From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic_map Date: Fri, 1 Jul 2016 17:06:23 +0200 Message-ID: References: <20160630205429.16480-1-rkrcmar@redhat.com> <20160630205429.16480-4-rkrcmar@redhat.com> <20160701124421.GA2301@potion> <20160701143827.GE27840@potion> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Andrew Honig , linux-kernel@vger.kernel.org, kvm , "Lan, Tianyu" , Igor Mammedov , Jan Kiszka , Peter Xu To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: In-Reply-To: <20160701143827.GE27840@potion> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 01/07/2016 16:38, Radim Kr=C4=8Dm=C3=A1=C5=99 wrote: >> > Should it? > Yes, x2APIC ID cannot be changed in hardware and is initialized to th= e > intitial APIC ID. > Letting LAPIC_SET change x2APIC ID would allow scenarios where usersp= ace > reuses old VMs instead of building new ones after reconfiguration. > I don't think it's a sensible use case and it it is currently broken, > because we don't exit to userspace when changing APIC mode, so KVM wo= uld > just set APIC ID to VCPU ID on any transition and userspace couldn't > amend it. >=20 >> > According to QEMU if you have e.g. 3 cores per socket = one >> > socket take 4 APIC IDs. For Knights Landing the "worst" prime fac= tor in >> > 288 is 3^2 so you need APIC IDs up to 288 * (4/3)^2 =3D 512. > The topology can result in sparse APIC ID and APIC ID is initialized > from VCPU ID, so userspace has to pick VCPU ID accordingly. Right, I was confusing KVM_MAX_VCPUS and KVM_MAX_VCPU_ID. So the overflow case cannot happen and neither can the 32GB allocation. On th= e other hand, I suspect you need to bump KVM_MAX_VCPU_ID beyond its current default setting (which is equal to KVM_MAX_VCPUS), up to 511 or 1023. Thanks, Paolo