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

[konrad wilk]

On 10/3/2014 12:06 PM, Akinobu Mita wrote:
> 2014-10-03 23:27 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> > 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?
>
> 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@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).
>
> 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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