linux-kernel - Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit

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Message-ID: <20180127042411.GB21508@localhost.localdomain>
Date:   Fri, 26 Jan 2018 23:24:13 -0500
From:   Konrad Rzeszutek Wilk <konrad@...nok.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, bp@...en8.de, rga@...zon.de,
        thomas.lendacky@....com
Subject: Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit

> Looks good to me.
> 
> Another IRC discussion was that Boris may eventually add a feature to
> the alternatives code to automatically insert such a jump if there are a
> lot of nops.

I vaguely recall that NOPs are skipped by the CPU so that it shouldn't
matter as compared to say jumping over them. But is that no the case
anymore? Or is it that we have so many NOPs that the prefetcher runs
out of space and stalls?