[PATCH for-5.2] s390x/pci: Unregister listeners before destroying IOMMU address space

[Matthew Rosato]

Hot-unplugging a vfio-pci device on s390x causes a QEMU crash:

qemu-system-s390x: ../softmmu/memory.c:2772:
 do_address_space_destroy: Assertion `QTAILQ_EMPTY(&as->listeners)' failed.

In s390, the IOMMU address space is freed during device unplug but the
associated vfio-pci device may not yet be finalized and therefore may
still have a listener registered to the IOMMU address space.

Commit a2166410ad74 ("spapr_pci: Unregister listeners before destroying
the IOMMU address space") previously resolved this issue for spapr_pci.
We are now seeing this in s390x; it would seem the possibility for this
issue was already present but based on a bisect commit 2d24a6466154
("device-core: use RCU for list of children of a bus") has now changed
the timing such that it is now readily reproducible.

Add logic to ensure listeners are removed before destroying the address
space.

Reported-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
 hw/s390x/s390-pci-bus.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
index 48a3be8..e0dc20c 100644
--- a/hw/s390x/s390-pci-bus.c
+++ b/hw/s390x/s390-pci-bus.c
@@ -732,6 +732,13 @@ static void s390_pci_iommu_free(S390pciState *s, PCIBus 
*bus, int32_t devfn)
 
     table->iommu[PCI_SLOT(devfn)] = NULL;
     g_hash_table_destroy(iommu->iotlb);
+    /*
+     * An attached PCI device may have memory listeners, eg. VFIO PCI.
+     * The associated subregion will already have been unmapped in
+     * s390_pci_iommu_disable in response to the guest deconfigure request.
+     * Remove the listeners now before destroying the address space.
+     */
+    address_space_remove_listeners(&iommu->as);
     address_space_destroy(&iommu->as);
     object_unparent(OBJECT(&iommu->mr));
     object_unparent(OBJECT(iommu));
-- 
1.8.3.1