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    SubjectRe: [PATCH v3 0/5] enhance DMA CMA on x86
    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?

    >> 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).

    Regards,
    Peter Hurley




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