Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752297AbeAEQh6 (ORCPT + 1 other); Fri, 5 Jan 2018 11:37:58 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:47696 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751774AbeAEQhx (ORCPT ); Fri, 5 Jan 2018 11:37:53 -0500 Message-ID: <1515170250.29312.144.camel@infradead.org> Subject: Re: [PATCH 0/7] IBRS patch series From: David Woodhouse To: Andrea Arcangeli Cc: "Van De Ven, Arjan" , Paul Turner , Linus Torvalds , Tim Chen , Thomas Gleixner , Andy Lutomirski , Greg KH , "Hansen, Dave" , Andi Kleen , Linux Kernel Mailing List In-Reply-To: <20180105160534.GR26807@redhat.com> References: <1515093549.29312.11.camel@infradead.org> <1515162514.29312.131.camel@infradead.org> <0575AF4FD06DD142AD198903C74E1CC87A56B309@ORSMSX103.amr.corp.intel.com> <1515166704.29312.140.camel@infradead.org> <20180105160534.GR26807@redhat.com> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-AZqQAxZbqGHBBbJJwWL7" Date: Fri, 05 Jan 2018 16:37:30 +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: --=-AZqQAxZbqGHBBbJJwWL7 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2018-01-05 at 17:05 +0100, Andrea Arcangeli wrote: > On Fri, Jan 05, 2018 at 03:38:24PM +0000, David Woodhouse wrote: > >=20 > > We had IBRS first, and especially on Broadwell and earlier, its > > performance really is painful. > >=20 > > Then came retpoline, purely as an optimisation. A very *important* > > performance improvement, but an optimisation nonetheless. > >=20 > > When looking at optimisations, it is rare for us to say "oh, well it > > opens up only a *small* theoretical security hole, but it's faster so > > that's OK". > I couldn't express any better than the above, the way I look at > repotlines. >=20 > Now seeing how this thing escalates with 2-way gcc code emission with > amd version that will not have ibrs at all to call it around the bios > etc... and IBRS skylake is still needed as a 3-way alternative and no > code exists to even patch it all at boot like that, and then qemu has > to be compiled with reptoline and 2-way alternative there too, to > achieve ibrs 2 ibpb 1, it's not looking like the way to go to me. >=20 > Not in the short term at least, and for the long term "reptoline" is > guaranteed a totally useless effort. >=20 > reptoline is like highmem kmap() etc... eventually nobody will ever > use reptoline. reptoline is more interesting for downstream in fact if > something where at least we won't have to deal with that forever where > there's more control on the toolchain used, and after a certain number > of years that code expires. >=20 > I can imagine we'll unfortunately have to deal with reptoline at some > point, but starting with reptoline at least looks backwards, > especially if it's the long term you're planning for. You are completely ignoring pre-Skylake here. On pre-Skylake, retpoline is perfectly sufficient and it's a *lot* faster than the IBRS option which is almost prohibitively slow. We didn't do it just for fun. 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