x86, pkeys: Documentation
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+Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU
+feature which will be found in future Intel CPUs.  The work here
+was done with the aid of simulators.
+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.