Re: [Qemu-devel] [PATCH] target/ppc: only save guest timebase once after stopping

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jul 26, 2018 at 09:44:11AM +0200, Laurent Vivier wrote:
> On 26/07/2018 07:07, David Gibson wrote:
> > On Thu, May 03, 2018 at 11:20:44PM -0500, Michael Roth wrote:
> >> In some cases (e.g. spapr) we record guest timebase after qmp_stop()
> >> via a runstate hook so we can restore it on qmp_cont(). If a migration
> >> occurs in between those events we end up saving it again, this time
> >> based on the current timebase the guest would be seeing had it been
> >> running. This has the effect of advancing the guest timebase while
> >> it is stopped, which is not what the code intends.
> >>
> >> Other than simple jumps in time, this has been seen to trigger what
> >> appear to be RCU-related crashes in recent kernels when the advance
> >> exceeds rcu_cpu_stall_timeout, and it can be triggered by fairly
> >> common operations such as `virsh migrate ... --timeout 60`.
> >>
> >> Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
> >> Cc: David Gibson <david@gibson.dropbear.id.au>
> >> Cc: Laurent Vivier <lvivier@redhat.com>
> >> Cc: qemu-ppc@nongnu.org
> >> Cc: qemu-stable@nongnu.org
> >> Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > 
> > Sorry, this fell off my radar for ages, but I've finally had a chance
> > to look at it properly.
> > 
> > I'm not totally convinced this handle all the possible edge cases
> > correctly, but I am convinced it gives behaviour that's more correct
> > than we have now.  It doesn't introduce changes to the interface or
> > migration stream that would break things in future, so I've applied it
> > to ppc-for-3.1.
> 
> See https://bugzilla.redhat.com/show_bug.cgi?id=1571230#c8
> 
> So you revert:
> 
> commit 42043e4f1241eeb77f87f5816b5cf0b6e9583ed7
> Author: Laurent Vivier <lvivier@redhat.com>
> Date:   Fri Jan 27 13:24:58 2017 +0100
> 
>     spapr: clock should count only if vm is running
> 
> and change the behaviour compared to x86_64
> 
>      6053a86 kvmclock: reduce kvmclock difference on migration
> 
> Is this what you want?

Ah, no, not really.  I've pulled it out of 3.1 again.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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