[Qemu-devel] PCI regression in 2.9

[Jeff Cody <jcody@redhat.com>]

I ran into this while running qemu iotests, and it looks like it is a
regression from 2.8.

Here is the reproducer:

$ ./x86_64-softmmu/qemu-system-x86_64 -drive if=virtio
qemu-system-x86_64: qemu-kvm/memory.c:2078: memory_region_del_subregion: Assertion `subregion->container == mr' failed
Aborted (core dumped)

The proper output is (with no abort):
qemu-system-x86_64: -drive if=virtio: Device needs media, but drive is empty

I bisected it down to this commit:

commit c53598ed18e40a9609573b21f2a361221ca0f806
Author: Alexey Kardashevskiy <aik@ozlabs.ru>
Date:   Mon Mar 27 15:40:30 2017 +1100

    pci: Add missing drop of bus master AS reference
    
    The recent introduction of a bus master container added
    memory_region_add_subregion() into the PCI device registering path but
    missed memory_region_del_subregion() in the unregistering path leaving
    a reference to the root memory region of the new container.
    
    This adds missing memory_region_del_subregion().
    
    Fixes: 3716d5902d743 ("pci: introduce a bus master container")
    Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
    Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>