How to enable/disable Intel VT-d on XEN

See: http://wiki.xen.org/wiki/VTd_HowTo

Usually, you just need 'iommu=1' to enable VT-d. At the same time, most of VT-d features (DMA remapping, snoop control, queued invalidation and interrupt remapping) are enabled by default if they are available. You can use 'no-xxx' to disable a feature, for example, 'iommu=no-snoop' disable snoop control.

When RMRR address range is not in reserved memory (BIOS issue), can use 'iommu_inclusive_mapping=1' to work around it.

The grub configuration is like:

title Xen-Linux (2.6.18-xen)

        root (hd0,0)

        kernel /boot/xen.gz iommu=1

        module /boot/vmlinuz-2.6.18.8-xen root=LABEL=/

        module /boot/initrd-2.6.18-xen.img