On Fri, Oct 03, 2014 at 08:08:33AM +0900, 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. > > > 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.
Right. That sounds like a good plan .. > > I'm not sure I read correctly, though. Can boot option 'cma=0' also > help avoiding CMA from IOMMU implementation? .. it would automatically done now instead of having to pass 'cma=0'. -- 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/
