[vfio-users] Users of X99-boards, can you do me a small favor?

Martin Schrodt martin at schrodt.org
Tue Oct 4 08:09:58 UTC 2016

Hi Hristo,

> No need to sleep/wake - my X99-based system starts with TSC disabled:
> $ dmesg | grep TSC
> [    0.000000] tsc: Fast TSC calibration using PIT
> [    0.077986] TSC deadline timer enabled
> [    0.203383] TSC synchronization [CPU#0 -> CPU#1]:
> [    0.203384] Measured 974558547804462 cycles TSC warp between CPUs,
> turning off TSC clock.
> [    0.203388] tsc: Marking TSC unstable due to check_tsc_sync_source
> failed
> Consequently, tsc is not among the clock sources listed in
> available_clocksource. KVM is not happy about that:
> [16739.200656] kvm: SMP vm created on host with unstable TSC; guest TSC
> will not be reliable

Ok, so an X99-board that behaves like this even on a fresh start.

> But I haven't observed any instabilities of the Windows 10 guest, which
> happily runs with 4 virtual CPUs (2 virtual hyperthreaded CPUs) bound to
> two cores of my i7-5820K.

This really makes me think there's something else involved in this
behaviour. Maybe the CPU configuration (I use "Skylake-Client") exposes
TSC to the guest, so if you put that on, it'll use it?

Can you check what kind of virtual CPU you use?


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