linux-kernel - Re: [PATCH] x86/alternatives: Fix optimize

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Message-ID: <1515584995.22302.117.camel@infradead.org>
Date:   Wed, 10 Jan 2018 11:49:55 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <andi@...stfloor.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking

On Wed, 2018-01-10 at 12:45 +0100, Borislav Petkov wrote:
> On Wed, Jan 10, 2018 at 11:36:41AM +0000, David Woodhouse wrote:
> > 
> > That fixed and understood, I shall remove the offending NOPs anyway,
> > because aligning instructions in the *altinstr* section is entirely
> > pointless as they might still not be aligned when they get copied into
> > place anyway.
>
> Yap. It was still a good experiment because we found this shortcoming!

Don't suppose you want to make the alignment actually *work*? :)

Paul, how much of a win was it really? And not just in a microbenchmark
of the retpoline itself, saying "look Ma! This goes faster if I make it
take three times as many icache lines!", but overall system benchmarks?
:)
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