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

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Date:	Fri, 3 Oct 2014 09:40:16 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Don Dutile <ddutile@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, iommu@...ts.linux-foundation.org,
	Greg KH <greg@...ah.com>
Subject: Re: [PATCH v3 0/5] enhance DMA CMA on x86

On Fri, Oct 03, 2014 at 08:08:33AM +0900, Akinobu Mita wrote:
> 2014-10-03 7:03 GMT+09:00 Peter Hurley <peter@...leysoftware.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.szyprowski@...sung.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@...sung.com>
> >     Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> >     CC: Michal Nazarewicz <mina86@...a86.com>
> >     Acked-by: Arnd Bergmann <arnd@...db.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'.
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