Translation Blocking is a required feature for Downstream Ports (Root
Ports or Switch Downstream Ports) that implement ACS.  When enabled, the
Port checks the Address Type (AT) of each upstream Memory Request it
receives.

The default AT (00b) means "untranslated" and the IOMMU can decide whether
to treat the address as I/O virtual or physical.

If AT is not the default, i.e., if the Memory Request contains an
already-translated (physical) address, the Port blocks the request and
reports an ACS error.

When enabling ACS, enable Translation Blocking for external-facing ports
and untrusted (external) devices.  This is to help prevent attacks from
external devices that initiate DMA with physical addresses that bypass the
IOMMU.

[bhelgaas: commit log, simplify setting bit and drop warning; TB is
required for Downstream Ports with ACS, so we should never see the warning]
Link: https://lore.kernel.org/r/20200707224604.3737893-4-rajatja@google.comSigned-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>