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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 9CF02C43603 for ; Wed, 11 Dec 2019 19:18:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6B2BE206A5 for ; Wed, 11 Dec 2019 19:18:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727334AbfLKTST (ORCPT ); Wed, 11 Dec 2019 14:18:19 -0500 Received: from mga18.intel.com ([134.134.136.126]:23830 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726312AbfLKTST (ORCPT ); Wed, 11 Dec 2019 14:18:19 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Dec 2019 11:18:18 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,302,1571727600"; d="scan'208";a="245399141" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by fmsmga002.fm.intel.com with ESMTP; 11 Dec 2019 11:18:17 -0800 Date: Wed, 11 Dec 2019 11:18:17 -0800 From: Sean Christopherson To: Jim Mattson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , kvm list , LKML Subject: Re: [PATCH 1/2] KVM: x86: Add build-time assertion on usage of bit() Message-ID: <20191211191817.GJ5044@linux.intel.com> References: <20191211175822.1925-1-sean.j.christopherson@intel.com> <20191211175822.1925-2-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: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 11, 2019 at 10:24:36AM -0800, Jim Mattson wrote: > On Wed, Dec 11, 2019 at 9:58 AM Sean Christopherson > wrote: > > > > Add build-time checks to ensure KVM isn't trying to do a reverse CPUID > > lookup on Linux-defined feature bits, along with comments to explain > > the gory details of X86_FEATUREs and bit(). > > > > Signed-off-by: Sean Christopherson > > --- > > > > Note, the premature newline in the first line of the second comment is > > intentional to reduce churn in the next patch. > > > > arch/x86/kvm/x86.h | 23 +++++++++++++++++++++-- > > 1 file changed, 21 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h > > index cab5e71f0f0f..4ee4175c66a7 100644 > > --- a/arch/x86/kvm/x86.h > > +++ b/arch/x86/kvm/x86.h > > @@ -144,9 +144,28 @@ static inline bool is_pae_paging(struct kvm_vcpu *vcpu) > > return !is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu); > > } > > > > -static inline u32 bit(int bitno) > > +/* > > + * Retrieve the bit mask from an X86_FEATURE_* definition. Features contain > > + * the hardware defined bit number (stored in bits 4:0) and a software defined > > + * "word" (stored in bits 31:5). The word is used to index into arrays of > > + * bit masks that hold the per-cpu feature capabilities, e.g. this_cpu_has(). > > + */ > > +static __always_inline u32 bit(int feature) > > { > > - return 1 << (bitno & 31); > > + /* > > + * bit() is intended to be used only for hardware-defined > > + * words, i.e. words whose bits directly correspond to a CPUID leaf. > > + * Retrieving the bit mask from a Linux-defined word is nonsensical > > + * as the bit number/mask is an arbitrary software-defined value and > > + * can't be used by KVM to query/control guest capabilities. > > + */ > > + BUILD_BUG_ON((feature >> 5) == CPUID_LNX_1); > > + BUILD_BUG_ON((feature >> 5) == CPUID_LNX_2); > > + BUILD_BUG_ON((feature >> 5) == CPUID_LNX_3); > > + BUILD_BUG_ON((feature >> 5) == CPUID_LNX_4); > > + BUILD_BUG_ON((feature >> 5) > CPUID_7_EDX); > > What is magical about CPUID_7_EDX? It's currently the last cpufeatures word. My thought was to force this to be updated in order to do reverse lookup on the next new word. I didn't want to use NCAPINTS because that gets updated when a new word is added to cpufeatures, i.e. wouldn't catch the case where the next new word is a Linux-defined word, which is extremely unlikely but theoretically possible. > > + > > + return 1 << (feature & 31); > > Why not BIT(feature & 31)? That's a very good question. > > } > > > > static inline u8 vcpu_virt_addr_bits(struct kvm_vcpu *vcpu) > > -- > > 2.24.0 > >