On Sat, 18 Nov 2017, Jan Kiszka wrote: > On 2017-11-17 23:49, Thomas Gleixner wrote: > > On Thu, 16 Nov 2017, Jan Kiszka wrote: > >> Calibrate the TSC and, where necessary, the APIC timer against the > >> TMTIMER. We need our own implementation as neither the PIC nor the HPET > >> are available, and the standard calibration routines try to make use of > >> them. > > > > Why is this needed at all? > > > > The host the frequency already. So this can be done w/o pmtimer and extra > > calibration routine. > > The hypervisor does not have the frequencies. It will never use the APIC > timer (it's owned by the guests), and it has no use case for the TSC so > far. Only the root cell (the Linux that booted the system) has that > data. Now we could > > - trust the root cell to provide the right values and export them during > startup to the hypervisor and from there to the non-root cells. > > - calculate the frequencies once and store them in the hyperivsor > config, just like other system-specific information, for re-export to > the cells. > > But I don't think option 1 will be ok for all use cases. Maybe a > combination of both, falling back to the root cell data if nothing is > defined in the config. Let me think about this.
Another question is whether systems which can support jailhouse, have the frequencies available via cpuid/msr and can avoid that calibration thing completely. Thanks, tglx