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

[Peter Hurley]

On 10/02/2014 07:08 PM, Akinobu Mita wrote:
> 2014-10-03 7:03 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>> 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@xxxxxxxxxxx>
>> 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@xxxxxxxxxxx>
>>     Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
>>     CC: Michal Nazarewicz <mina86@xxxxxxxxxx>
>>     Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
>>
>>
>> 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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