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 8BB34C169C4 for ; Wed, 6 Feb 2019 15:28:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5C2D32073F for ; Wed, 6 Feb 2019 15:28:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729867AbfBFP2b (ORCPT ); Wed, 6 Feb 2019 10:28:31 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:54502 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728658AbfBFP2b (ORCPT ); Wed, 6 Feb 2019 10:28:31 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id DA71780D; Wed, 6 Feb 2019 07:28:30 -0800 (PST) Received: from [10.1.196.75] (e110467-lin.cambridge.arm.com [10.1.196.75]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 80B053F675; Wed, 6 Feb 2019 07:28:29 -0800 (PST) Subject: Re: [PATCH 03/19] dma-iommu: don't use a scatterlist in iommu_dma_alloc To: Christoph Hellwig Cc: Joerg Roedel , Catalin Marinas , Will Deacon , Tom Lendacky , iommu@lists.linux-foundation.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20190114094159.27326-1-hch@lst.de> <20190114094159.27326-4-hch@lst.de> <5145b2f7-6fc8-6ed9-4cf2-9b7e1d33b0fe@arm.com> <20190201161638.GJ6532@lst.de> From: Robin Murphy Message-ID: Date: Wed, 6 Feb 2019 15:28:28 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190201161638.GJ6532@lst.de> 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 On 01/02/2019 16:16, Christoph Hellwig wrote: > On Fri, Feb 01, 2019 at 03:24:45PM +0000, Robin Murphy wrote: >> On 14/01/2019 09:41, Christoph Hellwig wrote: >>> Directly iterating over the pages makes the code a bit simpler and >>> prepares for the following changes. >> >> It also defeats the whole purpose of __iommu_dma_alloc_pages(), so I'm not >> really buying the simplification angle - you've *seen* that code, right? ;) > > How does it defeat the purpose of __iommu_dma_alloc_pages? Because if iommu_map() only gets called at PAGE_SIZE granularity, then the IOMMU PTEs will be created at PAGE_SIZE (or smaller) granularity, so any effort to get higher-order allocations matching larger IOMMU block sizes is wasted, and we may as well have just done this: for (i = 0; i < count; i++) { struct page *page = alloc_page(gfp); ... iommu_map(..., page_to_phys(page), PAGE_SIZE, ...); } Really, it's a shame we have to split huge pages for the CPU remap, since in the common case the CPU MMU will have a matching block size, but IIRC there was something in vmap() or thereabouts that explicitly chokes on them. Robin.