linux-kernel - Re: [PATCH] KVM: x86: Reduce retpoline performance impact in slot_handle_level

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Date:   Fri, 2 Feb 2018 11:10:19 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        KarimAllah Ahmed <karahmed@...zon.de>, sironi@...zon.de,
        "the arch/x86 maintainers" <x86@...nel.org>,
        KVM list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] KVM: x86: Reduce retpoline performance impact in slot_handle_level_range()

On Fri, Feb 2, 2018 at 10:50 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Will it make for bigger code? Yes. But probably not really all *that*
> much bigger, because of how it also will allow the compiler to
> simplify some things.

Actually, testing this with my fairly minimal config, it actually
makes for *smaller* code to inline those things.

That may be a quirk of my configuration, or maybe I screwed something
else up, but:

  [torvalds@i7 linux]$ size ~/mmu.o arch/x86/kvm/mmu.o
     text    data     bss     dec     hex filename
    85587    9310     120   95017   17329 /home/torvalds/mmu.o
    85531    9310     120   94961   172f1 arch/x86/kvm/mmu.o

so the attached patch actually shrank things down by about 50 bytes
because of the code simplification.

Of course, I have been known to screw up retpoline testing in the
past, so my numbers are suspect ;). Somebody should double-check me.

                    Linus

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