[tip:x86/urgent] x86/mm/pkeys: Add missing Documentation

[tip-bot for Dave Hansen]

Commit-ID:  591b1d8d86074ac3a3163d89bcfe7b232cf83902
Gitweb:     http://git.kernel.org/tip/591b1d8d86074ac3a3163d89bcfe7b232cf83902
Author:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Mon, 14 Dec 2015 11:06:34 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 29 Mar 2016 11:21:17 +0200

x86/mm/pkeys: Add missing Documentation

Stefan Richter noticed that the X86_INTEL_MEMORY_PROTECTION_KEYS option
in arch/x86/Kconfig references Documentation/x86/protection-keys.txt,
but the file does not exist.

This is a patch merging mishap: the final (v8) version of the pkeys
series did not include the documentation patch 32 and v7 included.
Add it now.

Reported-by: Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave@xxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Link: http://lkml.kernel.org/r/20151214190634.426BEE41@xxxxxxxxxxxxxxxxxx
[ Added changelog. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 Documentation/x86/protection-keys.txt | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt
new file mode 100644
index 0000000..c281ded
--- /dev/null
+++ b/Documentation/x86/protection-keys.txt
@@ -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.