Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751779AbaK0Ujv (ORCPT ); Thu, 27 Nov 2014 15:39:51 -0500 Received: from mail-wi0-f171.google.com ([209.85.212.171]:59299 "EHLO mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751082AbaK0Ujt convert rfc822-to-8bit (ORCPT ); Thu, 27 Nov 2014 15:39:49 -0500 Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 8.1 \(1993\)) Subject: Re: [PATCH 3/4] KVM: x86: allow 256 logical x2APICs again From: Nadav Amit In-Reply-To: <20141127201641.GB383@potion.brq.redhat.com> Date: Thu, 27 Nov 2014 22:39:45 +0200 Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Paolo Bonzini , Gleb Natapov , Nadav Amit Content-Transfer-Encoding: 8BIT Message-Id: References: <1417114994-25235-1-git-send-email-rkrcmar@redhat.com> <1417114994-25235-4-git-send-email-rkrcmar@redhat.com> <22F11C34-E2E4-459B-882B-A944AD0853FD@gmail.com> <20141127201641.GB383@potion.brq.redhat.com> To: =?utf-8?Q?Radim_Kr=C4=8Dm=C3=A1=C5=99?= X-Mailer: Apple Mail (2.1993) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Radim Krčmář wrote: > 2014-11-27 21:53+0200, Nadav Amit: >> Radim Krčmář wrote: >>> - new->cid_mask = (1 << KVM_X2APIC_CID_BITS) - 1; >>> - new->lid_mask = 0xffff; >>> + new->cid_mask = new->lid_mask = 0xffff; >> You set cid_mask to 0xffff, while there are only 16 clusters. I think it is >> risky (if you twist my hand would come with a scenario). > > Let's see :) APIC id is 8 bit, and we compute cluster part of LDR by > taking four upper bits, so 16 is enough. To clarify my concern - I am worried that some of the CPUs are still in xAPIC mode with LDR that does not follow x2APIC LDR scheme. > It isn't the safest programming practice, but we already fail to check > physical_map bounds and any boost to maximal APIC ID is going to require > a rewrite, thus I didn't bother to do it ... > > All uses should be covered with the following hunk, I will add it to v2 > after all reviews, > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index 6c2b8a5..30e4cc1 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -198,7 +198,7 @@ static void recalculate_apic_map(struct kvm *kvm) > cid = apic_cluster_id(new, ldr); > lid = apic_logical_id(new, ldr); > > - if (lid) > + if (lid && cid < ARRAY_SIZE(map->logical_map)) > new->logical_map[cid][ffs(lid) - 1] = apic; > } > out: > > >> Yet, why not to set >> cid_mask to (ARRAY_SIZE(map->logical_map) - 1) ? > > We would incorrectly deliver messages intended for high clusters, > it has to be 0xffff. >From the SDM, I am not sure you are correct, but your solution is fine. Nadav -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/