Messages in this thread | | | Date | Fri, 03 Oct 2014 12:33:56 -0400 | From | konrad wilk <> | Subject | Re: [PATCH v3 0/5] enhance DMA CMA on x86 |
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On 10/3/2014 12:06 PM, Akinobu Mita wrote: > 2014-10-03 23:27 GMT+09:00 Peter Hurley <peter@hurleysoftware.com>: >> On 10/02/2014 07:08 PM, Akinobu Mita wrote: >>> 2014-10-03 7:03 GMT+09:00 Peter Hurley <peter@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.szyprowski@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.szyprowski@samsung.com> >>>> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com> >>>> CC: Michal Nazarewicz <mina86@mina86.com> >>>> Acked-by: Arnd Bergmann <arnd@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? >
If you implement an DMA API producer - aka dma_ops (which is what Peter is thinking I believe) it won't matter which IOMMUs / DMA producers are selected right?
Or are you saying that CMA needs SWIOTLB to handle certain type of pages as a fallback mechanism - and hence there needs to be a tight relationship?
In which case I would look at making SWIOTLB be more library like - the Xen-SWIOTLB already does that by using certain parts of the SWIOTLB code which are exposed to the rest of the kernel.
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