Re: [PATCH v12 09/11] x86/tsc: prepare for early sched_clock

From: Peter Zijlstra
Date: Thu Jun 28 2018 - 07:47:10 EST


On Thu, Jun 28, 2018 at 12:43:59PM +0200, Thomas Gleixner wrote:
> init_hypervisor_platform()
> vmware:
> Retrieves frequency and store it for the
> calibration function
>
> khz = vmware_get_khz_magic()
> vmware_tsc_khz = khz
> calibrate_cpu = vmware_get_tsc_khz
> calibrate_tsc = vmware_get_tsc_khz
> preset_lpj(khz)
>
> hyperv:
> if special hyperv MSRs are available:
>
> calibrate_cpu = hv_get_tsc_khz
> calibrate_tsc = hv_get_tsc_khz
>
> MSR is readable already in this function
>
> jailhouse:
>
> Frequency is available in this function and store
> in a variable for the calibration function
>
> calibrate_cpu = jailhouse_get_tsc
> calibrate_tsc = jailhouse_get_tsc
>
> ...
>
> kvmclock_init()
>
> if (magic_conditions)
> calibrate_tsc = kvm_get_tsc_khz
> calibrate_cpu = kvm_get_tsc_khz
>
> kvm_get_preset_lpj()
> khz = kvm_get_tsc_khz()
> preset_lpj(khz);

Note that all these which get TSC values from a HV _should_ set
X86_FEATURE_TSC_KNOWN_FREQ to avoid the late recalibrate.

Calibrating against a virtual PIT/HPET/PMTIMER is utterly pointless.

> The generic initilizaiton does everything twice, which makes no sense,
> except for the unlikely case were no fast functions are available and the
> quick PIT calibration fails (PMTIMER/HPET) are not available in early
> calibration. HPET

Incomplete; but I suspect you want to talk about how we can make HPET
available early by putting it in a fixmap.

And only if we fail, do we at a later stage try again using PMTIMER.

Currently it all works by accident, since !hpet and acpi_pm_read_early()
returns 0, but really we should not be running the fallback crap at all
that early.