Backporting 325651::upstream/release_50

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r325651 | sdardis | 2018-02-21 00:05:05 +0000 (Wed, 21 Feb 2018) | 34 lines

[mips] Spectre variant two mitigation for MIPSR2

This patch provides mitigation for CVE-2017-5715, Spectre variant two,
which affects the P5600 and P6600. It provides the option
-mindirect-jump=hazard, which instructs the LLVM backend to replace
indirect branches with their hazard barrier variants.

This option is accepted when targeting MIPS revision two or later.

The migitation strategy suggested by MIPS for these processors is to
use two hazard barrier instructions. 'jalr.hb' and 'jr.hb' are hazard
barrier variants of the 'jalr' and 'jr' instructions respectively.

These instructions impede the execution of instruction stream until
architecturally defined hazards (changes to the instruction stream,
privileged registers which may affect execution) are cleared. These
instructions in MIPS' designs are not speculated past.

These instructions are used with the option -mindirect-jump=hazard
when branching indirectly and for indirect function calls.

These instructions are defined by the MIPS32R2 ISA, so this mitigation
method is not compatible with processors which implement an earlier
revision of the MIPS ISA.

Implementation note: I've opted to provide this as an
-mindirect-jump={hazard,...} style option in case alternative
mitigation methods are required for other implementations of the MIPS
ISA in future, e.g. retpoline style solutions.

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D43487

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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_50@327755 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 3 insertions, 0 deletions

diff --git a/include/clang/Driver/Options.td b/include/clang/Driver/Options.td
index 96cac8720c..3a622bf516 100644
--- a/include/clang/Driver/Options.td
+++ b/include/clang/Driver/Options.td
@@ -2016,6 +2016,9 @@ def mcheck_zero_division : Flag<["-"], "mcheck-zero-division">, Group<m_Group>;
 def mno_check_zero_division : Flag<["-"], "mno-check-zero-division">,
                               Group<m_Group>;
 def mcompact_branches_EQ : Joined<["-"], "mcompact-branches=">, Group<m_Group>;
+def mindirect_jump_EQ : Joined<["-"], "mindirect-jump=">,
+  Group<m_Group>,
+  HelpText<"Change indirect jump instructions to inhibit speculation">;
 def mdsp : Flag<["-"], "mdsp">, Group<m_Group>;
 def mno_dsp : Flag<["-"], "mno-dsp">, Group<m_Group>;
 def mdspr2 : Flag<["-"], "mdspr2">, Group<m_Group>;