PCIe Address Translation Services (ATS) provides a mechanism for a device
to provide an on-device caching translation agent (device IOTLB).  We
already have a means to disable support for this feature via the pci=noats
option.  For untrusted and externally facing devices, we not only disable
ATS support for the device, but we use Access Control Services (ACS)
Transaction Blocking to actively prevent devices from sending TLPs with
non-default AT field values.

Extend pci=noats to also make use of PCI_ACS_TB so that not only is ATS
disabled at the device, but blocked at the downstream ports.  This provides
a means to further lock-down ATS for cases such as device assignment, where
it may not be the hardware configuration of the device that makes it
untrusted, but the driver running on the device.

Link: https://lore.kernel.org/r/162404966325.2362347.12176138291577486015.stgit@omenSigned-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Rajat Jain <rajatja@google.com>