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 <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/x86/index.rst

···
    protection-keys
    intel_mpx
    amd-memory-encryption
+   pti

Documentation/x86/pti.txt Documentation/x86/pti.rst

···
+.. 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
···
 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
···
      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,
···
    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
 
···
    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