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    SubjectRe: [PATCH 05/10] x86: jailhouse: Set up timekeeping
    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

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