Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754107AbaJCQGX (ORCPT ); Fri, 3 Oct 2014 12:06:23 -0400 Received: from mail-la0-f48.google.com ([209.85.215.48]:53934 "EHLO mail-la0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753332AbaJCQGI (ORCPT ); Fri, 3 Oct 2014 12:06:08 -0400 MIME-Version: 1.0 In-Reply-To: <542EB242.4090102@hurleysoftware.com> References: <1397567329-3771-1-git-send-email-akinobu.mita@gmail.com> <5426CA0A.7000806@hurleysoftware.com> <54294C0B.1060705@hurleysoftware.com> <542ABF77.1020402@hurleysoftware.com> <542B5DC2.8020806@hurleysoftware.com> <20141002164121.GF1715@laptop.dumpdata.com> <542DCB9C.4020703@hurleysoftware.com> <542EB242.4090102@hurleysoftware.com> Date: Sat, 4 Oct 2014 01:06:07 +0900 Message-ID: Subject: Re: [PATCH v3 0/5] enhance DMA CMA on x86 From: Akinobu Mita To: Peter Hurley Cc: Konrad Rzeszutek Wilk , Thomas Gleixner , LKML , Andrew Morton , Marek Szyprowski , David Woodhouse , Don Dutile , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , x86@kernel.org, iommu@lists.linux-foundation.org, Greg KH Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2014-10-03 23:27 GMT+09:00 Peter Hurley : > On 10/02/2014 07:08 PM, Akinobu Mita wrote: >> 2014-10-03 7:03 GMT+09:00 Peter Hurley : >>> On 10/02/2014 12:41 PM, Konrad Rzeszutek Wilk wrote: >>>> On Tue, Sep 30, 2014 at 09:49:54PM -0400, Peter Hurley wrote: >>>>> On 09/30/2014 07:45 PM, Thomas Gleixner wrote: >> >>>>> Which is different than if the plan is to ship production units for x86; >>>>> then a general purpose solution will be required. >>>>> >>>>> As to the good design of a general purpose solution for allocating and >>>>> mapping huge order pages, you are certainly more qualified to help Akinobu >>>>> than I am. >>> >>> What Akinobu's patches intend to support is: >>> >>> phys_addr = dma_alloc_coherent(dev, 64 * 1024 * 1024, &bus_addr, GFP_KERNEL); >>> >>> which raises three issues: >>> >>> 1. Where do coherent blocks of this size come from? >>> 2. How to prevent fragmentation of these reserved blocks over time by >>> existing DMA users? >>> 3. Is this support generically required across all iommu implementations on x86? >>> >>> Questions 1 and 2 are non-trivial, in the general case, otherwise the page >>> allocator would already do this. Simply dropping in the contiguous memory >>> allocator doesn't work because CMA does not have the same policy and performance >>> as the page allocator, and is already causing performance regressions even >>> in the absence of huge page allocations. >> >> Could you take a look at the patches I sent? Can they fix these issues? >> https://lkml.org/lkml/2014/9/28/110 >> >> With these patches, normal alloc_pages() is used for allocation first >> and dma_alloc_from_contiguous() is used as a fallback. > > Sure, I can test these patches this weekend. > Where are the unit tests? Thanks a lot. I would like to know whether the performance regression you see will disappear or not with these patches as if CONFIG_DMA_CMA is disabled. >>> So that's why I raised question 3; is making the necessary compromises to support >>> 64MB coherent DMA allocations across all x86 iommu implementations actually >>> required? >>> >>> Prior to Akinobu's patches, the use of CMA by x86 iommu configurations was >>> designed to be limited to testing configurations, as the introductory >>> commit states: >>> >>> commit 0a2b9a6ea93650b8a00f9fd5ee8fdd25671e2df6 >>> Author: Marek Szyprowski >>> Date: Thu Dec 29 13:09:51 2011 +0100 >>> >>> X86: integrate CMA with DMA-mapping subsystem >>> >>> This patch adds support for CMA to dma-mapping subsystem for x86 >>> architecture that uses common pci-dma/pci-nommu implementation. This >>> allows to test CMA on KVM/QEMU and a lot of common x86 boxes. >>> >>> Signed-off-by: Marek Szyprowski >>> Signed-off-by: Kyungmin Park >>> CC: Michal Nazarewicz >>> Acked-by: Arnd Bergmann >>> >>> >>> Which brings me to my suggestion: if support for huge coherent DMA is >>> required only for a special test platform, then could not this support >>> be specific to a new iommu configuration, namely iommu=cma, which would >>> get initialized much the same way that iommu=calgary is now. >>> >>> The code for such a iommu configuration would mostly duplicate >>> arch/x86/kernel/pci-swiotlb.c and the CMA support would get removed from >>> the other x86 iommu implementations. >> >> I'm not sure I read correctly, though. Can boot option 'cma=0' also >> help avoiding CMA from IOMMU implementation? > > Maybe, but that's not an appropriate solution for distro kernels. > > Nor does this address configurations that want a really large CMA so > 1GB huge pages can be allocated (not for DMA though). Now I see the point of iommu=cma you suggested. But what should we do when CONFIG_SWIOTLB is disabled, especially for x86_32? Should we just introduce yet another flag to tell not using DMA_CMA instead of adding new swiotlb-like iommu implementation? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/