[Bug 195819] TSC calibration can hang FreeBSD in Hyper-V Generation 2 VM

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=195819

Dexuan Cui <decui at microsoft.com> changed:

           What    |Removed                     |Added
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                 CC|                            |decui at microsoft.com,
                   |                            |honzhan at microsoft.com

--- Comment #16 from Dexuan Cui <decui at microsoft.com> ---
(In reply to Mark Trettin from comment #11)
> I believe what is happening is the TSC calibration is dropping to the i8254 PIT to
> calibrate the TSC and I don't think the Generation 2 Hyper-V has that.

Hi Mark, you're right. In Hyper-V generation-2 Linux VM, the ACPI timer is used
to calibrate TSC.

With "set machdep.disable_tsc_calibration=1", the bootup can go further but the
VM still hangs later -- I guess this may be a bad side effect of disabling TSC
calibration and I guess the TSC has to be calibrated here, e.g., by the ACPI
timer?

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