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

[Paolo Bonzini]

On 07/12/20 18:41, Thomas Gleixner wrote:
> Right this happens still occasionally, but for quite some time this is
> 100% firmware sillyness and not a fundamental property of the hardware
> anymore.

It's still a fundamental property of old hardware.  Last time I tried to 
kill support for processors earlier than Core 2, I had to revert it. 
That's older than Nehalem.

> > 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).
>
> I just ran a guest on an old machine with unsynchronized TSCs and was
> able to observe clock monotonic going backwards between two threads
> pinned on two vCPUs, which _is_ bad. Getting unsynced clocks reliably
> under control is extremly hard.

Using kvmclock?  (Half serious: perhaps a good reason to have per-vCPU 
offsets is to be able to test what happens with unsynchronized TSCs...).

Paolo