From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eduardo Habkost Subject: Re: [PATCH 3/3] kvm-all: notice KVM of vcpu's TSC rate after migration Date: Tue, 29 Sep 2015 15:02:07 -0300 Message-ID: <20150929180207.GK4130@thinpad.lan.raisama.net> References: <1443418711-24106-1-git-send-email-haozhong.zhang@intel.com> <1443418711-24106-4-git-send-email-haozhong.zhang@intel.com> <20150928163734.GF4130@thinpad.lan.raisama.net> <20150929034334.GI3583@hzzhang-OptiPlex-9020.sh.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii To: qemu-devel@nongnu.org, kvm@vger.kernel.org, Paolo Bonzini , Marcelo Tosatti , Richard Henderson , libvir-list@redhat.com Return-path: Received: from mx1.redhat.com ([209.132.183.28]:40356 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755527AbbI2SCK (ORCPT ); Tue, 29 Sep 2015 14:02:10 -0400 Content-Disposition: inline In-Reply-To: <20150929034334.GI3583@hzzhang-OptiPlex-9020.sh.intel.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Sep 29, 2015 at 11:43:34AM +0800, Haozhong Zhang wrote: > On Mon, Sep 28, 2015 at 01:37:34PM -0300, Eduardo Habkost wrote: > > On Mon, Sep 28, 2015 at 01:38:31PM +0800, Haozhong Zhang wrote: [...] > > > static void do_kvm_cpu_synchronize_post_init(void *arg) > > > { > > > CPUState *cpu = arg; > > > + CPUX86State *env = &X86_CPU(cpu)->env; > > > + int r; > > > + > > > + /* > > > + * XXX: KVM_SET_TSC_KHZ must be done before kvm_arch_put_registers(). > > > > Could you explain where this requirement comes from? > > > > kvm_arch_put_registers() below will setup vcpu's MSR_IA32_TSC through > KVM ioctl KVM_SET_MSRS. KVM needs to know vcpu's TSC rate so as to > correctly scale the TSC value given by QEMU, especially when vcpu's > TSC rate is different than the host TSC rate (e.g. it's migrated from > another machine w/ different host TSC rate than the current one). Thanks. The comment above could contain a short version of this explanation, e.g.: "KVM needs KVM_SET_TSC_KHZ to be done before KVM_SET_MSRS sets MSR_IA32_TSC (done by kvm_arch_put_registers())". > [...] > > Or maybe we shouldn't treat this as VM state, but as configuration, and > > let management configure the TSC frequency explicitly if the user really > > needs it to stay the same during migration. > > > > (CCing libvir-list to see if they have feedback) > > > > Thanks for CC. I'll consider to add a command line option to control > the configuration of guest TSC fequency. It already exists, -cpu has a "tsc-freq" option. -- Eduardo