Documentation: x86: convert pti.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/x86/index.rst

@@ -21,3 +21,4 @@ x86-specific Documentation
    protection-keys
    intel_mpx
    amd-memory-encryption
+   pti

Documentation/x86/pti.rst

@@ -1,9 +1,15 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+Page Table Isolation (PTI)
+==========================
+
 Overview
 ========
 
-Page Table Isolation (pti, previously known as KAISER[1]) is a
+Page Table Isolation (pti, previously known as KAISER [1]_) is a
 countermeasure against attacks on the shared user/kernel address
-space such as the "Meltdown" approach[2].
+space such as the "Meltdown" approach [2]_.
 
 To mitigate this class of attacks, we create an independent set of
 page tables for use only when running userspace applications.  When
@@ -60,6 +66,7 @@ Protection against side-channel attacks is important.  But,
 this protection comes at a cost:
 
 1. Increased Memory Use
+
   a. Each process now needs an order-1 PGD instead of order-0.
      (Consumes an additional 4k per process).
   b. The 'cpu_entry_area' structure must be 2MB in size and 2MB
@@ -68,6 +75,7 @@ this protection comes at a cost:
      is decompressed, but no space in the kernel image itself.
 
 2. Runtime Cost
+
   a. CR3 manipulation to switch between the page table copies
      must be done at interrupt, syscall, and exception entry
      and exit (it can be skipped when the kernel is interrupted,
@@ -142,6 +150,7 @@ ideally doing all of these in parallel:
    interrupted, including nested NMIs.  Using "-c" boosts the rate of
    NMIs, and using two -c with separate counters encourages nested NMIs
    and less deterministic behavior.
+   ::
 
 	while true; do perf record -c 10000 -e instructions,cycles -a sleep 10; done
 
@@ -182,5 +191,5 @@ that are worth noting here.
    tended to be TLB invalidation issues.  Usually invalidating
    the wrong PCID, or otherwise missing an invalidation.
 
-1. https://gruss.cc/files/kaiser.pdf
-2. https://meltdownattack.com/meltdown.pdf
+.. [1] https://gruss.cc/files/kaiser.pdf
+.. [2] https://meltdownattack.com/meltdown.pdf