From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751238AbeAEKzz (ORCPT + 1 other); Fri, 5 Jan 2018 05:55:55 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:50772 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751008AbeAEKzy (ORCPT ); Fri, 5 Jan 2018 05:55:54 -0500 Message-ID: <1515149738.29312.104.camel@infradead.org> Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support From: David Woodhouse To: Paul Turner Cc: Alexei Starovoitov , Linus Torvalds , Andi Kleen , LKML , Greg Kroah-Hartman , Tim Chen , Dave Hansen , Thomas Gleixner , Kees Cook , Rik van Riel , Peter Zijlstra , Andy Lutomirski , Jiri Kosina , One Thousand Gnomes In-Reply-To: <20180105102824.GA247671@google.com> References: <1515058213.12987.89.camel@amazon.co.uk> <20180104143710.8961-1-dwmw@amazon.co.uk> <20180104181744.komdplek7nfdvlsw@ast-mbp> <20180104183559.wlqoxmp7rf4d44ku@ast-mbp> <1515094078.29312.17.camel@infradead.org> <20180105102824.GA247671@google.com> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-elN2FfCutSbmohanJ3QZ" Date: Fri, 05 Jan 2018 10:55:38 +0000 Mime-Version: 1.0 X-Mailer: Evolution 3.18.5.2-0ubuntu3.2 X-SRS-Rewrite: SMTP reverse-path rewritten from by twosheds.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: --=-elN2FfCutSbmohanJ3QZ Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2018-01-05 at 02:28 -0800, Paul Turner wrote: > On Thu, Jan 04, 2018 at 07:27:58PM +0000, David Woodhouse wrote: > > On Thu, 2018-01-04 at 10:36 -0800, Alexei Starovoitov wrote: > > >=C2=A0 > > > Pretty much. > > > Paul's writeup: https://support.google.com/faqs/answer/7625886 > > > tldr: jmp *%r11 gets converted to: > > > call set_up_target; > > > capture_spec: > > > =C2=A0 pause; > > > =C2=A0 jmp capture_spec; > > > set_up_target: > > > =C2=A0 mov %r11, (%rsp); > > > =C2=A0 ret; > > > where capture_spec part will be looping speculatively. > >=C2=A0 > > That is almost identical to what's in my latest patch set, except that > > the capture_spec loop has 'lfence' instead of 'pause'. >=20 > When choosing this sequence I benchmarked several alternatives here, incl= uding > (nothing, nops, fences, and other serializing instructions such as cpuid)= . >=20 > The "pause; jmp" sequence proved minutely faster than "lfence;jmp" which = is why > it was chosen. >=20 > =C2=A0 "pause; jmp" 33.231 cycles/call 9.517 ns/call > =C2=A0 "lfence; jmp" 33.354 cycles/call 9.552 ns/call >=20 > (Timings are for a complete retpolined indirect branch.) Yeah, I studiously ignored you here and went with only what Intel had *assured* me was correct and put into the GCC patches, rather than chasing those 35 picoseconds ;) The GCC patch set already had about four different variants over time, with associated "oh shit, that one doesn't actually work; try this". What we have in my patch set is precisely what GCC emits at the moment. I'm all for optimising it further, but maybe not this week. Other than that, is there any other development from your side that I haven't captured in the latest (v4) series? http://git.infradead.org/users/dwmw2/linux-retpoline.git/ --=-elN2FfCutSbmohanJ3QZ Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu dGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQTAeFw0xNzEyMjEwMDAwMDBaFw0xODEyMjEyMzU5 NTlaMCQxIjAgBgkqhkiG9w0BCQEWE2R3bXcyQGluZnJhZGVhZC5vcmcwggEiMA0GCSqGSIb3DQEB AQUAA4IBDwAwggEKAoIBAQDgzLNWa18DNpGUj/ZeH0Sgz53ESIbzdPw3OJeuNP6jZhxZojbyfxbM hETscxI/Hj6UZ4a7sHm5BkVjlsB1Af2Za/PXUt8MmLAcPMHkMPGunvkUibEvblDvpqMkQZlaZM+t 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