From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH] KVM: x86: Reduce retpoline performance impact in slot_handle_level_range() Date: Fri, 02 Feb 2018 19:17:13 +0000 Message-ID: <1517599033.31953.71.camel@infradead.org> References: <1517583559-424-1-git-send-email-dwmw@amazon.co.uk> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-TUgWKi/OVXZiDIPRPbmO" Cc: Thomas Gleixner , KarimAllah Ahmed , sironi@amazon.de, the arch/x86 maintainers , KVM list , Paolo Bonzini , Linux Kernel Mailing List , Borislav Petkov , Peter Zijlstra To: Linus Torvalds Return-path: Received: from twosheds.infradead.org ([90.155.92.209]:47436 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753099AbeBBTRV (ORCPT ); Fri, 2 Feb 2018 14:17:21 -0500 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: --=-TUgWKi/OVXZiDIPRPbmO 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 > 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. 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