pci: fix locking around vPCI removal in pci_remove_device()

Currently vpci_deassign_device() is called without holding the per-domain
pci_lock in pci_remove_device(), which leads to:

Assertion 'rw_is_write_locked(&pdev->domain->pci_lock)' failed at ../drivers/vpci/vpci.c:47
[...]
Xen call trace:
   [<ffff82d040260eac>] R vpci_deassign_device+0x10d/0x1b9
   [<ffff82d04027932f>] S pci_remove_device+0x2b1/0x380
   [<ffff82d040260bd0>] F pci_physdev_op+0x197/0x19e
   [<ffff82d04032272d>] F do_physdev_op+0x342/0x12aa
   [<ffff82d0402f067a>] F pv_hypercall+0x58e/0x62b
   [<ffff82d0402012ba>] F lstar_enter+0x13a/0x140

Move the existing block that removes the device from the domain pdev_list ahead
and also issue the call to vpci_deassign_device() there.  It's fine to remove
the device from the domain list of assigned devices, as further functions only
care that the pdev domain field is correctly set to the owner of the device
about to be removed.

Moving the vpci_deassign_device() past the pci_cleanup_msi() call can be
dangerous, as doing the MSI cleanup ahead of having removed the vPCI handlers
could lead to stale data in vPCI MSI(-X) internal structures.

Fixes: 4f78438b45 ('vpci: use per-domain PCI lock to protect vpci structure')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stewart Hildebrand <stewart.hildebrand@amd.com>

xen/drivers/passthrough/pci.c

@@ -817,15 +817,15 @@ int pci_remove_device(u16 seg, u8 bus, u8 devfn)
     list_for_each_entry ( pdev, &pseg->alldevs_list, alldevs_list )
         if ( pdev->bus == bus && pdev->devfn == devfn )
         {
-            vpci_deassign_device(pdev);
-            pci_cleanup_msi(pdev);
-            ret = iommu_remove_device(pdev);
             if ( pdev->domain )
             {
                 write_lock(&pdev->domain->pci_lock);
+                vpci_deassign_device(pdev);
                 list_del(&pdev->domain_list);
                 write_unlock(&pdev->domain->pci_lock);
             }
+            pci_cleanup_msi(pdev);
+            ret = iommu_remove_device(pdev);
             printk(XENLOG_DEBUG "PCI remove device %pp\n", &pdev->sbdf);
             free_pdev(pseg, pdev);
             break;