linux-kernel - Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch II

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Date:   Mon, 8 Jan 2018 17:49:16 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>
Cc:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows
 on context switch II

> > On Skylake and Broadwell when the RSB underflows it will fall back to the 
> > indirect branch predictor, which can be poisoned and we try to avoid
> > using with retpoline. So we try to avoid underflows, and this filling
> > helps us with that.
> 
> That's no longer true for Broadwell with the latest microcode, right?

That's right.

-Andi