linux-kernel - Re: [PATCH] x86/retpoline: Fill return stack buffer on vmexit

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Date:   Wed, 10 Jan 2018 15:22:35 -0800 (PST)
From:   David Lang <david@...g.hm>
To:     David Woodhouse <dwmw@...zon.co.uk>
cc:     Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, bp@...en8.de, rga@...zon.de
Subject: Re: [PATCH] x86/retpoline: Fill return stack buffer on vmexit

I somewhat hate to ask this, but for those of us following at home, what does 
this add to the overhead?

I am remembering an estimate from mid last week that put retpoline at replacing 
a 3 clock 'ret' with 30 clocks of eye-bleed code