linux-kernel - Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc

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Date:	Wed, 10 Sep 2008 14:48:22 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc_coherent

On Wed, Sep 10, 2008 at 09:38:11PM +0900, FUJITA Tomonori wrote:
> On Wed, 10 Sep 2008 14:03:10 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
> > It needs a fix anyway and the
> > right solution here is to fall back to one of the software iommu
> > implementations. The stackable dma_ops patches I have currently in work
> > will do exactly that.
> 
> I'm not sure you need the stackable dma_ops support. Calgary IOMMU had
> the same problem and already solved it with dma_ops-per-device option.

We need stackable dma_ops anyway for paravirt IOMMU support in KVM. And
they will fix this issue too.

> > These flags are already removed in the dma_alloc_coherent function which
> > calls this one. Further I think in the case of a remapping IOMMU like
> 
> Not true about x86/tip/iommu. dma_alloc_coherent in dma-mapping.h does
> that so that swiotlb and pci-nommu don't need the gfp hack. Clearing
> the gfp flags is much simpler than setting up the flags correctly
> mainly because of the fallback device, setting up the flags is really
> difficult.

Yes, dma_alloc_coherent in dma-mapping.h clears the flags. And this
function also calls ops->alloc_coherent which points to the AMD IOMMUs
alloc_coherent function if the driver is in place.

Joerg

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