Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965909AbeAJP4n (ORCPT + 1 other); Wed, 10 Jan 2018 10:56:43 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:57772 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964872AbeAJP4j (ORCPT ); Wed, 10 Jan 2018 10:56:39 -0500 Message-ID: <1515599784.22302.200.camel@infradead.org> Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code From: David Woodhouse To: Andrea Arcangeli Cc: Dave Hansen , Thomas Gleixner , Jiri Kosina , "asit.k.mallick" , "Van De Ven, Arjan" , Peter Zijlstra , LKML , Linus Torvalds , x86@kernel.org, Borislav Petkov , Tim Chen , Andi Kleen , Greg KH , Andy Lutomirski In-Reply-To: <20180110154729.GK9417@redhat.com> References: <1515587384.22302.132.camel@infradead.org> <20180110124119.GG9706@redhat.com> <20180110125710.GH9706@redhat.com> <1515589641.22302.145.camel@infradead.org> <20180110141047.GD9417@redhat.com> <1610a587-fe9e-28bd-dcd1-b9ec940c07ef@intel.com> <20180110151317.GI9417@redhat.com> <1515597857.22302.190.camel@infradead.org> <20180110154729.GK9417@redhat.com> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-ohweT3MuxpRLW9QVTAbh" Date: Wed, 10 Jan 2018 15:56:24 +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: --=-ohweT3MuxpRLW9QVTAbh Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2018-01-10 at 16:47 +0100, Andrea Arcangeli wrote: > On Wed, Jan 10, 2018 at 03:24:17PM +0000, David Woodhouse wrote: > > Since it achieves nothing=C2=B9 but to make userspace run slower, there= 's no > > need to=C2=A0write it again on returning to userspace. It will perform = that > > function just fine without doing so. >=20 > Ok, very glad we are on the same page now. >=20 > Note that as far as I can tell there was no way to answer the above > question by reading the spec. The spec does, I concede, leave something to be desired. This sentence in particular =E2=80=94 and it really is a single sentence =E2=80=94 caused= me to throw my toys out of the pram and demand verbal explanations (which is perhaps the only reason I managed to work it out): "If IBRS is set, near returns and near indirect jumps/calls will not allow = their speculative target address to be controlled by code that executed in a less privileged prediction mode before the IBRS mode was last written with a value of 1 or on another logical processor so long as all RSB entries from = the previous less privileged prediction mode are overwritten" But it *does* have the words "before the IBRS mode was last written with a value of 1" in there somewhere... ;) --=-ohweT3MuxpRLW9QVTAbh Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw 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