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 CBDC4C433E0 for ; Wed, 17 Feb 2021 17:03:29 +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 75E8164E42 for ; Wed, 17 Feb 2021 17:03:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 75E8164E42 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com 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.86377.162162 (Exim 4.92) (envelope-from ) id 1lCQEO-0006BC-8F; Wed, 17 Feb 2021 17:03:16 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 86377.162162; Wed, 17 Feb 2021 17:03:16 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lCQEO-0006B5-58; Wed, 17 Feb 2021 17:03:16 +0000 Received: by outflank-mailman (input) for mailman id 86377; Wed, 17 Feb 2021 17:03:14 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lCQEM-0006B0-RC for xen-devel@lists.xenproject.org; Wed, 17 Feb 2021 17:03:14 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id ff620281-393a-4fce-a474-807e649b114f; Wed, 17 Feb 2021 17:03:13 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id D249CB7AE; Wed, 17 Feb 2021 17:03:12 +0000 (UTC) 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: ff620281-393a-4fce-a474-807e649b114f X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613581392; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ofAHBmxiXPw1XkhhaHsSb0iNu3YjM+dKi/K/AamBjAo=; b=fa6pjqKtXTEqcyIiun3ywb4rGaRjbEvMOHurmwb0Uuzu7zD/3w63IAvVXFDrGWkpgomLcJ SJdjaHg6S2BUTX/yjQnX3ri/OE0ia+espKFYJKtPVAd8mip4JkGMmNehWF/fw/keIEc+ZR y9YgRDH/o8j2Sauq6uJBsaHJY5cXb7s= Subject: Re: [RFC] xen/arm: introduce XENFEAT_ARM_dom0_iommu To: Bertrand Marquis Cc: Julien Grall , "xen-devel@lists.xenproject.org" , "Volodymyr_Babchuk@epam.com" , Rahul Singh , Stefano Stabellini References: <538F9995-6D8D-4B02-A9B6-7C5F26F95657@arm.com> From: Jan Beulich Message-ID: Date: Wed, 17 Feb 2021 18:03:12 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <538F9995-6D8D-4B02-A9B6-7C5F26F95657@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 17.02.2021 16:34, Bertrand Marquis wrote: >> On 17 Feb 2021, at 02:00, Stefano Stabellini wrote: >> Today Linux uses the swiotlb-xen driver (drivers/xen/swiotlb-xen.c) to >> translate addresses for DMA operations in Dom0. Specifically, >> swiotlb-xen is used to translate the address of a foreign page (a page >> belonging to a domU) mapped into Dom0 before using it for DMA. >> >> This is important because although Dom0 is 1:1 mapped, DomUs are not. On >> systems without an IOMMU swiotlb-xen enables PV drivers to work as long >> as the backends are in Dom0. Thanks to swiotlb-xen, the DMA operation >> ends up using the MFN, rather than the GFN. >> >> >> On systems with an IOMMU, this is not necessary: when a foreign page is >> mapped in Dom0, it is added to the Dom0 p2m. A new GFN->MFN translation >> is enstablished for both MMU and SMMU. Dom0 could safely use the GFN >> address (instead of the MFN) for DMA operations and they would work. It >> would be more efficient than using swiotlb-xen. >> >> If you recall my presentation from Xen Summit 2020, Xilinx is working on >> cache coloring. With cache coloring, no domain is 1:1 mapped, not even >> Dom0. In a scenario where Dom0 is not 1:1 mapped, swiotlb-xen does not >> work as intended. >> >> >> The suggested solution for both these issues is to add a new feature >> flag "XENFEAT_ARM_dom0_iommu" that tells Dom0 that it is safe not to use >> swiotlb-xen because IOMMU translations are available for Dom0. If >> XENFEAT_ARM_dom0_iommu is set, Linux should skip the swiotlb-xen >> initialization. I have tested this scheme with and without cache >> coloring (hence with and without 1:1 mapping of Dom0) on ZCU102 and it >> works as expected: DMA operations succeed. > > Wouldn’t it be easier to name the flag XENFEAT_ARM_swiotlb_needed ? Except that "swiotlb" is Linux terminology, which I don't think a Xen feature should be named after. Names should be generic, except maybe when they're really targeting exactly one kind of guest (which imo would better never be the case). Jan