tree 543fdabf46799fd3045a8cad090682692dc6e824
parent 334f080ab3c1bfbb13601a4f404b9c97e2294eb9
author Simon Dardis <simon.dardis@mips.com> 1521238860 +0000
committer Simon Dardis <simon.dardis@mips.com> 1521238860 +0000

Backporting 325651::
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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