On Tue, Oct 09, 2012 at 04:26:32PM +0200, Avi Kivity wrote:
> On 10/09/2012 04:24 PM, Marcelo Tosatti wrote:
> > On Tue, Oct 09, 2012 at 02:12:18PM +0200, Avi Kivity wrote:
> >> On 10/08/2012 07:30 PM, Marcelo Tosatti wrote:
> >> >
> >> > From Intel's manual:
> >> >
> >> > • If an execution of WRMSR to the IA32_TIME_STAMP_COUNTER MSR adds (or
> >> > subtracts) value X from the TSC,
> >> > the logical processor also adds (or subtracts) value X from the
> >> > IA32_TSC_ADJUST MSR.
> >> >
> >> > This is not handled in the patch.
> >> >
> >> > To support migration, it will be necessary to differentiate between
> >> > guest initiated and userspace-model initiated msr write. That is,
> >> > only guest initiated TSC writes should affect the value of
> >> > IA32_TSC_ADJUST MSR.
> >> >
> >> > Avi, any better idea?
> >> >
> >>
> >> I think we need that anyway, since there are some read-only MSRs that
> >> need to be configured by the host (nvmx capabilities). So if we add
> >> that feature it will be useful elsewhere. I don't think it's possible
> >> to do it in any other way:
> >>
> >> "Local offset value of the IA32_TSC for a
> >> logical processor. Reset value is Zero. A
> >> write to IA32_TSC will modify the local
> >> offset in IA32_TSC_ADJUST and the
> >> content of IA32_TSC, but does not affect
> >> the internal invariant TSC hardware."
> >>
> >> What we want to do is affect the internal invariant TSC hardware, so we
> >> can't do that through the normal means.
> >>
> >> btw, will tsc writes from userspace (after live migration) cause tsc
> >> skew? If so we should think how to model a guest-wide tsc.
> >
> > No because there is an easy shortcut:
> >
> > if (level == KVM_PUT_FULL_STATE) {
> > /*
> > * KVM is yet unable to synchronize TSC values of multiple VCPUs
> > * on
> > * writeback. Until this is fixed, we only write the offset to
> > * SMP
> > * guests after migration, desynchronizing the VCPUs, but
> > * avoiding
> > * huge jump-backs that would occur without any writeback at
> > * all.
> > */
> > if (smp_cpus == 1 || env->tsc != 0) {
> > kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
> > }
> > }
>
> Still we write back after migration. So this needs to be fixed (or I
> misunderstood you).
Handled by kvm_write_tsc() in x86.c. Is this what you mean?
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