Guest OS migration and lost IPIs

["Paul E. McKenney" <paulmck@kernel.org>]

Hello, Paolo!

We are seeing occasional odd hangs, but only in cases where guest OSes
are being migrated.  Migrating more often makes the hangs happen more
frequently.

Added debug showed that the hung CPU is stuck trying to send an IPI (e.g.,
smp_call_function_single()).  The hung CPU thinks that it has sent the
IPI, but the destination CPU has interrupts enabled (-not- disabled,
enabled, as in ready, willing, and able to take interrupts).  In fact,
the destination CPU usually is going about its business as if nothing
was wrong, which makes me suspect that the IPI got lost somewhere along
the way.

I bumbled a bit through the qemu and KVM source, and didn't find anything
synchronizing IPIs and migrations, though given that I know pretty much
nothing about either qemu or KVM, this doesn't count for much.

The guest OS is running v5.2, so reasonably recent.  It is using
QEMU Guest Agent 2.12.0.  The host is also running v5.2 and providing
qemu-system-x86_64 version 2.11.0.

Is this a known problem?  Is there some debugging options I should enable?
Any other patch I should apply?

							Thanx, Paul