[PATCH 1/9] x86, pkeys: Documentation

From: Dave Hansen
Date: Wed Mar 09 2016 - 17:00:25 EST



From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Give a high-level overview of Protection Keys from a hardware
perspective, as well as some description since we referred to
this from the Kconfig text.

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

b/Documentation/x86/protection-keys.txt | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)

diff -puN /dev/null Documentation/x86/protection-keys.txt
--- /dev/null 2016-03-01 16:49:31.662341041 -0800
+++ b/Documentation/x86/protection-keys.txt 2016-03-09 13:55:19.273412780 -0800
@@ -0,0 +1,28 @@
+Memory Protection Keys for User pages is a CPU feature which will
+first appear on Skylake Servers, but will also be supported on
+future non-server parts.
+
+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.
_