but allow to opt in to >> use FPU even if it's known to break some bits for workloads where >> users would need speed over accuracy and would be happy to live with >> the limitation. > > About the only comparison I can think of is the thread=3Dsingle:multi > flags for TCG which is mostly there to help developers eliminate causes > of bugs. The default for MTTCG is it is enabled when it's safe. If you > enable it via the command line where QEMU hasn't defaulted it on you > will get lots of loud warnings about potential instability. The most > commonly used case is thread=3Dsingle when you want to check it's not a > MTTCG bug. > > I'm as cautious as Peter here about adding a "faster but broken" comman= d > line flag because users will invariably read up to the "faster" and the= n > spend a lot of time scratching their heads when things break. OK understood. However this is specifically about PPC emulation where som= e=20 bits exists in FPSCR that's hard to emulate with hardfloat but until=20 recently these weren't correctly emulated in my understanding and things=20 still ran OK so it's believed that these bits are seldom used. It was onl= y=20 fixed due to some tests revealing it not because of actual negative=20 effects seen. Unfortunately enabling hardfloat without any other changes would make the= =20 default softfloat case a bit slower so that's why I was asking about also= =20 making hardfloat case the default. I can try to try some other approach to avoid always setting inexact bit=20 and only keep it set as other archs do but not sure when will I have time= =20 to get there. Some help in this would be appreciated by those who know PP= C=20 FPU and interested in running it faster in QEMU. Regards, BALATON Zoltan --3866299591-712543236-1582722443=:81576--