Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Vitaly Kuznetsov]

Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:

>
> But other than that I don't mind making TSC offset global per VM thing.
> Paulo, what do you think about this?
>

Not Paolo here but personally I'd very much prefer we go this route but
unsynchronized TSCs are, unfortunately, still a thing: I was observing
it on an AMD Epyc server just a couple years ago (cured with firmware
update). We try to catch such situation in KVM instead of blowing up but
this may still result in subtle bugs I believe. Maybe we would be better
off killing all VMs in case TSC ever gets unsynced (by default).

Another thing to this bucket is kvmclock which is currently per-cpu. If
we forbid TSC to un-synchronize (he-he), there is no point in doing
that. We can as well use e.g. Hyper-V TSC page method which is
per-VM. Creating another PV clock in KVM may be a hard sell as all
modern x86 CPUs support TSC scaling (in addition to TSC offsetting which
is there for a long time) and when it's there we don't really need a PV
clock to make migration possible.

-- 
Vitaly