Re: [Qemu-devel] [PATCH v15 23/26] sched: early boot clock

[Pavel Tatashin]

On 19-01-25 21:04:10, Jon DeVree wrote:
> 
> * KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is always set by the hypervisor
>   starting with Linux v2.6.35 - 371bcf646d17 ("KVM: x86: Tell the guest
>   we'll warn it about tsc stability")
> * PVCLOCK_TSC_STABLE_BIT is set starting in Linux v3.8 but only if the
>   clocksource is the TSC - d828199e8444 ("KVM: x86: implement
>   PVCLOCK_TSC_STABLE_BIT pvclock flag")
> 
> I changed the clocksource of a hypervisor that wasn't having issues from
> TSC to HPET and when I started up a guest VM the bug suddenly appeared.
> I shut down the guest, set the hypervisor's clocksource back to TSC,
> started up the guest and the bug went away again.
> 
> You don't actually have to reboot the guest before the bug is visible
> either, just letting the guest sit at the GRUB menu for a minute or two
> before loading Linux is enough to make the bug plainly visible in the
> printk timestamps.
> 
> I don't know enough to actually fix the bug, but hopefully this is
> enough to allow everyone else to reproduce it and come up with a fix.

Thank you very much for your analysis, I am now able to reproduce the
problem by setting clocksource on my machine to hpet. I will soon submit a
patch with a fix.

Pasha

> 
> -- 
> Jon
> X(7): A program for managing terminal windows. See also screen(1) and tmux(1).