svn commit: r361064 - head/sys/amd64/vmm

[Konstantin Belousov]

On Thu, May 14, 2020 at 10:18:12PM +0000, Peter Grehan wrote:
> Author: grehan
> Date: Thu May 14 22:18:12 2020
> New Revision: 361064
> URL: https://svnweb.freebsd.org/changeset/base/361064
> 
> Log:
>   Hide host CPUID 0x15 TSC/Crystal ratio/freq info from guest
>   
>   In recent Linux (5.3+) and OpenBSD (6.6+) kernels, and with hosts that
>   support CPUID 0x15, the local APIC frequency is determined directly
>   from the reported crystal clock to avoid calibration against the 8254
>   timer.
>   
>   However, the local APIC frequency implemented by bhyve is 128MHz, where
>   most h/w systems report frequencies around 25MHz. This shows up on
>   OpenBSD guests as repeated keystrokes on the emulated PS2 keyboard
>   when using VNC, since the kernel's timers are now much shorter.
>   
>   Fix by reporting all-zeroes for CPUID 0x15. This allows guests to fall
>   back to using the 8254 to calibrate the local APIC frequency.
>   
>   Future work could be to compute values returned for 0x15 that would
>   match the host TSC and bhyve local APIC frequency, though all dependencies
>   on this would need to be examined (for example, Linux will start using
>   0x16 for some hosts).

FreeBSD uses 0x15 (and 0x16 if there is no 0x15) as well.  We recheck
against 8254 and decide that there is no ISA timer if calibration differs
too much from CPUID numbers.