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 16972C2D0BF for ; Wed, 18 Dec 2019 20:10:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DD2912176D for ; Wed, 18 Dec 2019 20:10:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727541AbfLRUKD (ORCPT ); Wed, 18 Dec 2019 15:10:03 -0500 Received: from mga04.intel.com ([192.55.52.120]:9870 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726636AbfLRUKD (ORCPT ); Wed, 18 Dec 2019 15:10:03 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by fmsmga104.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 18 Dec 2019 12:10:02 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,330,1571727600"; d="scan'208";a="218256493" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by orsmga003.jf.intel.com with ESMTP; 18 Dec 2019 12:10:02 -0800 Date: Wed, 18 Dec 2019 12:10:02 -0800 From: Sean Christopherson To: Jim Mattson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , kvm list , LKML , Weijiang Yang Subject: Re: [RFC PATCH] KVM: x86: Disallow KVM_SET_CPUID{2} if the vCPU is in guest mode Message-ID: <20191218201002.GE25201@linux.intel.com> References: <20191218174255.30773-1-sean.j.christopherson@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Dec 18, 2019 at 11:38:43AM -0800, Jim Mattson wrote: > On Wed, Dec 18, 2019 at 9:42 AM Sean Christopherson > wrote: > > > > Reject KVM_SET_CPUID{2} with -EBUSY if the vCPU is in guest mode (L2) to > > avoid complications and potentially undesirable KVM behavior. Allowing > > userspace to change a guest's capabilities while L2 is active would at > > best result in unexpected behavior in the guest (L1 or L2), and at worst > > induce bad KVM behavior by breaking fundamental assumptions regarding > > transitions between L0, L1 and L2. > > This seems a bit contrived. As long as we're breaking the ABI, can we > disallow changes to CPUID once the vCPU has been powered on? I can at least concoct scenarios where changing CPUID after KVM_RUN provides value, e.g. effectively creating a new VM/vCPU without destroying the kernel's underlying data structures and without putting the file descriptors, for performance (especially if KVM avoids its hardware on/off paths) or sandboxing (process has access to a VM fd, but not /dev/kvm). A truly contrived, but technically architecturally accurate, scenario would be modeling SGX interaction with the machine check architecutre. Per the SDM, #MCs or clearing bits in IA32_MCi_CTL disable SGX, which is reflected in CPUID: Any machine check exception (#MC) that occurs after Intel SGX is first enables causes Intel SGX to be disabled, (CPUID.SGX_Leaf.0:EAX[SGX1] == 0) It cannot be enabled until after the next reset. Any act of clearing bits from '1 to '0 in any of the IA32_MCi_CTL register may disable Intel SGX (set CPUID.SGX_Leaf.0:EAX[SGX1] to 0) until the next reset. I doubt a userspace VMM would actively model that behavior, but it's at least theoretically possible. Yes, it would technically be possible for SGX to be disabled while L2 is active, but I don't think it's unreasonable to require userspace to first force the vCPU out of L2.