On Mon, 20 Nov 2017, Jan Kiszka wrote: > On 2017-11-20 12:24, Thomas Gleixner wrote: > > 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. > > OK, some may (not Xeons, though), and we would not exploit it with this > approach.
Ok. Let's expose pmtimer it's not a big issue. Thanks, tglx