From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 94C9DC73C66 for ; Thu, 11 Jul 2019 17:45:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 76C8121537 for ; Thu, 11 Jul 2019 17:45:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728759AbfGKRpO (ORCPT ); Thu, 11 Jul 2019 13:45:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45050 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728479AbfGKRpN (ORCPT ); Thu, 11 Jul 2019 13:45:13 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 647DE5D5E6; Thu, 11 Jul 2019 17:45:13 +0000 (UTC) Received: from treble (ovpn-122-237.rdu2.redhat.com [10.10.122.237]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E7C82600CD; Thu, 11 Jul 2019 17:45:10 +0000 (UTC) Date: Thu, 11 Jul 2019 12:45:07 -0500 From: Josh Poimboeuf To: Thomas Gleixner Cc: Paolo Bonzini , "Lendacky, Thomas" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , Andrew Cooper , Pu Wen , Borislav Petkov Subject: Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC Message-ID: <20190711174507.wrwfxohzutfsgbgl@treble> References: <45f247d2-80f5-6c8c-d11e-965a3da8a88f@amd.com> <4a13c6a3-a13e-d3e5-0008-41a6d47a6eff@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Thu, 11 Jul 2019 17:45:13 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 10, 2019 at 01:33:48PM +0200, Thomas Gleixner wrote: > On Wed, 10 Jul 2019, Paolo Bonzini wrote: > > On 08/07/19 21:32, Lendacky, Thomas wrote: > > >> AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC). > > >> They've both had it for a long time, and AMD has had it enabled in Linux > > >> since Spectre v1 was announced. > > >> > > >> Back then, there was a proposal to remove the serializing mfence feature > > >> bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have > > >> serializing lfence. At the time, it was (ahem) speculated that some > > >> hypervisors might not yet support its removal, so it remained for the > > >> time being. > > >> > > >> Now a year-and-a-half later, it should be safe to remove. > > > > > > I vaguely remember a concern from a migration point of view, maybe? Adding > > > Paolo to see if he has any concerns. > > > > It would be a problem to remove the conditional "if > > (boot_cpu_has(X86_FEATURE_LFENCE_RDTSC))" from svm_get_msr_feature. But > > removing support for X86_FEATURE_MFENCE_RDTSC essentially amounts to > > removing support for hypervisors that haven't been updated pre-Spectre. > > That's fair enough, I think. > > Yes, they have other more interesting problems :) Great. Anyone care to give an ACK? :-) -- Josh