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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 B7329C48BCF for ; Wed, 9 Jun 2021 14:12:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9659061351 for ; Wed, 9 Jun 2021 14:12:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237486AbhFIOOn (ORCPT ); Wed, 9 Jun 2021 10:14:43 -0400 Received: from mga18.intel.com ([134.134.136.126]:46704 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234601AbhFIOOl (ORCPT ); Wed, 9 Jun 2021 10:14:41 -0400 IronPort-SDR: OTAlQFD/zhxbr+nwIDGYA9SZzALG+vxW/ZOzbJIUCncP2RyvrYAVcaTaaIFaQLf9Ps44muMh5f 0ICHO6FgJclg== X-IronPort-AV: E=McAfee;i="6200,9189,10010"; a="192394313" X-IronPort-AV: E=Sophos;i="5.83,261,1616482800"; d="scan'208";a="192394313" Received: from orsmga001.jf.intel.com ([10.7.209.18]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Jun 2021 07:12:46 -0700 IronPort-SDR: sAqzFnBRv/4y30ntscj2FSIhspuU1b7lMKEr7SPViE3xpfyl54+nwimQrgY1Ouxi3zGQMeCX6c hzVvj62gR10w== X-IronPort-AV: E=Sophos;i="5.83,261,1616482800"; d="scan'208";a="482400162" Received: from dspaldin-mobl.amr.corp.intel.com (HELO [10.212.158.45]) ([10.212.158.45]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Jun 2021 07:12:46 -0700 Subject: Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest To: Kuppuswamy Sathyanarayanan , Peter Zijlstra , Andy Lutomirski , Tony Luck , Dan Williams Cc: Andi Kleen , Kirill Shutemov , Kuppuswamy Sathyanarayanan , Raj Ashok , Sean Christopherson , linux-kernel@vger.kernel.org References: <20210609011030.751451-1-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: <20210609011030.751451-1-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 On 6/8/21 6:10 PM, Kuppuswamy Sathyanarayanan wrote: > Since cache is always coherent in TDX guests, making wbinvd as > noop should not cause any issues in above mentioned code path. > The end-behavior is the same as KVM guest (treat as noops). I don't see anything in the specs to back up such a broad statement. For Secure-EPT, I see in the TDX "EAS" that "Ignore PAT" is "Set to 1". This, presumably along with the "TD VMCS Guest MSRs... IA32_PAT" being set to 0x0007040600070406 (I didn't decode it, I'm just guessing), ensures that guests using Secure-EPT have no architectural way of creating non-coherent mappings using the guest x86 page tables. That covers one of the memory types to which guests have access. Guests can also access TD-shared memory. Those mappings are controlled by the VMM and not mapped by Secure-EPT. This is the part that concerns me and is not consistent with the statement above. Is it architecturally impossible for a VMM to create an non-coherent mapping and expose it to a guest? If it is impossible, please include citations of the spec or the logic behind this so that a reader can understand, just as I did above. If it is possible to have non-coherent mappings in a guest, then please remove the above statement.