Re: [PATCH] Enabling IA32_TSC_ADJUST for guest VM

[Marcelo Tosatti <mtosatti@redhat.com>]

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);
        }
    }