Re: [RFC] block layer support for DMA IOMMU bypass mode

From: James Bottomley (
Date: Tue Jul 01 2003 - 12:28:47 EST

On Tue, 2003-07-01 at 12:09, Andi Kleen wrote:
> I assume on 2.5 has this problem, not 2.4, right?

Yes, sorry, I'm so focussed on 2.5 I keep forgetting 2.4.

> But a mask is not good for AMD64 because there is no guarantee
> that the bypass/iommu address is checkable using a mask
> (K8 uses an memory hole for IOMMU purposes and for various
> reasons the hole can be anywhere in the address space)
> This means x86_64 needs an function. Also the name is quite weird and
> the issue is not really BIO specific. How about just calling it
> iommu_address() ?

The name was simply to be consistent with BIO_VMERGE_BOUNDARY which is
another asm/io.h setting for this.

Could you elaborate more on the amd64 IOMMU window. Is this a window
where IOMMU mapping always takes place?

I'm a bit reluctant to put a function like this in because the block
layer does a very good job of being separate from the dma layer.
Maintaining this separation is one of the reasons I added a dma_mask to
the request_queue, not a generic device pointer.


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