Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752447AbcLFMx0 (ORCPT ); Tue, 6 Dec 2016 07:53:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53412 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751932AbcLFMxV (ORCPT ); Tue, 6 Dec 2016 07:53:21 -0500 Date: Tue, 6 Dec 2016 13:52:31 +0100 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Paolo Bonzini , Igor Mammedov Subject: Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff Message-ID: <20161206125231.GC7972@potion> References: <20161202194401.10038-1-rkrcmar@redhat.com> <20161202194401.10038-5-rkrcmar@redhat.com> <20161205160228.GA8660@potion> <20161205205728.GB7972@potion> <6f14273e-8429-18dc-6785-5f001b194bf1@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6f14273e-8429-18dc-6785-5f001b194bf1@redhat.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Tue, 06 Dec 2016 12:52:35 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1423 Lines: 34 2016-12-06 10:37+0100, David Hildenbrand: >> I think the agreement is to embrace compatibility, so we pile new >> mistakes to hide known ones. >> (Rewriting the past requires far more power than accepting it: >> If we didn't force unfixed kernels out of existence, then userspace >> couldn't tell if hotplug up to high VCPU ID limit is supported.) > > I agree, the question is how old the bug is (you should know better than me > :) ) Just half a year old, since v4.7. > and if introducing a capability is strictly necessary. Do we have to do > the check in QEMU or can we simply fix implementations out there silently. This fix is too big for stable and I don't think that patches outside of stable get backported much. > (especially as hotplugging cpuid > 255 doesn't sound like setups wildly used > already today - and it doesn't work ;) ). Yes, it seems that no-one using high APIC ID noticed/cared. > But as I said, I don't know the > history, so you decide if this check in QEMU is necessary. QEMU can decide not to check (I actually expect it won't :]). I think the option to check is worth two lines of code in KVM, though. > Fix all QEMUs (introduce capability check) vs fix all relevant kernels > (limiting VCPU id to 255). APID ID over 255 works without hotplug and has few users, so lowering the limit would regress cases that are more important, IMO.