[PATCH 32/32] x86, pkeys: Documentation

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>


Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---

 b/Documentation/x86/protection-keys.txt |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff -puN /dev/null Documentation/x86/protection-keys.txt
--- /dev/null	2015-12-10 15:28:13.322405854 -0800
+++ b/Documentation/x86/protection-keys.txt	2015-12-14 10:43:30.185926457 -0800
@@ -0,0 +1,27 @@
+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.
_
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