x86/tsc: fetch frequency from CPUID when running on Xen

[royger]

Summary

Introduce a helper to fetch the TSC frequency from CPUID when running
under Xen.

Sponsored by: Citrix Systems R&D

[markj]

sys/x86/x86/tsc.c

157

Since this routine is using virtualized cpuid leaf to obtain a fequency value, why not simply inline it into tsc_freq_cpuid_vm() or call this routine from there?

[royger]

sys/x86/x86/tsc.c

157

I was mostly following what's done for tsc_freq_vmware(), but I could see about unifying all those into a single helper if that's preferred.

[markj]

This needs to be rebased on top of 075e2779aca7cbd8f201ce0e1bb60318d0cbd8b8 .

sys/x86/x86/tsc.c

157

Ok, on second thought your approach seems good. The probe_tsc_freq() helpers are a bit inconsistent (e.g., whether or not they assign to tsc_freq directly or not, whether they emit the bootverbose message or not) and could maybe benefit from some refactoring, but that's a separate change.

159

Should we verify hv_high >= 0x40000003 first?

[royger]

sys/x86/x86/tsc.c

159

It's a non-optional leaf so not having it would mean something has gone very wrong. I had a KASSERT originally, but ended up removing it. I can add it.

[royger]

• Rebase over changes in tsc.c.
• Add KASSERT.
• Call tsc_init from Xen PVH early_clock_source_init hook.

[royger]

It might be better to pull tsc_init() call out of the hook implementation, now that all of them call it anyway.

[royger]

Pull out tsc_init calls from the early_clock_source_init() handlers.