2014-10-03 23:27 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: > On 10/02/2014 07:08 PM, Akinobu Mita wrote: >> 2014-10-03 7:03 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: >>> 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 <m.szyprow...@samsung.com> >>> 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 <m.szyprow...@samsung.com> >>> Signed-off-by: Kyungmin Park <kyungmin.p...@samsung.com> >>> CC: Michal Nazarewicz <min...@mina86.com> >>> Acked-by: Arnd Bergmann <a...@arndb.de> >>> >>> >>> 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 majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
