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RE: [Xen-devel] VP problematic for backend drivers on IA64?

To: "Muli Ben-Yehuda" <mulix@xxxxxxxxx>, "Magenheimer, Dan \(HP Labs Fort Collins\)" <dan.magenheimer@xxxxxx>
Subject: RE: [Xen-devel] VP problematic for backend drivers on IA64?
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Wed, 25 Jan 2006 01:35:31 -0000
Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, okrieg@xxxxxxxxxx
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Thread-topic: [Xen-devel] VP problematic for backend drivers on IA64?
> > Actually, after thinking about this, it's a bit more complicated 
> > because of the possibility that a DMA may address more than 
> one page.  
> > If so, a simple DMA may need to be translated into a scatter-gather 
> > (or a scatter-gather into a more complex scatter-gather).
> > 
> > Not impossible, obviously because Xen/x86 handles this -- 
> by changing 
> > Linux, correct?
> Correct. Specifically, it's handled by Xen's version of swiotlb.c.

swiotlb just handles the rare cases where things straddle a page. Other
modifications we've made try to avoid this happening (preventing block
request merging across boundries that aren't machine and phys
contiguous, skb slab cache).

> > Do hardware IOMMU's in general handle this complication?
> Yes. You can use a HW IOMMU to map a scatter-gather list of 
> machine pages into a contigous range in the IO space. It can 
> also do the reverse, but that's less interesting.
> > E.g. is there a cleanly defined interface that can be 
> applied to a VP 
> > domain "Xen IOMMU"?

The existing linux one will suffice.


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