Re: [libvirt] [PATCH 3/3] kvm-all: notice KVM of vcpu's TSC rate after migration

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.

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