Re: [PATCH 0/2] RFC: Precise TSC migration

[Thomas Gleixner]

On Mon, Nov 30 2020 at 16:16, Marcelo Tosatti wrote:
> Not really. The synchronization logic tries to sync TSCs during
> BIOS boot (and CPU hotplug), because the TSC values are loaded
> sequentially, say:
>
> CPU		realtime	TSC val
> vcpu0		0 usec		0
> vcpu1		100 usec	0
> vcpu2		200 usec	0

That's nonsense, really.

> And we'd like to see all vcpus to read the same value at all times.

Providing guests with a synchronized and stable TSC on a host with a
synchronized and stable TSC is trivial.

Write the _same_ TSC offset to _all_ vcpu control structs and be done
with it. It's not rocket science.

The guest TSC read is:

    hostTSC + vcpu_offset

So if the host TSC is synchronized then the guest TSCs are synchronized
as well.

If the host TSC is not synchronized, then don't even try.

Thanks,

        tglx