Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stoppedby host

[Dor Laor]

On 12/07/2011 04:41 PM, Avi Kivity wrote:
> On 12/05/2011 10:18 PM, Eric B Munson wrote:
> > Changes from V4:
> > Rename KVM_GUEST_PAUSED to KVMCLOCK_GUEST_PAUSED
> > Add description of KVMCLOCK_GUEST_PAUSED ioctl to api.txt
> >
> > Changes from V3:
> > Include CC's on patch 3
> > Drop clear flag ioctl and have the watchdog clear the flag when it is reset
> >
> > Changes from V2:
> > A new kvm functions defined in kvm_para.h, the only change to pvclock is the
> > initial flag definition
> >
> > Changes from V1:
> > (Thanks Marcelo)
> > Host code has all been moved to arch/x86/kvm/x86.c
> > KVM_PAUSE_GUEST was renamed to KVM_GUEST_PAUSED
> >
> > When a guest kernel is stopped by the host hypervisor it can look like a soft
> > lockup to the guest kernel.  This false warning can mask later soft lockup
> > warnings which may be real.  This patch series adds a method for a host
> > hypervisor to communicate to a guest kernel that it is being stopped.  The
> > final patch in the series has the watchdog check this flag when it goes to
> > issue a soft lockup warning and skip the warning if the guest knows it was
> > stopped.
> >
> > It was attempted to solve this in Qemu, but the side effects of saving and
> > restoring the clock and tsc for each vcpu put the wall clock of the guest behind
> > by the amount of time of the pause.  This forces a guest to have ntp running
> > in order to keep the wall clock accurate.

Guests need to run NTP regardless, not only the virtualization layer add 
some skew, the physical world is not that perfect.
btw: traditional NTP client won't sync the time automatically if the 
diff is > 0.5%.

> Having this controlled from userspace means it doesn't work for SIGSTOP
> or for long scheduling delays.  What about doing this automatically
> based on preempt notifiers?

Isn't it solved by steal time?
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