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 Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 793D1C433F5 for ; Wed, 12 Jan 2022 09:23:25 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.256414.439985 (Exim 4.92) (envelope-from ) id 1n7ZqT-0004Yp-5w; Wed, 12 Jan 2022 09:23:05 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 256414.439985; Wed, 12 Jan 2022 09:23:05 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1n7ZqT-0004Yi-1U; Wed, 12 Jan 2022 09:23:05 +0000 Received: by outflank-mailman (input) for mailman id 256414; Wed, 12 Jan 2022 09:23:04 +0000 Received: from se1-gles-sth1-in.inumbo.com ([159.253.27.254] helo=se1-gles-sth1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1n7ZqR-0004Yb-Ue for xen-devel@lists.xenproject.org; Wed, 12 Jan 2022 09:23:03 +0000 Received: from ppsw-42.csi.cam.ac.uk (ppsw-42.csi.cam.ac.uk [131.111.8.142]) by se1-gles-sth1.inumbo.com (Halon) with ESMTPS id 3ce9f228-7389-11ec-9ce5-af14b9085ebd; Wed, 12 Jan 2022 10:23:03 +0100 (CET) Received: from hades.srcf.societies.cam.ac.uk ([131.111.179.67]:47142) by ppsw-42.csi.cam.ac.uk (ppsw.cam.ac.uk [131.111.8.138]:25) with esmtps (TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256) id 1n7ZqO-000QjG-6l (Exim 4.95) (return-path ); Wed, 12 Jan 2022 09:23:00 +0000 Received: from [192.168.1.10] (host-92-12-61-86.as13285.net [92.12.61.86]) (Authenticated sender: amc96) by hades.srcf.societies.cam.ac.uk (Postfix) with ESMTPSA id A7F4A1FD4A; Wed, 12 Jan 2022 09:22:59 +0000 (GMT) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 3ce9f228-7389-11ec-9ce5-af14b9085ebd X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: https://help.uis.cam.ac.uk/email-scanner-virus Message-ID: Date: Wed, 12 Jan 2022 09:22:59 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Content-Language: en-GB To: Jan Beulich , "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <01baee92-9d7f-5a2c-d63f-1de390bc10e2@suse.com> From: Andrew Cooper Subject: Re: [PATCH] x86: replace a few do_div() uses In-Reply-To: <01baee92-9d7f-5a2c-d63f-1de390bc10e2@suse.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 12/01/2022 09:00, Jan Beulich wrote: > When the macro's "return value" is not used, the macro use can be > replaced by a simply division, avoiding some obfuscation. > > According to my observations, no change to generated code. > > Signed-off-by: Jan Beulich I like this change in principle, but see below. do_div() needs to be deleted, because it's far too easy screw up.  At a bare minimum, it should be replaced with a static inline that takes it's first parameter by pointer, because then at least every callsite reads correctly in terms of the C language. > --- a/xen/arch/x86/time.c > +++ b/xen/arch/x86/time.c > @@ -610,8 +610,7 @@ static uint64_t xen_timer_cpu_frequency( > struct vcpu_time_info *info = &this_cpu(vcpu_info)->time; > uint64_t freq; > > - freq = 1000000000ULL << 32; > - do_div(freq, info->tsc_to_system_mul); > + freq = (1000000000ULL << 32) / info->tsc_to_system_mul; > if ( info->tsc_shift < 0 ) > freq <<= -info->tsc_shift; do_div()'s output is consumed here.  I don't think this hunk is safe to convert. ~Andrew