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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.com Signed-off-by: Rajat Jain <rajatja@google.com> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.