From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752622AbcGAODJ (ORCPT ); Fri, 1 Jul 2016 10:03:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57770 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751193AbcGAODI (ORCPT ); Fri, 1 Jul 2016 10:03:08 -0400 Subject: Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic_map To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= References: <20160630205429.16480-1-rkrcmar@redhat.com> <20160630205429.16480-4-rkrcmar@redhat.com> <20160701124421.GA2301@potion> Cc: Andrew Honig , linux-kernel@vger.kernel.org, kvm , "Lan, Tianyu" , Igor Mammedov , Jan Kiszka , Peter Xu From: Paolo Bonzini Message-ID: Date: Fri, 1 Jul 2016 16:03:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 MIME-Version: 1.0 In-Reply-To: <20160701124421.GA2301@potion> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Fri, 01 Jul 2016 14:03:07 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/07/2016 14:44, Radim Krčmář wrote: > 2016-07-01 10:42+0200, Paolo Bonzini: >> On 01/07/2016 00:15, Andrew Honig wrote: >>>>> + /* kvm_apic_map_get_logical_dest() expects multiples of 16 */ >>>>> + size = round_up(max_id + 1, 16); >>> Now that you're using the full range of apic_id values, could this >>> calculation overflow? Perhaps max_id could be u64? >> >> Good point, but I wonder if it's a good idea to let userspace allocate >> 32 GB of memory. :) > > Yes, both could happen. I'll change it to u64 to make it future proof. It's not necessary to change it to u64 if you put a limit, but you can add a WARN_ON(size == 0). Also if kvm_apic_map_get_logical_dest() expects multiples of 16, it should warn whenever the invariant is not respected. >> Let's put a limit on the maximum supported APIC ID, and report it >> through KVM_CHECK_EXTENSION on the new KVM_CAP_X2APIC_API capability. >> If 767 is enough for Knights Landing, the allocation below fits in two >> pages. If you need to make it higher, please change the allocation to >> use kvm_kvzalloc and kvfree. > > We sort of have a capability for maximum APIC ID, KVM_MAX_VCPU_ID, > because VCPU ID is initial APIC ID and x2APIC ID should always be the > initial APIC ID. Should it? According to QEMU if you have e.g. 3 cores per socket one socket take 4 APIC IDs. For Knights Landing the "worst" prime factor in 288 is 3^2 so you need APIC IDs up to 288 * (4/3)^2 = 512. Paolo