Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature

which will be found on future Intel CPUs.

Memory Protection Keys provides a mechanism for enforcing page-based

protections, but without requiring modification of the page tables

when an application changes protection domains. It works by

dedicating 4 previously ignored bits in each page table entry to a

"protection key", giving 16 possible keys.

There is also a new user-accessible register (PKRU) with two separate

bits (Access Disable and Write Disable) for each key. Being a CPU

register, PKRU is inherently thread-local, potentially giving each

thread a different set of protections from every other thread.

There are two new instructions (RDPKRU/WRPKRU) for reading and writing

to the new register. The feature is only available in 64-bit mode,

even though there is theoretically space in the PAE PTEs. These

permissions are enforced on data access only and have no effect on

instruction fetches.

=========================== Config Option ===========================

This config option adds approximately 1.5kb of text. and 50 bytes of

data to the executable. A workload which does large O_DIRECT reads

of holes in XFS files was run to exercise get_user_pages_fast(). No

performance delta was observed with the config option

enabled or disabled.