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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> > 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.

Ian

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