2014-10-03 7:03 GMT+09:00 Peter Hurley <[email protected]>: > 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. > 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 <[email protected]> > 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 <[email protected]> > Signed-off-by: Kyungmin Park <[email protected]> > CC: Michal Nazarewicz <[email protected]> > Acked-by: Arnd Bergmann <[email protected]> > > > 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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
