1Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
 2which will be found on future Intel CPUs.
 3
 4Memory Protection Keys provides a mechanism for enforcing page-based
 5protections, but without requiring modification of the page tables
 6when an application changes protection domains.  It works by
 7dedicating 4 previously ignored bits in each page table entry to a
 8"protection key", giving 16 possible keys.
 9
10There is also a new user-accessible register (PKRU) with two separate
11bits (Access Disable and Write Disable) for each key.  Being a CPU
12register, PKRU is inherently thread-local, potentially giving each
13thread a different set of protections from every other thread.
14
15There are two new instructions (RDPKRU/WRPKRU) for reading and writing
16to the new register.  The feature is only available in 64-bit mode,
17even though there is theoretically space in the PAE PTEs.  These
18permissions are enforced on data access only and have no effect on
19instruction fetches.
20
21=========================== Config Option ===========================
22
23This config option adds approximately 1.5kb of text. and 50 bytes of
24data to the executable.  A workload which does large O_DIRECT reads
25of holes in XFS files was run to exercise get_user_pages_fast().  No
26performance delta was observed with the config option
27enabled or disabled.