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

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <1057080529.2003.62.camel@mulgrave>
By author:    James Bottomley <James.Bottomley@steeleye.com>
In newsgroup: linux.dev.kernel
> 
> 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
bypass.

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.

	-hpa
-- 
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