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

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Date:   Mon, 8 Jan 2018 16:15:51 -0800
From:   Paul Turner <pjt@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        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>
Subject: Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch

On Mon, Jan 8, 2018 at 2:25 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> So pjt did alignment, a single unroll and per discussion earlier today
>> (CET) or late last night (PST), he only does 16.
>
> I used the Intel recommended sequence, which recommends 32.
>
> Not sure if alignment makes a difference. I can check.
>
>> Why is none of that done here? Also, can we pretty please stop using
>> those retarded number labels, they make this stuff unreadable.
>
> Personally I find the magic labels with strange ASCII characters
> far less readable than a simple number.
>
> But can change it if you insist.
>
>> Also, pause is unlikely to stop speculation, that comment doesn't make
>> sense. Looking at PJT's version there used to be a speculation trap in
>> there, but I can't see that here.
>
> My understanding is that it stops speculation. But could also
> use LFENCE.
>

Neither pause nor lfence stop speculation.

> -Andi