[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

--- Comment #18 from Dexuan Cui <decui at microsoft.com> ---
(In reply to Ed Maste from comment #17)
> When machdep.disable_tsc_calibration=1 is set the TSC frequency should be
> determined from the CPU identification instead.
OK, got it.

> Are you able to try a verbose boot with machdep.disable_tsc_calibration=1 set, and
> attach the boot log
Chris Lee has attached the log/screen in Comment 14 when the kernel hangs. I
got the same log. It looks the kernel is in some busy loop forever.
NB: I guess Chris uses SMP VM.

If I use only 1 vCPU, FreeBSD 10.2 VM's kernel can boot fine and it finally
stops at the Welcome screen for the installation (I'll attach the screen) --
the (virtual) keyboard doesn't work, so we can't go further...
This is because Generation-2 Hyper-V VM doesn't have the legacy 8042 keyboard,
so a driver for the Hyper-V synthetic keyboard device is required inside the
FreeBSD VM (BTW, Linux already has the driver) -- a new TODO. :-)

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