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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 807FBC3F2D2 for ; Thu, 5 Mar 2020 21:51:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5FBFC20848 for ; Thu, 5 Mar 2020 21:51:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726485AbgCEVvv (ORCPT ); Thu, 5 Mar 2020 16:51:51 -0500 Received: from mga02.intel.com ([134.134.136.20]:5375 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726141AbgCEVvv (ORCPT ); Thu, 5 Mar 2020 16:51:51 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga101.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 05 Mar 2020 13:51:50 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,519,1574150400"; d="scan'208";a="234564919" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by orsmga008.jf.intel.com with ESMTP; 05 Mar 2020 13:51:50 -0800 Date: Thu, 5 Mar 2020 13:51:50 -0800 From: Sean Christopherson To: Jim Mattson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , kvm list , LKML , Pu Wen Subject: Re: [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor and Centaur classes Message-ID: <20200305215149.GS11500@linux.intel.com> References: <20200305013437.8578-1-sean.j.christopherson@intel.com> <20200305013437.8578-5-sean.j.christopherson@intel.com> <20200305192532.GN11500@linux.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: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 05, 2020 at 01:10:57PM -0800, Jim Mattson wrote: > On Thu, Mar 5, 2020 at 11:25 AM Sean Christopherson > wrote: > > > > On Thu, Mar 05, 2020 at 10:43:51AM -0800, Jim Mattson wrote: > > > On Wed, Mar 4, 2020 at 5:34 PM Sean Christopherson > > > wrote: > > > > > > > > Rework the masking in the out-of-range CPUID logic to handle the > > > > Hypervisor sub-classes, as well as the Centaur class if the guest > > > > virtual CPU vendor is Centaur. > > > > > > > > Masking against 0x80000000 only handles basic and extended leafs, which > > > > results in Hypervisor range checks being performed against the basic > > > > CPUID class, and Centuar range checks being performed against the > > > > Extended class. E.g. if CPUID.0x40000000.EAX returns 0x4000000A and > > > > there is no entry for CPUID.0x40000006, then function 0x40000006 would > > > > be incorrectly reported as out of bounds. > > > > > > > > While there is no official definition of what constitutes a class, the > > > > convention established for Hypervisor classes effectively uses bits 31:8 > > > > as the mask by virtue of checking for different bases in increments of > > > > 0x100, e.g. KVM advertises its CPUID functions starting at 0x40000100 > > > > when HyperV features are advertised at the default base of 0x40000000. > > > > > > > > The bad range check doesn't cause functional problems for any known VMM > > > > because out-of-range semantics only come into play if the exact entry > > > > isn't found, and VMMs either support a very limited Hypervisor range, > > > > e.g. the official KVM range is 0x40000000-0x40000001 (effectively no > > > > room for undefined leafs) or explicitly defines gaps to be zero, e.g. > > > > Qemu explicitly creates zeroed entries up to the Cenatur and Hypervisor > > > > limits (the latter comes into play when providing HyperV features). > > > > > > > > The bad behavior can be visually confirmed by dumping CPUID output in > > > > the guest when running Qemu with a stable TSC, as Qemu extends the limit > > > > of range 0x40000000 to 0x40000010 to advertise VMware's cpuid_freq, > > > > without defining zeroed entries for 0x40000002 - 0x4000000f. > > > > > > > > Note, documentation of Centaur/VIA CPUs is hard to come by. Designating > > > > 0xc0000000 - 0xcfffffff as the Centaur class is a best guess as to the > > > > behavior of a real Centaur/VIA CPU. > > > > > > Don't forget Transmeta's CPUID range at 0x80860000 through 0x8086FFFF! > > > > Hmm, is it actually needed here? KVM doesn't advertise support for that > > range in KVM_GET_SUPPORTED_CPUID. > > That's also why I limited the Centaur > > range to vendor==CENTAUR, as KVM_GET_SUPPORTED_CPUID enumerates the > > Centaur range if and only if the host CPU is Centaur. Actually, I take this part of that comment back. I limited this to vendor==CENTAUR so that it wouldn't conflict with an actual Intel CPU if Intel adds a CPUID leaf in the 0xc0000000 range. > Ah. So cross-vendor CPUID specifications are not supported? Cross-vendor CPUID is sort of allowed? E.g. this plays nice with creating a Centaur CPU on an Intel platform. My interpretation of GET_SUPPORTED... is that KVM won't prevent enumerating what you want in CPUID, but it only promises to correctly support select leafs.