[PATCHv4 0/4]: dma, x86, xen: reduce SWIOTLB usage in Xen guests

[David Vrabel <david.vrabel@citrix.com>]

On systems where DMA addresses and physical addresses are not 1:1
(such as Xen PV guests), the generic dma_get_required_mask() will not
return the correct mask (since it uses max_pfn).

Some device drivers (such as mptsas, mpt2sas) use
dma_get_required_mask() to set the device's DMA mask to allow them to use
only 32-bit DMA addresses in hardware structures.  This results in
unnecessary use of the SWIOTLB if DMA addresses are more than 32-bits,
impacting performance significantly.

This series allows Xen PV guests to override the default
dma_get_required_mask() with one that calculates the DMA mask from the
maximum MFN (and not the PFN).

Changes in v4:
- Assume 64-bit mask is required.

Changes in v3:
- fix off-by-one in xen_dma_get_required_mask()
- split ia64 changes into separate patch.

Changes in v2:
- split x86 and xen changes into separate patches

David