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

From: H. Peter Anvin (
Date: Tue Jul 01 2003 - 12:54:31 EST

Followup to: <1057080529.2003.62.camel@mulgrave>
By author: James Bottomley <>
In newsgroup:
> 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?

It's a window (in the form of a BAR - base and mask) within which
IOMMU mapping always takes place. Outside the window everything is

This applies to all x86-64 machines and some i386 machines, in
particular those i386 chipsets with "full GART" support as opposed to
"AGP only GART" (my terminology.)

Andi likes to say this isn't a real IOMMU (mostly because it doesn't
solve the legacy region problem), but I disagree with that view. It
still would be nicer if it covered more address space, though.

I don't know if it would be worthwhile to support "full GART" on the
i386 systems which support it.


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