Re: [PATCH] Enabling IA32_TSC_ADJUST for guest VM

[Avi Kivity <avi@redhat.com>]

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).


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