From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stefan Fritsch Subject: Re: [PATCH] KVM: add KVM_CAP_VMX_APICV to advertise hardware apic-v support Date: Wed, 17 Dec 2014 23:17:54 +0100 Message-ID: <5914325.jABlNffIti@k> References: <1418285221-14256-1-git-send-email-zhang.zhanghailiang@huawei.com> <548993D8.6010004@huawei.com> <5489D13B.3000703@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: zhanghailiang , kvm@vger.kernel.org, gleb@kernel.org, peter.huangpeng@huawei.com To: Paolo Bonzini Return-path: Received: from eru.sfritsch.de ([188.40.99.202]:40862 "EHLO eru.sfritsch.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751189AbaLQWqa (ORCPT ); Wed, 17 Dec 2014 17:46:30 -0500 In-Reply-To: <5489D13B.3000703@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thursday 11 December 2014 18:15:39, Paolo Bonzini wrote: > On 11/12/2014 13:53, zhanghailiang wrote: > >> I think it's a Windows bug---it should prefer x2apic to hv-vapic > >> if both are available. > > > > No, i don't think it is a windows bug, it has nothing to do with > > x2apic, > hv-vapic MSRs doesn't provide any performance improvement over > x2apic MSRs. So Windows should use x2apic MSRs if both are > available. Windows can use x2apic MSRs together with its EOI > optimization, like Linux does. > > There definitely are Windows versions that know how to use x2apic > (e.g. 2008R2). > The Hyper-V specification (V3) has different bits for "supported" and "recommended". qemu always sets this bit (HV_X64_APIC_ACCESS_RECOMMENDED): Bit 3: Recommend using MSRs for accessing APIC registers EOI, ICR and TPR rather than their memory-mapped counterparts. But it does not seem to set this bit: Bit 8: Recommend using x2APIC MSRs. I guess that is why Windows does not use x2apic and qemu should always set this bit. But the HV_X64_APIC_ACCESS_RECOMMENDED bit should probably not be set if the host supports apic-v. I havn't done any testing, though. > > but apic-v (need hardware support, i.e. Haswell cpu). > > APICv can use the x2apic MSRs. > > > When we don't passthough host cpu model to Guest os, > > it has no idea about whether it supports apic-v in host, > > The presence of APICv should be totally transparent to the guest. > > > Actually, qemu has a option 'hv_vapic' for -cpu, we can choose not > > to configure it if we know there is apic-v support in host. But > > IMHO, we'd better to do it automatically. > > ... and cause the CPUID to change under the guest's feet if you > migrate from a non-APICv to an APICv machines, or vice versa.