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

[Haozhong Zhang]

On Tue, Sep 29, 2015 at 03:02:07PM -0300, Eduardo Habkost wrote:
> 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())".
>

will include this in the comment

> > 
> [...]
> > > 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.
>

What I'm considering is to add a "-keep-tsc-freq" option to control
whether the TSC frequency should be migrated if "tsc-freq" is not
presented. Compared to "tsc-freq", "-keep-tsc-freq" can free users
from figuring out the guest TSC frequency manually in the migration.

- Haozhong

> -- 
> Eduardo
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