Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host

On Tue, Sep 20, 2011 at 09:03:51PM +0100, Jamie Lokier wrote:
> Marcelo Tosatti wrote:
> > In case the VM stops for whatever reason, the host system is not
> > supposed to adjust time related hardware state to compensate, in an
> > attempt to present apparent continuous time.
> > 
> > If you save a VM and then restore it later, it is the guest
> > responsability to adjust its time representation.
> 
> If the guest doesn't know it's been stopped, then its time
> representation will be wrong until it finds out, e.g. after a few
> minutes with NTP, or even a seconds can be too long.
> 
> That is sad when it happens because it breaks the coherence of any
> timed-lease caching the guest is involved in.  I.e. where the guest
> acquires a lock on some data object (like a file in NFS) that it can
> efficiently access without network round trips (similar to MESI), with
> all nodes having agreed that it's coherent for, say, 5 seconds before
> renewing or breaking.  (It's just a way to reduce latency.)
> 
> But we can't trust CLOCK_MONOTONIC when a VM is involved, it's just
> one of those facts of life.  So instead the effort is to try and
> detect when a VM is involved and then distrust the clock.
> 
> (Non-VM) suspend/resume is similar, but there's usually a way to
> be notified about that as it happens.
> 
> -- Jamie

Thats pretty bad.

Time leased caching cannot be enabled in conjunction with save/restore
without VM involvement (the hypervisor management application must
notify the guest so it can drop such cache/writeback pending data).

But, if paravirtual clock is available (Linux VMs), it should be
possible to add an offset similarly to how suspend/resume is performed,
via a high priority channel, before resuming operation. We should look
into that possibility.

Thanks