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

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Date:   Mon, 8 Jan 2018 14:25:12 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andi Kleen <andi@...stfloor.org>, dwmw@...zon.co.uk,
        pjt@...gle.com, linux-kernel@...r.kernel.org,
        gregkh@...ux-foundation.org, tim.c.chen@...ux.intel.com,
        dave.hansen@...el.com, tglx@...utronix.de, luto@...capital.net
Subject: Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context
 switch

> 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.

-Andi