From mboxrd@z Thu Jan 1 00:00:00 1970 From: Radim =?utf-8?B?S3LEjW3DocWZ?= Subject: Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff Date: Wed, 7 Dec 2016 16:47:16 +0100 Message-ID: <20161207154715.GB15611@potion> References: <20161202194401.10038-1-rkrcmar@redhat.com> <20161202194401.10038-5-rkrcmar@redhat.com> <8ea7ddb3-84dd-e205-a728-cc0912462362@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Igor Mammedov To: Paolo Bonzini Return-path: Content-Disposition: inline In-Reply-To: <8ea7ddb3-84dd-e205-a728-cc0912462362@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 2016-12-07 13:03+0100, Paolo Bonzini: > On 02/12/2016 20:44, Radim Krčmář wrote: >> LAPIC after reset is in xAPIC mode, which poses a problem for hotplug of >> VCPUs with high APIC ID, because reset VCPU is waiting for INIT/SIPI, >> but there is no way to uniquely address it using xAPIC. >> >> From many possible options, we chose the one that also works on real >> hardware: accepting interrupts addressed to LAPIC's x2APIC ID even in >> xAPIC mode. >> >> KVM intentionally differs from real hardware, because real hardware >> (Knights Landing) does just "x2apic_id & 0xff" to decide whether to >> accept the interrupt in xAPIC mode and it can deliver one interrupt to >> more than one physical destination, e.g. 0x123 to 0x123 and 0x23. >> >> Add a capability to let userspace know that we do something now. > > I wouldn't even bother with the capability. It's a bit borderline for > stable, but I think it's okay. We can always Cc and let the maintainer > reject it. Ok, David also mentioned that I should tone down compatibility ... Thanks to both, I'll prepare v2.