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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 E5F97C47076 for ; Fri, 21 May 2021 16:11:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C120160C3E for ; Fri, 21 May 2021 16:11:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237097AbhEUQNG (ORCPT ); Fri, 21 May 2021 12:13:06 -0400 Received: from mga11.intel.com ([192.55.52.93]:12553 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229586AbhEUQNF (ORCPT ); Fri, 21 May 2021 12:13:05 -0400 IronPort-SDR: lRJmwh/H3Wm5zhNGve59golWKJ9eCoxtnGn7l26unzvk+eJfNrv7SVgjskBNMjWdo9Ao7EEwZT PSKOl+xzkEMA== X-IronPort-AV: E=McAfee;i="6200,9189,9990"; a="198433378" X-IronPort-AV: E=Sophos;i="5.82,319,1613462400"; d="scan'208";a="198433378" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 May 2021 09:11:42 -0700 IronPort-SDR: N2wRmQ4YpHad6kE8Lb6o+s/pL6HM2ZRul2ah+AD1P8YG9VWci6vKCUaFRyIAx+SkqM8gaZPlmU y4XQlooFFOfw== X-IronPort-AV: E=Sophos;i="5.82,319,1613462400"; d="scan'208";a="613304857" Received: from pburton-mobl.amr.corp.intel.com (HELO [10.209.36.169]) ([10.209.36.169]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 May 2021 09:11:41 -0700 Subject: Re: [RFC v2-fix-v2 1/1] x86/boot: Avoid #VE during boot for TDX platforms To: Kuppuswamy Sathyanarayanan , Peter Zijlstra , Andy Lutomirski Cc: Tony Luck , Andi Kleen , Kirill Shutemov , Kuppuswamy Sathyanarayanan , Dan Williams , Raj Ashok , Sean Christopherson , linux-kernel@vger.kernel.org, Sean Christopherson References: <20210521143524.2527690-1-sathyanarayanan.kuppuswamy@linux.intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210521143524.2527690-1-sathyanarayanan.kuppuswamy@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Avoid operations which will inject #VE during boot process. > They're easy to avoid and it is less complex than handling > the exceptions. This puts the solution before the problem. I'd also make sure to clearly connect this solution to the problem. For instance, if you refer to register "modification", ensure that you reflect that language here. Don't call them "modifications" in one part of the changelog and "operations" here. I'd also qualify them as "superfluous". Please reorder this in the following form: 1. Background 2. Problem 3. Solution Please do this for all of your patches. > There are a few MSRs and control register bits which the > kernel normally needs to modify during boot. But, TDX > disallows modification of these registers to help provide > consistent security guarantees ( and avoid generating #VE > when updating them). No, the TDX architecture does not avoid generating #VE. The *kernel* does that. This sentence conflates those two things. > Fortunately, TDX ensures that these are > all in the correct state before the kernel loads, which means > the kernel has no need to modify them. > > The conditions we need to avoid are: > > * Any writes to the EFER MSR > * Clearing CR0.NE > * Clearing CR3.MCE Sathya, there have been repeated issues in your changelogs with "we's". Remember, speak in imperative voice. Please fix this in your tooling to find these so that reviewers don't have to. > + /* > + * Preserve current value of EFER for comparison and to skip > + * EFER writes if no change was made (for TDX guest) > + */ > + movl %eax, %edx > btsl $_EFER_SCE, %eax /* Enable System Call */ > btl $20,%edi /* No Execute supported? */ > jnc 1f > btsl $_EFER_NX, %eax > btsq $_PAGE_BIT_NX,early_pmd_flags(%rip) > -1: wrmsr /* Make changes effective */ > > + /* Avoid writing EFER if no change was made (for TDX guest) */ > +1: cmpl %edx, %eax > + je 1f > + xor %edx, %edx > + wrmsr /* Make changes effective */ > +1: Just curious, but what if this goes wrong? Say the TDX firmware didn't set up EFER correctly and this code does the WRMSR. What ends up happening? Do we get anything out on the console, or is it essentially undebuggable? > > + /* > + * Skip writing to EFER if the register already has desiered > + * value (to avoid #VE for TDX guest). > + */ spelling ^ There are lots of editors that can do spell checking, even in C comments. You might want to look into that for your editor.