linux-kernel - Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch

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Date:   Tue, 09 Jan 2018 00:48:02 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Turner <pjt@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/retpoline: Avoid return buffer underflows on
 context switch

On Tue, 2018-01-09 at 00:44 +0000, Woodhouse, David wrote:
> On IRC, Arjan assures me that 'pause' here really is sufficient as a
> speculation trap. If we do end up returning back here as a
> misprediction, that 'pause' will stop the speculative execution on
> affected CPUs even though it isn't *architecturally* documented to do
> so.
> 
> Arjan, can you confirm that in email please?


That actually doesn't make sense to me. If 'pause' alone is sufficient,
then why in $DEITY's name would we need a '1:pause;jmp 1b' loop in the
retpoline itself?

Arjan?
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