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Re: [PATCH 2/5] x86: Add -mindirect-branch-loop=

From: Florian Weimer <fw at deneb dot enyo dot de>
To: "H.J. Lu" <hjl dot tools at gmail dot com>
Cc: gcc-patches at gcc dot gnu dot org
Date: Mon, 08 Jan 2018 09:20:35 +0100
Subject: Re: [PATCH 2/5] x86: Add -mindirect-branch-loop=
Authentication-results: sourceware.org; auth=none
References: <20180107225904.11535-1-hjl.tools@gmail.com> <20180107225904.11535-3-hjl.tools@gmail.com>

* H. J. Lu:

> Add -mindirect-branch-loop= option to control loop filler in call and
> return thunks generated by -mindirect-branch=.  'lfence' uses "lfence"
> as loop filler.  'pause' uses "pause" as loop filler.  'nop' uses "nop"
> as loop filler.  The default is 'lfence'.

Why is the loop needed?  Doesn't ud2 or cpuid stop speculative
execution?

Follow-Ups:
- Re: [PATCH 2/5] x86: Add -mindirect-branch-loop=
  - From: H.J. Lu
- Re: [PATCH 2/5] x86: Add -mindirect-branch-loop=
  - From: David Woodhouse

References:
- [PATCH 0/5] x86: CVE-2017-5715, aka Spectre
  - From: H.J. Lu
- [PATCH 2/5] x86: Add -mindirect-branch-loop=
  - From: H.J. Lu

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