From: Dave Hansen <dave.han...@linux.intel.com> 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.han...@linux.intel.com> --- b/Documentation/x86/protection-keys.txt | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 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 2016-02-22 17:09:22.811272277 -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. _
