Re: [Qemu-devel] [RFC 0/2] memory/vfio: notify region_del() when unregister listeners

[Paolo Bonzini]

On 19/01/2018 09:42, Peter Xu wrote:
> I encountered an event loss problem during unplugging vfio devices:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=1531393
> 
> I thought it should be a simple VT-d issue but I was wrong.  The whole
> debugging leads me to these patches.
> 
> Basically I think what we missed is that when unregistering memory
> listeners, we don't really call region_del() at all.  Instead we just
> remove ourselves from the listener list.  IMHO that's not enough.  A
> clean unregister should undo all possible changes that have done
> during region_add().  That's patch 1.
> 
> Patch 2 fixes a vfio issue when patch 1 is applied.

It makes sense, though of course patch 1 must come second for
bisectability.  Of the other listeners, most do not implement
region_del, but commit must be audited as well.  What matters is whether
the listener is unregistered, and only few are:

- kvm_arm_machine_init_done must unregister the listener after the
QSLIST_FOREACH_SAFE loop.

- Xen seems okay

- vhost needs to remove the memory_region_unref loop after unregistering
the listener - and this must be done at the same time as patch 1, not before

- virtio seems okay

Thanks,

Paolo