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=-2.2 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 35265C73C6D for ; Wed, 10 Jul 2019 06:20:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 018222054F for ; Wed, 10 Jul 2019 06:20:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726261AbfGJGUk (ORCPT ); Wed, 10 Jul 2019 02:20:40 -0400 Received: from mail-ed1-f67.google.com ([209.85.208.67]:42300 "EHLO mail-ed1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725844AbfGJGUk (ORCPT ); Wed, 10 Jul 2019 02:20:40 -0400 Received: by mail-ed1-f67.google.com with SMTP id v15so930391eds.9 for ; Tue, 09 Jul 2019 23:20:38 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=OCbt8NMT/t7R6kfvNLimxFCsLyfy0iic4GSSQ/N6KvU=; b=mDGrk4gChxjRQnFWUYxN7ep3qYhTCdjq6Qiv0Oo769qTD+smhDwGaHqQDHK2lfSE70 /kmD/r+BDHDKI9CXOymg30ZiVpGOut3LiK4JMc6EZxV1mOjrRY40a5DusXZlqe6Rp6QO Z+6kSNYVPkUaDkpRg1RDMTWEbnOJui3JZXxZkheT3s4rUpZz1+QQf+fpiC1TmpAsGisD bgZqdQ6RSXPs8Gt0yx9kWwrVWG/Dcfgg3T2Fm5i900UrTPd0uUkSvsJA0/2E/iQYTyM9 Ep3UvtrvZWnpdiXBLKvRw4b+S8ym7QAT+zuKSuCCxnintTeEUJMNExwE1JLhqS0r2E6k DJSQ== X-Gm-Message-State: APjAAAX9c4PXxktHOAGtB+TNAz5IZzWFuEmrVCCaZxw/QZUfPNmpA8bM jtCym47BBArVZE2k+FjblGagKA== X-Google-Smtp-Source: APXvYqxkN9dHXefZERuD5bLK8SGMMlwY90+yq4ilhOu3Q/nF/K+Quq3Y6EozLJvP683WAaYA99tCCA== X-Received: by 2002:a50:9489:: with SMTP id s9mr23062595eda.86.1562739638294; Tue, 09 Jul 2019 23:20:38 -0700 (PDT) Received: from ?IPv6:2001:b07:6468:f312:19db:ad53:90ea:9423? ([2001:b07:6468:f312:19db:ad53:90ea:9423]) by smtp.gmail.com with ESMTPSA id n1sm910730wrx.39.2019.07.09.23.20.36 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Tue, 09 Jul 2019 23:20:37 -0700 (PDT) Subject: Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC To: "Lendacky, Thomas" , Josh Poimboeuf , "x86@kernel.org" Cc: "linux-kernel@vger.kernel.org" , Andrew Cooper , Thomas Gleixner , Pu Wen , Borislav Petkov References: <45f247d2-80f5-6c8c-d11e-965a3da8a88f@amd.com> From: Paolo Bonzini Message-ID: <4a13c6a3-a13e-d3e5-0008-41a6d47a6eff@redhat.com> Date: Wed, 10 Jul 2019 08:20:40 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <45f247d2-80f5-6c8c-d11e-965a3da8a88f@amd.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Paolo