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=-12.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, 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 8AF9DC64E7B for ; Wed, 2 Dec 2020 14:11:28 +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 17F5821D7F for ; Wed, 2 Dec 2020 14:11:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 17F5821D7F 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.42759.76943 (Exim 4.92) (envelope-from ) id 1kkSqc-0000J0-3s; Wed, 02 Dec 2020 14:11:10 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 42759.76943; Wed, 02 Dec 2020 14:11:10 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kkSqb-0000It-VC; Wed, 02 Dec 2020 14:11:09 +0000 Received: by outflank-mailman (input) for mailman id 42759; Wed, 02 Dec 2020 14:11:08 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kkSqa-0000In-Of for xen-devel@lists.xenproject.org; Wed, 02 Dec 2020 14:11:08 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kkSqa-0002Z0-D7; Wed, 02 Dec 2020 14:11:08 +0000 Received: from [54.239.6.188] (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 1kkSqa-0001AL-3l; Wed, 02 Dec 2020 14:11:08 +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=JhAUXmvFa2YAXnelqSqr2MrMrVaCGJG+Ro8or6l66i8=; b=qs0dzSljClMkN2Q9r++2ip3B/l pYtK4hBzwJN5KKEG0Ie9PiQZwT/XIsTGh9GVJNKBDwC4m+CeCgfB4IJ6oH/LJjuv8v4tqRjqRYa6T ODRxHlCdRspSBalVjmnGzLv5NWpKwciNbX1/yxSA7DRFh+VL+odxJSpJQrGoUm2iH4gs=; Subject: Re: [PATCH v2 4/8] xen/arm: Remove support for MSI on SMMUv3 To: Rahul Singh , Stefano Stabellini Cc: "xen-devel@lists.xenproject.org" , Bertrand Marquis , Volodymyr Babchuk References: From: Julien Grall Message-ID: <5689cfe7-ca16-6540-d394-00d3f60f4f5f@xen.org> Date: Wed, 2 Dec 2020 14:11:06 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 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 Rahul, On 02/12/2020 13:12, Rahul Singh wrote: > Hello Stefano, > >> On 2 Dec 2020, at 12:40 am, Stefano Stabellini wrote: >> >> On Tue, 1 Dec 2020, Stefano Stabellini wrote: >>> On Thu, 26 Nov 2020, Rahul Singh wrote: >>>> XEN does not support MSI on ARM platforms, therefore remove the MSI >>>> support from SMMUv3 driver. >>>> >>>> Signed-off-by: Rahul Singh >>> >>> I wonder if it makes sense to #ifdef CONFIG_MSI this code instead of >>> removing it completely. >> >> One more thought: could this patch be achieved by reverting >> 166bdbd23161160f2abcea70621adba179050bee ? If this patch could be done >> by a couple of revert, it would be great to say it in the commit >> message. >> > Ok will add in next version. > >> >>> In the past, we tried to keep the entire file as textually similar to >>> the original Linux driver as possible to make it easier to backport >>> features and fixes. So, in this case we would probably not even used an >>> #ifdef but maybe something like: >>> >>> if (/* msi_enabled */ 0) >>> return; >>> >>> at the beginning of arm_smmu_setup_msis. >>> >>> >>> However, that strategy didn't actually work very well because backports >>> have proven difficult to do anyway. So at that point we might as well at >>> least have clean code in Xen and do the changes properly. It was difficult because Linux decided to rework how IOMMU drivers works. I agree the risk is still there and therefore clean code would be better with some caveats (see below). > > Main reason to remove the feature/code that is not usable in XEN is to have a clean code. I agree that short term this feature will not be usable. However, I think we need to think about {medium, long}-term plan to avoid extra effort in the future because the driver evolve in a way making the revert of revert impossible. Therefore I would prefer to keep both the MSI and PCI ATS present as they are going to be useful/necessary on some platforms. It doesn't matter that they don't work because the driver will be in tech preview. Cheers, -- Julien Grall