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

[Jamie Lokier]

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
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