Re: [RFC/DISCUSSION] x86/tsc: Handling legacy PIT fallback calibration on modern platforms

[Ping K. Jiang]

> We have recently observed an issue regarding TSC calibration fallback on
> modern client/server platforms (Specifically Raptor Lake and newer) when
> running inside restricted virtualized environments.
>
> On modern platforms, however, legacy port I/O cycle latency has increased
> substantially. In benchmark comparisons between older platforms (e.g.
> Broadwell-DE) and modern silicon (Raptor Lake), 1024 PIT reads took ~2.7M
> cycles on BDX compared to ~21.3M cycles on RPL (~8x-10x slower, yielding
> ~10-20 us per read).
>

Dear Dr. Hahn,

I was reproducing this on i5-14600KF. My PIT delay wass ~13000 cycles,
resulting in TSC calibration with 30 ppm error, tested by running
`sudo dmesg | grep TSC` on host and VM.

I wonder why I can't reproduce it.
My motherboard is 'B760M-PLUS WIFI II'.

Thanks!
Ping Jiang