Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754803AbeALK2b (ORCPT + 1 other); Fri, 12 Jan 2018 05:28:31 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:57488 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754713AbeALK23 (ORCPT ); Fri, 12 Jan 2018 05:28:29 -0500 Message-ID: <1515752898.22302.432.camel@infradead.org> Subject: Re: [PATCH v8 03/12] x86/retpoline: Add initial retpoline support From: David Woodhouse To: Tom Lendacky , Andi Kleen Cc: Paul Turner , LKML , Linus Torvalds , Greg Kroah-Hartman , Tim Chen , Dave Hansen , tglx@linutronix.de, Kees Cook , Rik van Riel , Peter Zijlstra , Andy Lutomirski , Jiri Kosina , gnomes@lxorguk.ukuu.org.uk, x86@kernel.org, Josh Poimboeuf In-Reply-To: References: <1515707194-20531-1-git-send-email-dwmw@amazon.co.uk> <1515707194-20531-4-git-send-email-dwmw@amazon.co.uk> Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwBAMAAAClLOS0AAAAG1BMVEUHBwcUFBQpKSlGRkZhYWF9fX2Xl5eysrLMzMxFF+rXAAACaElEQVQ4y21UQXbbIBQE9wJALmAg6ToWON22FrhZthHgbvssUPathC7QWMful2JHSmtWwGg+zPxBCE0DU4QoJQgRgsg4w2gJjBNE8PjFBZgnQMBs+uZ1NQNQjZO3BV4AGDFC0f+l4DBG0VUAM4yv7SO8IgRdHXQ+A78HKL5OAeCfNQV5cHX8DsBUyIJKtYbt98BKaGNCKjfgFVkqYVLbkHKsRsbSCSa0T6npIqLrpRBgQKHUpQmgs9eEKaiUcooE8WWfCGVnBiUcn1uF2XhbfmN9apKnmMP2K4kizKkQWxuaVNOpU2cACIyxO1Po8ETHcXEDMVnozcejkAYA9iaD4pU0ZvNQ8VurNnTuFAYVtuIPUZW25PjDIjQAlGyffIiRQxoWAZBmJ0LTdW2Nyc0iP3DqRhxizvGJkBWZmyFVyZkddWzmBoIBVMpCCJ1CFzl98xav4VJKSSD45KbUT75ixikTphDSRh8+Uz7JLgUTAgAFwzqzjxc/nDY7WUApqY0OMdTwCKZSXplSKkgIRCHElCp8ZnhnKqXuwcNbk1L0VXE+I9alUXoHlLHl3mv7/dWQlJwtjREC7mu9L/U2jQyMUuO2EDS4q9Kl2ddm232bxIE5pjJuVwiljNn/Cfv25/T0cu5cZbwHGVq7h/zp0B4n3S99V/utD+Uo8BiGx9xCsOAV5z7/tjo4Z4z1Lvb90KZ7eFOoOeXOukqF2seo234YYuaQPpRP+cVZU5adT1Edun5Iz3z8fTz3+eSDh0Ip1c7zx1MaijGzTd/3MbRuBHz8cvcVgCMBRpOHvgu59WDhoat+nIZm+LWm9C/aaaGq5DCP9QAAAABJRU5ErkJggg== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-m2mBOM5X84UA8zhoObUC" Date: Fri, 12 Jan 2018 10:28:18 +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: --=-m2mBOM5X84UA8zhoObUC Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2018-01-11 at 17:58 -0600, Tom Lendacky wrote: >=20 > > + * These are the bare retpoline primitives for indirect jmp and call. > > + * Do not use these directly; they only exist to make the ALTERNATIVE > > + * invocation below less ugly. > > + */ > > +.macro RETPOLINE_JMP reg:req > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0call=C2=A0=C2=A0=C2=A0=C2=A0.Ldo_rop_\@ > > +.Lspec_trap_\@: > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0pause Note that we never use that one on AMD. You just get 'lfence; jmp *reg' instead because you promised us that would work.... while Intel said it would work for a month or two and then said "er, oops, no it doesn't in all cases." =E2=80=94 so we're half-waiting for you lot to do the same thin= g :) You *do* get the RSB-stuffing one though, which is the same. So... > Talked with our engineers some more on using pause vs. lfence.=C2=A0 Paus= e is > not serializing on AMD, so the pause/jmp loop will use power as it is > speculated over waiting for return to mispredict to the correct target. > Can this be changed back to lfence?=C2=A0 It looked like a very small > difference in cycles/time. That seems reasonable, although at this stage I'm also tempted to suggest we can do that kind of fine-tuning in a followup patch. Like the bikeshedding about numbers vs. readable labels. We really need the IBRS and IBPB patches to be landing on top of this as soon as possible. Paul, the lfence=E2=86=92pause change was only a tiny micro-optimisation on Intel, wasn't it? Are you happy with changing the implementations of the RSB stuffing code to use lfence again (or what about 'hlt')? It currently looks like this... the capture loop is using 'jmp' to match the retpoline instead of 'call' as in your examples: #define __FILL_RETURN_BUFFER(reg, nr, sp, uniq) \ mov $(nr/2), reg; \ .Ldo_call1_ ## uniq: \ call .Ldo_call2_ ## uniq; \ .Ltrap1_ ## uniq: \ pause; \ jmp .Ltrap1_ ## uniq; \ .Ldo_call2_ ## uniq: \ call .Ldo_loop_ ## uniq; \ .Ltrap2_ ## uniq: \ pause; \ jmp .Ltrap2_ ## uniq; \ .Ldo_loop_ ## uniq: \ dec reg; \ jnz .Ldo_call1_ ## uniq; \ add $(BITS_PER_LONG/8) * nr, sp; --=-m2mBOM5X84UA8zhoObUC Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu 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