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The ARM architecture has a "Top Byte Ignore" (TBI) option that makes the MMU mask out bits [63:56] of an address, allowing a userspace application to store data in its pointers. This option is incompatible with PCI ATS. If TBI is enabled in the SMMU and userspace triggers DMA transactions on tagged pointers, the endpoint might create ATC entries for addresses that include a tag. Software would then have to send ATC invalidation packets for each 255 possible alias of an address, or just wipe the whole address space. This is not a viable option, so disable TBI. The impact of this change is unclear, since there are very few users of tagged pointers, much less SVA. But the requirement introduced by this patch doesn't seem excessive: a userspace application using both tagged pointers and SVA should now sanitize addresses (clear the tag) before using them for device DMA. Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com> Signed-off-by: Will Deacon <will.deacon@arm.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.