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Message-ID: <1517599033.31953.71.camel@infradead.org>
Subject: Re: [PATCH] KVM: x86: Reduce retpoline performance impact in
 slot_handle_level_range()
From:   David Woodhouse <dwmw2@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        KarimAllah Ahmed <karahmed@amazon.de>, sironi@amazon.de,
        the arch/x86 maintainers <x86@kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>
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Date:   Fri, 02 Feb 2018 19:17:13 +0000
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Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, 2018-02-02 at 11:10 -0800, Linus Torvalds wrote:
> On Fri, Feb 2, 2018 at 10:50 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >=20
> >=20
> > 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.
>=20
> That may be a quirk of my configuration, or maybe I screwed something
> else up, but:
>=20
> =C2=A0 [torvalds@i7 linux]$ size ~/mmu.o arch/x86/kvm/mmu.o
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0text=C2=A0=C2=A0=C2=A0=C2=A0data=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0bss=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0dec=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0hex filename
> =C2=A0=C2=A0=C2=A0=C2=A085587=C2=A0=C2=A0=C2=A0=C2=A09310=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0120=C2=A0=C2=A0=C2=A095017=C2=A0=C2=A0=C2=A017329 /home/torv=
alds/mmu.o
> =C2=A0=C2=A0=C2=A0=C2=A085531=C2=A0=C2=A0=C2=A0=C2=A09310=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0120=C2=A0=C2=A0=C2=A094961=C2=A0=C2=A0=C2=A0172f1 arch/x86/k=
vm/mmu.o
>=20
> so the attached patch actually shrank things down by about 50 bytes
> because of the code simplification.
>=20
> Of course, I have been known to screw up retpoline testing in the
> past, so my numbers are suspect ;). Somebody should double-check me.

I got this:

=C2=A0 =C2=A0text	=C2=A0=C2=A0=C2=A0data	=C2=A0=C2=A0=C2=A0=C2=A0bss	=C2=A0=
=C2=A0=C2=A0=C2=A0dec	=C2=A0=C2=A0=C2=A0=C2=A0hex=09
filename
=C2=A0 87167	=C2=A0=C2=A0=C2=A09310	=C2=A0=C2=A0=C2=A0=C2=A0120	=C2=A0=C2=
=A096597	=C2=A0=C2=A017955=09
arch/x86/kvm/mmu.o
=C2=A0 88299	=C2=A0=C2=A0=C2=A09310	=C2=A0=C2=A0=C2=A0=C2=A0120	=C2=A0=C2=
=A097729	=C2=A0=C2=A017dc1=09
arch/x86/kvm/mmu-inline.o

But then, I'd also done kvm_handle_hva() and kvm_handle_hva_range().

Either way, that does look like a reasonable answer. I had looked at
the various one-line wrappers around slot_handle_level_range() and
thought "hm, those should be inline", but I hadn't made the next step
and pondered putting the whole thing inline. We'll give it a spin and
work out where the next performance bottleneck is. Thanks.
--=-TUgWKi/OVXZiDIPRPbmO
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