Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754805AbeALKJW (ORCPT + 1 other); Fri, 12 Jan 2018 05:09:22 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:56184 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754626AbeALKJR (ORCPT ); Fri, 12 Jan 2018 05:09:17 -0500 Message-ID: <1515751748.22302.421.camel@infradead.org> Subject: Re: [PATCH 3/5] x86/ibrs: Add direct access support for MSR_IA32_SPEC_CTRL From: David Woodhouse To: Peter Zijlstra , Dave Hansen Cc: Ashok Raj , linux-kernel@vger.kernel.org, Thomas Gleixner , Tim Chen , Andy Lutomirski , Linus Torvalds , Greg KH , Andrea Arcangeli , Andi Kleen , Arjan Van De Ven , Dan Williams , Paolo Bonzini , Jun Nakajima , Asit Mallick In-Reply-To: <20180112095148.GP3040@hirez.programming.kicks-ass.net> References: <1515720739-43819-1-git-send-email-ashok.raj@intel.com> <1515720739-43819-4-git-send-email-ashok.raj@intel.com> <20180112095148.GP3040@hirez.programming.kicks-ass.net> 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="=-8OgNpCi6yYkAIO6Fvk89" Date: Fri, 12 Jan 2018 10:09:08 +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: --=-8OgNpCi6yYkAIO6Fvk89 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2018-01-12 at 10:51 +0100, Peter Zijlstra wrote: > On Thu, Jan 11, 2018 at 05:58:11PM -0800, Dave Hansen wrote: > > On 01/11/2018 05:32 PM, Ashok Raj wrote: > > > +static void save_guest_spec_ctrl(struct vcpu_vmx *vmx) > > > +{ > > > +=C2=A0=C2=A0=C2=A0if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) { > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0vm= x->spec_ctrl =3D spec_ctrl_get(); > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0sp= ec_ctrl_restriction_on(); > > > +=C2=A0=C2=A0=C2=A0} else > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0rm= b(); > > > +} > >=C2=A0 > > Does this need to be "ifence()"?=C2=A0 Better yet, do we just need to c= reate > > a helper for boot_cpu_has(X86_FEATURE_SPEC_CTRL) that does the barrier? >=20 > static_cpu_has() + hard asm-goto requirement. Please drop all the above > nonsense on the floor hard. Peter, NO! static_cpu_has() + asm-goto is NOT SUFFICIENT. It's still *possible* for a missed optimisation in GCC to still leave us with a conditional branch around the wrmsr, letting the CPU speculate around it too. Come on, Peter, we've learned this lesson long and hard since the 1990s when every GCC update would break some stupid=C2=B9 shit we did. Don't regress. WE DO NOT RELY ON UNGUARANTEED BEHAVIOUR OF THE COMPILER. Devise a sanity check which will force a build-fail if GCC ever misses the optimisation, and that's fine. (Or point me to an existing way that it's guaranteed to fail, if such is true already. Don't just ignore the objection.) Do not just ASSUME that GCC will always and forever manage to make that optimisation in every case. --=C2=A0 dwmw2 =C2=B9 In our defence, back then there was often no *other* way to do some =C2=A0 of the things we did, except the "stupid" way. These days, it's much =C2=A0 easier to add features to GCC and elicit guarantees. Although I'm =C2=A0 getting rather pissed off at them bikeshedding the retpoline patches =C2=A0 *so* hard they can't even agree on a command-line option and the nam= e =C2=A0 of the thunk symbol, regardless of the internal implementation =C2=A0 details we don't give a shit about. --=-8OgNpCi6yYkAIO6Fvk89 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 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