iommu/vt-d: Calculate mask for non-aligned flushes

Calculate the appropriate mask for non-size-aligned page selective
invalidation. Since psi uses the mask value to mask out the lower order
bits of the target address, properly flushing the iotlb requires using a
mask value such that [pfn, pfn+pages) all lie within the flushed
size-aligned region.  This is not normally an issue because iova.c
always allocates iovas that are aligned to their size. However, iovas
which come from other sources (e.g. userspace via VFIO) may not be
aligned.

To properly flush the IOTLB, both the start and end pfns need to be
equal after applying the mask. That means that the most efficient mask
to use is the index of the lowest bit that is equal where all higher
bits are also equal. For example, if pfn=0x17f and pages=3, then
end_pfn=0x181, so the smallest mask we can use is 8. Any differences
above the highest bit of pages are due to carrying, so by xnor'ing pfn
and end_pfn and then masking out the lower order bits based on pages, we
get 0xffffff00, where the first set bit is the mask we want to use.

Fixes: 6fe1010d6d9c ("vfio/type1: DMA unmap chunking")
Cc: stable@vger.kernel.org
Signed-off-by: David Stevens <stevensd@chromium.org>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20220401022430.1262215-1-stevensd@google.com
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20220410013533.3959168-2-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

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