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

[Grant Grundler]

On Wed, Jul 02, 2003 at 07:20:26PM +0200, Andi Kleen wrote:
...
> > Can two adjacent IOMMU entries be used to map two 1K buffers?
> > Assume the 1st buffer ends on a 4k alignment and the next one
> > starts on a 4k alignment.
> 
> Yes, it could. But is that situation likely/worth to handle? 

Probably.  It would reduce the number of mappings by 25% (3 instead of 4).
My assumption is two adjecent IOMMU entries have contigious bus addresses.

I was trying to figure out if x86-64 should be setting
BIO_VMERGE_BOUNDARY to 0 or 4k.

It sounds like x86-64 could support "#define BIO_VMERGE_BOUNDARY 4096"
if the IOMMU code will return one DMA address for two SG list entries
in the above example.

hth,
grant