x86: dynamic indirect branch promotion

This is a revised version of optpolines (formerly named retpolines) for
dynamic indirect branch promotion in order to reduce retpoline overheads
[1].

This version address some of the concerns that were raised before.
Accordingly, the code was slightly simplified and patching is now done
using the regular int3/breakpoint mechanism.

Outline optpolines for multiple targets was added. I do not think the
way I implemented it is the correct one. In my original (private)
version, if there are more targets than the outline block can hold, the
outline block is completely removed. However, I think this is
more-or-less how Josh wanted it to be.

The code modifications are now done using a gcc-plugin. This allows to
easily ignore code from init and other code sections. I think it should
also allow us to add opt-in/opt-out support for each branch, for example
by marking function pointers using address-space attributes.

All of these changes required some optimizations to go away to keep the
code simple. I have still did not run the benchmarks again. 

So I might have not addressed all the open issues, but it is rather hard
to finish the implementation since some still open high-level decisions
affect the way in which optimizations should be done.

Specifically:

- Is it going to be the only indirect branch promotion mechanism? If so,
  it probably should also provide interface similar to Josh's
  "static-calls" with annotations.

- Should it also be used when retpolines are disabled (in the config)?
  This does complicate the implementation a bit (RFC v1 supported it).

- Is it going to be opt-in or opt-out? If it is an opt-out mechanism,
  memory and performance optimizations need to be more aggressive.

- Do we use periodic learning or not? Josh suggested to reconfigure the
  branches whenever a new target is found. However, I do not know at
  this time how to do learning efficiently, without making learning much
  more expensive.

[1] https://lore.kernel.org/patchwork/cover/1001332/

Nadav Amit (6):
  x86: introduce kernel restartable sequence
  objtool: ignore instructions
  x86: patch indirect branch promotion
  x86: interface for accessing indirect branch locations
  x86: learning and patching indirect branch targets
  x86: outline optpoline

 arch/x86/Kconfig                             |    4 +
 arch/x86/entry/entry_64.S                    |   16 +-
 arch/x86/include/asm/nospec-branch.h         |   83 ++
 arch/x86/include/asm/sections.h              |    2 +
 arch/x86/kernel/Makefile                     |    1 +
 arch/x86/kernel/asm-offsets.c                |    9 +
 arch/x86/kernel/nospec-branch.c              | 1293 ++++++++++++++++++
 arch/x86/kernel/traps.c                      |    7 +
 arch/x86/kernel/vmlinux.lds.S                |    7 +
 arch/x86/lib/retpoline.S                     |   83 ++
 include/linux/cpuhotplug.h                   |    1 +
 include/linux/module.h                       |    9 +
 kernel/module.c                              |    8 +
 scripts/Makefile.gcc-plugins                 |    3 +
 scripts/gcc-plugins/x86_call_markup_plugin.c |  329 +++++
 tools/objtool/check.c                        |   21 +-
 16 files changed, 1872 insertions(+), 4 deletions(-)
 create mode 100644 arch/x86/kernel/nospec-branch.c
 create mode 100644 scripts/gcc-plugins/x86_call_markup_plugin.c

-- 
2.17.1