The ARM architecture has a "Top Byte Ignore" (TBI) option that makes the
MMU mask out bits [63:56] of an address, allowing a userspace application
to store data in its pointers. This option is incompatible with PCI ATS.

If TBI is enabled in the SMMU and userspace triggers DMA transactions on
tagged pointers, the endpoint might create ATC entries for addresses that
include a tag. Software would then have to send ATC invalidation packets
for each 255 possible alias of an address, or just wipe the whole address
space. This is not a viable option, so disable TBI.

The impact of this change is unclear, since there are very few users of
tagged pointers, much less SVA. But the requirement introduced by this
patch doesn't seem excessive: a userspace application using both tagged
pointers and SVA should now sanitize addresses (clear the tag) before
using them for device DMA.
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>