Re: [Intel IOMMU][patch 8/8] Preserve some Virtual Address when devices cannot address entire range.

[David Miller <davem@davemloft.net>]

From: Roland Dreier <rdreier@cisco.com>
Date: Tue, 24 Apr 2007 14:32:42 -0700

>  > My suggestion would be to allocate top-down in the 32-bit IOMMU space.
> 
> I think that's good for normal things, but it's not unreasonable to
> want to map > 4 GB of memory at once for an Infiniband device.

That's what the DAC interfaces were for, but people wanted to
remoe that since no in-tree users existed.

Devices like that want to essentially map the entire address space,
via pass-thru, not IOMMU mappings.

Clustering cards, such as those made by Dolphin, are another example.

I realize that this doesn't work when we absolutely must use an IOMMU
such as for virtualization which is the whole impetus of the Intel
IOMMU. :-)

> So maybe we would want some heuristics about the size of the mapping
> being requested or the amount of 32-bit space left to decide whether a
> mapping should be below 4 GB.

Perhaps, but as I said heuristics might not be enough here.
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