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 45C70C48BCD for ; Wed, 9 Jun 2021 21:03:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 23E72613EF for ; Wed, 9 Jun 2021 21:03:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229845AbhFIVFR (ORCPT ); Wed, 9 Jun 2021 17:05:17 -0400 Received: from mga14.intel.com ([192.55.52.115]:44296 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229536AbhFIVFO (ORCPT ); Wed, 9 Jun 2021 17:05:14 -0400 IronPort-SDR: AHkqv2dVpMCwmLx31KWyrtLAWc7sMDuqfrc6eyhw+TV6nuXZLbiT/z19uk5dWYgagb1h1vA1ED rlc+E6RKdCvg== X-IronPort-AV: E=McAfee;i="6200,9189,10010"; a="204981450" X-IronPort-AV: E=Sophos;i="5.83,261,1616482800"; d="scan'208";a="204981450" Received: from orsmga001.jf.intel.com ([10.7.209.18]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Jun 2021 14:03:18 -0700 IronPort-SDR: JU1YjG4pfXFY5kHBBNI4hC8Ung5d2uGOP1HB7ea73X9oySnTHwIyFf9EinIyHMDwsL9mYoWJsF scQLbhZJ6lNA== X-IronPort-AV: E=Sophos;i="5.83,261,1616482800"; d="scan'208";a="482540018" 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 14:03:18 -0700 Subject: Re: [RFC v2-fix-v5 1/1] x86: Skip WBINVD instruction for VM 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: <973add45-9fd2-7abc-3a97-96a26c263ea0@linux.intel.com> <20210609194926.1949859-1-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: <20210609194926.1949859-1-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 This changelog lacks both clear problem statements and a clear solution implemented within the patch. Here's a proposed changelog. It clearly spells out the two problems caused by WBINVD within a guest, and the proposed solution which fixes those two problems. Is this missing anything? -- VM guests that support ACPI use standard ACPI mechanisms to signal sleep state entry to the host. To ACPI, reboot is simply another sleep state. ACPI specifies that the platform preserve memory contents over (some) sleep states. It does not specify any requirements for data preservation in CPU caches. The ACPI specification mandates the use of WBINVD to flush the contents of the CPU caches to memory before entering specific sleep states, thus ensuring data in caches can survive sleep state transitions.e Unlike when entering sleep states bare metal, no actions within a guest can cause data in processor caches to be lost. That makes these WBINVD invocations harmless but superfluous within a guest. (<--- problem #1) In TDX guests, these WBINVD operations cause #VE exceptions. For debug, it would be ideal for the #VE handler to be able to WARN() when an unexpected WBINVD occurs. (<--- problem #2) Avoid WBINVD for all ACPI cache-flushing operations which occur while running under a hypervisor, which includes TDX guests. This both avoids TDX warnings and optimizes away superfluous WBINVD invocations. (<---- solution)