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=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,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 482C9C433B4 for ; Wed, 21 Apr 2021 19:53:06 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id EE3AC613B6 for ; Wed, 21 Apr 2021 19:53:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EE3AC613B6 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=xen.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.114940.219105 (Exim 4.92) (envelope-from ) id 1lZIty-0005xw-IX; Wed, 21 Apr 2021 19:52:46 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 114940.219105; Wed, 21 Apr 2021 19:52:46 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lZIty-0005xp-Fc; Wed, 21 Apr 2021 19:52:46 +0000 Received: by outflank-mailman (input) for mailman id 114940; Wed, 21 Apr 2021 19:52:45 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lZItx-0005xk-I7 for xen-devel@lists.xenproject.org; Wed, 21 Apr 2021 19:52:45 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lZItv-0007Gr-Kt; Wed, 21 Apr 2021 19:52:43 +0000 Received: from [54.239.6.186] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1lZItv-00015Y-BU; Wed, 21 Apr 2021 19:52:43 +0000 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" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=diCTTYxIlC0gKnHznR2F6gUqkZGyW/gzx+1PjTLAaAM=; b=h5DIK7z6Zost4jgsncWVBQMljz 6vzgGwkJxvY0V9Qo4E4aShaB/ORpbo0n0m/87c1gR2xF/qAyiT+CHCacKpnIRrQ9g/Li1wWrc94oz 7HRj210/OVgYGRvdHQNcT0bq34zx33gRCRrnWcAjCRYVqg9asr1j240MVoo+X97xqJfQ=; Subject: Re: [PATCH v3] evtchn/fifo: don't enforce higher than necessary alignment To: Jan Beulich , "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini References: <2a08aa31-fdbf-89ee-cd49-813f818b709a@suse.com> From: Julien Grall Message-ID: <69766ecb-d234-eebb-9b31-1533389a502e@xen.org> Date: Wed, 21 Apr 2021 20:52:41 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi, On 21/04/2021 15:36, Jan Beulich wrote: > Neither the code nor the original commit provide any justification for > the need to 8-byte align the struct in all cases. Enforce just as much > alignment as the structure actually needs - 4 bytes - by using alignof() > instead of a literal number. I had another fresh look today at this patch. The 32-bit padding is right after the field 'ready'. I can't for sure tell how the second half is going to ever be used and how. However, one possibility would be to extend the field 'ready' to 64-bit. With the current code, we could easily make a single 64-bit access without having to know whether the guest is able to interpret the top half. With your approach, we may need to have different path depending on the padding and ensure the new extension cannot be enabled if the padding is 4-byte. Otherwise, the atomicity would be broken. > While relaxation of the requirements is intended here, the primary goal > is to simply get rid of the hard coded number as well its lack of > connection to the structure that is is meant to apply to. Based on what I wrote above, I think the relaxation should not be done to give us more flexibility about possible extension to the structure. Although, I would be worth documenting the reasoning in the code. Cheers, -- Julien Grall