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 BEBCCC2BA83 for ; Fri, 7 Feb 2020 22:03:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9EE5D21775 for ; Fri, 7 Feb 2020 22:03:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727130AbgBGWD0 (ORCPT ); Fri, 7 Feb 2020 17:03:26 -0500 Received: from mga12.intel.com ([192.55.52.136]:23731 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727031AbgBGWD0 (ORCPT ); Fri, 7 Feb 2020 17:03:26 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Feb 2020 14:03:26 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,415,1574150400"; d="scan'208";a="280100123" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by FMSMGA003.fm.intel.com with ESMTP; 07 Feb 2020 14:03:25 -0800 Date: Fri, 7 Feb 2020 14:03:25 -0800 From: Sean Christopherson To: Peter Xu Cc: Paolo Bonzini , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Christoffer Dall , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= Subject: Re: [PATCH v5 17/19] KVM: Terminate memslot walks via used_slots Message-ID: <20200207220325.GO2401@linux.intel.com> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-18-sean.j.christopherson@intel.com> <20200206210944.GD700495@xz-x1> <20200207183325.GI2401@linux.intel.com> <20200207203909.GE720553@xz-x1> <20200207211016.GN2401@linux.intel.com> <20200207214623.GF720553@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200207214623.GF720553@xz-x1> 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 Fri, Feb 07, 2020 at 04:46:23PM -0500, Peter Xu wrote: > On Fri, Feb 07, 2020 at 01:10:16PM -0800, Sean Christopherson wrote: > > On Fri, Feb 07, 2020 at 03:39:09PM -0500, Peter Xu wrote: > > > On Fri, Feb 07, 2020 at 10:33:25AM -0800, Sean Christopherson wrote: > > > > On Thu, Feb 06, 2020 at 04:09:44PM -0500, Peter Xu wrote: > > > > > On Tue, Jan 21, 2020 at 02:31:55PM -0800, Sean Christopherson wrote: > > > > > > @@ -9652,13 +9652,13 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size) > > > > > > if (IS_ERR((void *)hva)) > > > > > > return PTR_ERR((void *)hva); > > > > > > } else { > > > > > > - if (!slot->npages) > > > > > > + if (!slot || !slot->npages) > > > > > > return 0; > > > > > > > > > > > > - hva = 0; > > > > > > + hva = slot->userspace_addr; > > > > > > > > > > Is this intended? > > > > > > > > Yes. It's possible to allow VA=0 for userspace mappings. It's extremely > > > > uncommon, but possible. Therefore "hva == 0" shouldn't be used to > > > > indicate an invalid slot. > > > > > > Note that this is the deletion path in __x86_set_memory_region() not > > > allocation. IIUC userspace_addr won't even be used in follow up code > > > path so it shouldn't really matter. Or am I misunderstood somewhere? > > > > No, but that's precisely why I don't want to zero out @hva, as doing so > > implies that '0' indicates an invalid hva, which is wrong. > > > > What if I change this to > > > > hva = 0xdeadull << 48; > > > > and add a blurb in the changelog about stuff hva with a non-canonical value > > to indicate it's being destroyed. > > IMO it's fairly common to have the case where "when A is XXX then > parameters B is invalid" happens in C. I'm not arguing that's not the case. My point is that there's nothing special about '0', so why use it? E.g. "hva = 1" would also be ok from a functional perspective, but more obviously "wrong".