PCI/ATS: Only enable ATS for trusted devices

Add pci_ats_supported(), which checks whether a device has an ATS
capability, and whether it is trusted.  A device is untrusted if it is
plugged into an external-facing port such as Thunderbolt and could be
spoofing an existing device to exploit weaknesses in the IOMMU
configuration.  PCIe ATS is one such weaknesses since it allows
endpoints to cache IOMMU translations and emit transactions with
'Translated' Address Type (10b) that partially bypass the IOMMU
translation.

The SMMUv3 and VT-d IOMMU drivers already disallow ATS and transactions
with 'Translated' Address Type for untrusted devices.  Add the check to
pci_enable_ats() to let other drivers (AMD IOMMU for now) benefit from
it.

By checking ats_cap, the pci_ats_supported() helper also returns whether
ATS was globally disabled with pci=noats, and could later include more
things, for example whether the whole PCIe hierarchy down to the
endpoint supports ATS.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20200520152201.3309416-2-jean-philippe@linaro.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>

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