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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 9E04EC282C3 for ; Fri, 25 Jan 2019 02:06:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 768F4218D2 for ; Fri, 25 Jan 2019 02:06:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728542AbfAYCGl (ORCPT ); Thu, 24 Jan 2019 21:06:41 -0500 Received: from mga18.intel.com ([134.134.136.126]:52907 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727983AbfAYCGl (ORCPT ); Thu, 24 Jan 2019 21:06:41 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga001.jf.intel.com ([10.7.209.18]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 24 Jan 2019 18:06:40 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,519,1539673200"; d="scan'208";a="132914672" Received: from allen-box.sh.intel.com (HELO [10.239.159.136]) ([10.239.159.136]) by orsmga001.jf.intel.com with ESMTP; 24 Jan 2019 18:06:38 -0800 Cc: baolu.lu@linux.intel.com, David Woodhouse , ashok.raj@intel.com, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Liu Yi L , Jacob Pan Subject: Re: [PATCH 1/1] iommu/vt-d: Leave scalable mode default off To: Joerg Roedel References: <20190124023132.20410-1-baolu.lu@linux.intel.com> <20190124132250.k3rnlpaqazmatzaf@8bytes.org> From: Lu Baolu Message-ID: Date: Fri, 25 Jan 2019 10:02:04 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190124132250.k3rnlpaqazmatzaf@8bytes.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Joerg, On 1/24/19 9:22 PM, Joerg Roedel wrote: > On Thu, Jan 24, 2019 at 10:31:32AM +0800, Lu Baolu wrote: >> Commit 765b6a98c1de3 ("iommu/vt-d: Enumerate the scalable >> mode capability") enables VT-d scalable mode if hardware >> advertises the capability. As we will bring up different >> features and use cases to upstream in different patch >> series, it will leave some intermediate kernel versions >> which support partial features. Hence, end user might run >> into problems when they use such kernels on bare metals >> or virtualization environments. > > I don't get it, can you be more specific about the problems that users > might run into? Sorry, I didn't make it clear in the message. Around VT-d scalable mode, we plan to enable several features. For example, (1)basic scalable mode support; (2)aux domain; (3)system level pasid allocation; .... Since they will be submitted in different patch series for reviewing and merging, users will face compatible problems. For example, when users run kernel v5.0, they might fail to assign an ADI (Assignable Device Interface) to a VM because the aux domain is not included yet. They will complain "I have a kernel claimed to support scalable mode, but when I tried to assign an ADI to a VM, ...". So we decide to leave it off by default, and turn it default on later when all the features get merged. Users could try scalable mode features with "intel-iommu=sm_on" in kernel command line. > And is this patch needed as a fix for v5.0 or is it just > a precaution because future patches might break something for users? It will be better if it can be a fix for v5.0. Best regards, Lu Baolu