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References: <20180108201531.6782-1-andi@firstfloor.org> <20180108221121.GU29822@worktop.programming.kicks-ass.net>
 <20180108222512.GC6718@tassilo.jf.intel.com>
From: Paul Turner <pjt@google.com>
Date: Mon, 8 Jan 2018 16:15:51 -0800
Message-ID: <CAPM31RKRqJcAKYx3z2T3L7-gsPO859C8KLrCpUH+eW5MS18EnQ@mail.gmail.com>
Subject: Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch
To: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Andi Kleen <andi@firstfloor.org>,
        "Woodhouse, David" <dwmw@amazon.co.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@amacapital.net>
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Sender: linux-kernel-owner@vger.kernel.org

On Mon, Jan 8, 2018 at 2:25 PM, Andi Kleen <ak@linux.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