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 2B14ECDB474 for ; Mon, 23 Oct 2023 09:20:15 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.621121.967217 (Exim 4.92) (envelope-from ) id 1qur6O-0007K2-8D; Mon, 23 Oct 2023 09:20:00 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 621121.967217; Mon, 23 Oct 2023 09:20:00 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1qur6O-0007Jv-5g; Mon, 23 Oct 2023 09:20:00 +0000 Received: by outflank-mailman (input) for mailman id 621121; Mon, 23 Oct 2023 09:19:59 +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 1qur6N-0007Jp-Gd for xen-devel@lists.xenproject.org; Mon, 23 Oct 2023 09:19:59 +0000 Received: from support.bugseng.com (mail.bugseng.com [162.55.131.47]) by se1-gles-sth1.inumbo.com (Halon) with ESMTPS id 556d3b58-7185-11ee-98d5-6d05b1d4d9a1; Mon, 23 Oct 2023 11:19:57 +0200 (CEST) Received: from support.bugseng.com (support.bugseng.com [162.55.131.47]) by support.bugseng.com (Postfix) with ESMTPA id C68D14EE0740; Mon, 23 Oct 2023 11:19:57 +0200 (CEST) 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: 556d3b58-7185-11ee-98d5-6d05b1d4d9a1 MIME-Version: 1.0 Date: Mon, 23 Oct 2023 11:19:57 +0200 From: Nicola Vetrini To: Jan Beulich Cc: sstabellini@kernel.org, michal.orzel@amd.com, xenia.ragiadakou@amd.com, ayan.kumar.halder@amd.com, consulting@bugseng.com, andrew.cooper3@citrix.com, roger.pau@citrix.com, Simone Ballarin , Doug Goldstein , George Dunlap , Julien Grall , Wei Liu , xen-devel@lists.xenproject.org Subject: Re: [XEN PATCH][for-4.19 v3 1/8] xen/include: add macro LOWEST_BIT In-Reply-To: References: <546cf30aa43d6d0687a9a6c6d23b11128e5783e8.1697815135.git.nicola.vetrini@bugseng.com> User-Agent: Roundcube Webmail/1.4.3 Message-ID: X-Sender: nicola.vetrini@bugseng.com Organization: BUGSENG s.r.l. Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit On 23/10/2023 09:48, Jan Beulich wrote: > On 20.10.2023 17:28, Nicola Vetrini wrote: >> --- a/automation/eclair_analysis/ECLAIR/deviations.ecl >> +++ b/automation/eclair_analysis/ECLAIR/deviations.ecl >> @@ -246,6 +246,12 @@ constant expressions are required.\"" >> "any()"} >> -doc_end >> >> +-doc_begin="The macro LOWEST_BIT encapsulates a well-known pattern to >> obtain the value >> +2^ffs(x) for unsigned integers on two's complement architectures >> +(all the architectures supported by Xen satisfy this requirement)." >> +-config=MC3R1.R10.1,reports+={safe, >> "any_area(any_loc(any_exp(macro(^LOWEST_BIT$))))"} >> +-doc_end > > This being deviated this way (rather than by SAF-* comments) wants > justifying in the description. You did reply to my respective > comment on v2, but such then (imo) needs propagating into the actual > patch as well. > Sure, will do. >> --- a/docs/misra/deviations.rst >> +++ b/docs/misra/deviations.rst >> @@ -192,6 +192,13 @@ Deviations related to MISRA C:2012 Rules: >> See automation/eclair_analysis/deviations.ecl for the full >> explanation. >> - Tagged as `safe` for ECLAIR. >> >> + * - R10.1 >> + - The well-known pattern (x & -x) applied to unsigned integer >> values on 2's >> + complement architectures (i.e., all architectures supported by >> Xen), used >> + to obtain the value 2^ffs(x), where ffs(x) is the position of >> the first >> + bit set. If no bits are set, zero is returned. >> + - Tagged as `safe` for ECLAIR. > > In such an explanation there shall not be any ambiguity. Here I see > an issue with ffs() returning 1-based bit position numbers, which > isn't in line with the use in 2^ffs(x). (Arguably use of ^ itself is > also problematic, as that's XOR in C, not POW. I'd suggest 2^^ffs(x) > or 2**ffs(x).) > Makes sense, I think I'll use an plain english explanation to avoid any confusion about notation. >> --- a/xen/include/xen/macros.h >> +++ b/xen/include/xen/macros.h >> @@ -8,8 +8,11 @@ >> #define DIV_ROUND(n, d) (((n) + (d) / 2) / (d)) >> #define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d)) >> >> -#define MASK_EXTR(v, m) (((v) & (m)) / ((m) & -(m))) >> -#define MASK_INSR(v, m) (((v) * ((m) & -(m))) & (m)) >> +/* Returns the 2^ffs(x) or 0, where ffs(x) is the index of the lowest >> set bit */ >> +#define LOWEST_BIT(x) ((x) & -(x)) > > I'm afraid my concern regarding this new macro's name (voiced on v2) > hasn't > been addressed (neither verbally nor by finding a more suitable name). > > Jan I didn't come up with much better names, so I left it as is. Here's a few ideas: - LOWEST_SET - MASK_LOWEST_SET - MASK_LOWEST_BIT - MASK_FFS_0 - LOWEST_ONE and also yours, included here for convenience, even though you felt it was too long: - ISOLATE_LOW_BIT() All maintainers from THE REST are CC-ed, so we can see if anyone has any suggestion. -- Nicola Vetrini, BSc Software Engineer, BUGSENG srl (https://bugseng.com)