From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752596AbeAJHPl (ORCPT + 1 other); Wed, 10 Jan 2018 02:15:41 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:47250 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752444AbeAJHPj (ORCPT ); Wed, 10 Jan 2018 02:15:39 -0500 Message-ID: <1515568506.22302.72.camel@infradead.org> Subject: Re: [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip From: David Woodhouse To: Linus Torvalds , Andi Kleen Cc: Thomas Gleixner , the arch/x86 maintainers , Linux Kernel Mailing List , Paul Turner , Andrew Lutomirski , Peter Zijlstra , Tom Lendacky , Tim Chen , Greg Kroah-Hartman , Dave Hansen , Jiri Kosina , Andi Kleen In-Reply-To: References: <20180110003139.10531-1-andi@firstfloor.org> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-KZF3eSZYQgIM/NKAeNEt" Date: Wed, 10 Jan 2018 07:15:07 +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: --=-KZF3eSZYQgIM/NKAeNEt Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2018-01-09 at 16:39 -0800, Linus Torvalds wrote: > On Tue, Jan 9, 2018 at 4:31 PM, Andi Kleen > wrote: > >=20 > >=20 > > The following patch fixes it for me. Something doesn't > > seem to work with ALTERNATIVE_2. It adds only a few bytes > > more code, so seems acceptable. > Ugh. It's kind of stupid, though. >=20 > Why is the code sequence not simply: >=20 > =C2=A0 ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE_AMD > =C2=A0 ALTERNATIVE __stringify(jmp *\reg), __stringify(RETPOLINE_JMP > \reg), > X86_FEATURE_RETPOLINE >=20 > ie make that X86_FEATURE_RETPOLINE_AMD _only_ emit the "lfence", and > simply fall through to what will be the "jmp *\reg" of the > non-RETPOLINE version. >=20 > Then just make sure X86_FEATURE_RETPOLINE_AMD disables > X86_FEATURE_RETPOLINE. >=20 > That is both simpler and smaller, no? Not smaller, as the lfence *could* have gone in all the space left by turning the whole retpoline thing into a single 'jmp'. But I'll certainly give you simpler. I'll retest and merge Andi's latest patches for that; thanks. I'd really like to know what went wrong though. Did we merge Borislav's attempt to peek at jumps inside alternatives, perchance? 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