Re: Guest OS migration and lost IPIs

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

On Fri, Aug 07, 2020 at 02:36:17PM +0200, Paolo Bonzini wrote:
> On 05/08/20 02:07, Paul E. McKenney wrote:
> > 
> > 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 code migrating the interrupt controller is in
> kvm_x86_ops.sync_pir_to_irr (which calls vmx_sync_pir_to_irr) and
> kvm_apic_get_state.  kvm_apic_get_state is called after CPUs are stopped.
> 
> It's possible that we're missing a kvm_x86_ops.sync_pir_to_irr call
> somewhere.  It would be surprising but it would explain the symptoms
> very well.

Thank you for the info, Paolo!  I will see what I can find.  ;-)

							Thanx, Paul