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=-10.9 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,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 B9271C4708F for ; Tue, 1 Jun 2021 16:42:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 96710613AD for ; Tue, 1 Jun 2021 16:42:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234495AbhFAQoS (ORCPT ); Tue, 1 Jun 2021 12:44:18 -0400 Received: from foss.arm.com ([217.140.110.172]:54144 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233904AbhFAQoQ (ORCPT ); Tue, 1 Jun 2021 12:44:16 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0DA9B101E; Tue, 1 Jun 2021 09:42:35 -0700 (PDT) Received: from [10.57.73.64] (unknown [10.57.73.64]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EFABC3F719; Tue, 1 Jun 2021 09:42:33 -0700 (PDT) Subject: Re: [PATCH] iommu: Print default strict or lazy mode at init time To: John Garry , joro@8bytes.org, will@kernel.org Cc: iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, hch@lst.de References: <1622209074-37899-1-git-send-email-john.garry@huawei.com> <3ee986a0-29c1-100c-c72f-360f919caf7d@arm.com> From: Robin Murphy Message-ID: <58ea001c-4c37-9c5a-77be-38ac3bea2476@arm.com> Date: Tue, 1 Jun 2021 17:42:27 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-06-01 16:50, John Garry wrote: > On 01/06/2021 10:09, Robin Murphy wrote: >>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c >>> index 808ab70d5df5..f25fae62f077 100644 >>> --- a/drivers/iommu/iommu.c >>> +++ b/drivers/iommu/iommu.c >>> @@ -138,6 +138,11 @@ static int __init iommu_subsys_init(void) >>>           (iommu_cmd_line & IOMMU_CMD_LINE_DMA_API) ? >>>               "(set via kernel command line)" : ""); >>> +    pr_info("Default DMA domain mode: %s %s\n", >> >> Nit: I think this might be a little unclear for end-users - *I'm* not >> even sure whether "Default" here is meant to refer to the mode setting >> itself or to default domains (of DMA type). Maybe something like "DMA >> domain TLB invalidation policy"? Certainly it seems like a good idea >> to explicitly mention invalidation to correlate with the documentation >> of the "iommu.strict" parameter. >> >> Ack to the general idea though. > > ok, so I'll go with this: > > pr_info("DMA domain default TLB invalidation policy: %s mode %s\n", >               iommu_dma_strict ? "strict" : "lazy", >                (iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ? >                        "(set via kernel command line)" : ""); > > I think it's worth mentioning "default" somewhere, as not all IOMMUs or > devices will use lazy mode even if it's default. But that's part of what I think is misleading - I boot and see that the default is something, so I reboot with iommu.strict to explicitly set it the other way, but now that's the default... huh? The way I see it, we're saying what the current IOMMU API policy is - the value of iommu_dma_strict at any given time is fact - but we're not necessarily saying how widely that policy is enforced. We similarly report the type for default domains from global policy even though that may also be overridden per-group by drivers and/or userspace later; we don't say it's the *default* default domain type. However, having now debugged the AMD issue from another thread, I think doing this at subsys_initcall is in fact going to be too early to be meaningful, since it ignores drivers' ability to change the global policy :( Robin.