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,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 A57E8C3F2C6 for ; Tue, 3 Mar 2020 18:01:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 82DBD20836 for ; Tue, 3 Mar 2020 18:01:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387991AbgCCSB2 (ORCPT ); Tue, 3 Mar 2020 13:01:28 -0500 Received: from mga18.intel.com ([134.134.136.126]:23690 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731827AbgCCSB0 (ORCPT ); Tue, 3 Mar 2020 13:01:26 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 03 Mar 2020 10:01:23 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,511,1574150400"; d="scan'208";a="440678485" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by fmsmga006.fm.intel.com with ESMTP; 03 Mar 2020 10:01:22 -0800 Date: Tue, 3 Mar 2020 10:01:22 -0800 From: Sean Christopherson To: Jim Mattson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , kvm list , LKML , Jan Kiszka , Xiaoyao Li Subject: Re: [PATCH 2/6] KVM: x86: Fix CPUID range check for Centaur and Hypervisor ranges Message-ID: <20200303180122.GO1439@linux.intel.com> References: <20200302195736.24777-1-sean.j.christopherson@intel.com> <20200302195736.24777-3-sean.j.christopherson@intel.com> <20200303045838.GF27842@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 Tue, Mar 03, 2020 at 09:42:42AM -0800, Jim Mattson wrote: > Unfathomable was the wrong word. I dunno, one could argue that the behavior of Intel CPUs for CPUID is unfathomable and I was just trying to follow suit :-D > I can see what you're trying to do. I > just don't think it's defensible. I suspect that Intel CPU architects > will be surprised and disappointed to find that the maximum effective > value of CPUID.0H:EAX is now 255, and that they have to define > CPUID.100H:EAX as the "maximum leaf between 100H and 1FFH" if they > want to define any leaves between 100H and 1FFH. Hmm, ya, I agree that applying a 0xffffff00 mask to all classes of CPUID ranges is straight up wrong. > Furthermore, AMD has only ceded 4000_0000h through 4000_00FFh to > hypervisors, so kvm's use of 40000100H through 400001FFH appears to be > a land grab, akin to VIA's unilateral grab of the C0000000H leaves. > Admittedly, one could argue that the 40000000H leaves are not AMD's to > apportion, since AMD and Intel appear to have reached a detente by > splitting the available space down the middle. Intel, who seems to be > the recognized authority for this range, declares the entire range > from 40000000H through 4FFFFFFFH to be invalid. Make of that what you > will. > > In any event, no one has ever documented what's supposed to happen if > you leave gaps in the 4xxxxxxxH range when defining synthesized CPUID > leaves under kvm. Probably stating the obvious, but for me, the least suprising thing is for such leafs to output zeros. It also feels safer, e.g. a guest that's querying hypervisor support is less likely to be led astray by all zeros than by a random feature bits being set. What about something like this? Along with a comment and documentation... static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function) { struct kvm_cpuid_entry2 *max; if (function >= 0x40000000 && function <= 0x4fffffff) max = kvm_find_cpuid_entry(vcpu, function & 0xffffff00, 0); else max = kvm_find_cpuid_entry(vcpu, function & 0x80000000, 0); return max && function <= max->eax; } > On Mon, Mar 2, 2020 at 8:58 PM Sean Christopherson > wrote: > > > > On Mon, Mar 02, 2020 at 08:25:31PM -0800, Jim Mattson wrote: > > > On Mon, Mar 2, 2020 at 7:25 PM Jim Mattson wrote: > > > > > > > > On Mon, Mar 2, 2020 at 11:57 AM Sean Christopherson > > > > wrote: > > > > > > > > > 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. > > > > > > > > I think it could be reasonably argued that this is a userspace bug. > > > > Clearly, when userspace explicitly supplies the results for a leaf, > > > > those results override the default CPUID values for that leaf. But I > > > > haven't seen it documented anywhere that leaves *not* explicitly > > > > supplied by userspace will override the default CPUID values, just > > > > because they happen to appear in some magic range. > > > > > > In fact, the more I think about it, the original change is correct, at > > > least in this regard. Your "fix" introduces undocumented and > > > unfathomable behavior. > > > > Heh, the takeaway from this is that whatever we decide on needs to be > > documented somewhere :-) > > > > I wouldn't say it's unfathomable, conceptually it seems like the intent > > of the hypervisor range was to mimic the basic and extended ranges. The > > whole thing is arbitrary behavior. Of course if Intel CPUs would just > > return 0s on undefined leafs it would be a lot less arbitrary :-) > > > > Anyways, I don't have a strong opinion on whether this patch stays or goes.