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

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Message-ID: <20110920200351.GH6690@jl-vm1.vm.bytemark.co.uk>
Date:	Tue, 20 Sep 2011 21:03:51 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Eric B Munson <emunson@...bm.net>,
	Anthony Liguori <anthony@...emonkey.ws>, avi@...hat.com,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, arnd@...db.de,
	riel@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, kvm-ppc@...r.kernel.org,
	aliguori@...ibm.com, raharper@...ibm.com, kvm-ia64@...r.kernel.org,
	Glauber Costa <glommer@...il.com>, mjwolf@...ibm.com
Subject: Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host

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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