Revert "virtio-pci: fix migration for pci bus master"

This reverts commit 4d43d3f3.

Reported to break PPC guests.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -86,6 +86,9 @@
  * 12 is historical, and due to x86 page size. */
 #define VIRTIO_PCI_QUEUE_ADDR_SHIFT    12
 
+/* Flags track per-device state like workarounds for quirks in older guests. */
+#define VIRTIO_PCI_FLAG_BUS_MASTER_BUG  (1 << 0)
+
 static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
                                VirtIOPCIProxy *dev);
 
@@ -320,6 +323,14 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
                                      proxy->pci_dev.config[PCI_COMMAND] |
                                      PCI_COMMAND_MASTER, 1);
         }
+
+        /* Linux before 2.6.34 sets the device as OK without enabling
+           the PCI device bus master bit. In this case we need to disable
+           some safety checks. */
+        if ((val & VIRTIO_CONFIG_S_DRIVER_OK) &&
+            !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
+            proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
+        }
         break;
     case VIRTIO_MSI_CONFIG_VECTOR:
         msix_vector_unuse(&proxy->pci_dev, vdev->config_vector);
@@ -469,18 +480,13 @@ static void virtio_write_config(PCIDevice *pci_dev, uint32_t address,
     VirtIOPCIProxy *proxy = DO_UPCAST(VirtIOPCIProxy, pci_dev, pci_dev);
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
 
-    uint8_t cmd = proxy->pci_dev.config[PCI_COMMAND];
-
     pci_default_write_config(pci_dev, address, val, len);
 
     if (range_covers_byte(address, len, PCI_COMMAND) &&
         !(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER) &&
-        (cmd & PCI_COMMAND_MASTER)) {
-        /* Bus driver disables bus mastering - make it act
-         * as a kind of reset to render the device quiescent. */
+        !(proxy->flags & VIRTIO_PCI_FLAG_BUS_MASTER_BUG)) {
         virtio_pci_stop_ioeventfd(proxy);
-        virtio_reset(vdev);
-        msix_unuse_all_vectors(&proxy->pci_dev);
+        virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
     }
 }
 
@@ -889,19 +895,11 @@ static void virtio_pci_vmstate_change(DeviceState *d, bool running)
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
 
     if (running) {
-        /* Linux before 2.6.34 drives the device without enabling
-           the PCI device bus master bit. Enable it automatically
-           for the guest. This is a PCI spec violation but so is
-           initiating DMA with bus master bit clear.
-           Note: this only makes a difference when migrating
-           across QEMU versions from an old QEMU, as for new QEMU
-           bus master and driver bits are always in sync.
-           TODO: consider enabling conditionally for compat machine types. */
-        if (vdev->status & (VIRTIO_CONFIG_S_ACKNOWLEDGE |
-                            VIRTIO_CONFIG_S_DRIVER)) {
-            pci_default_write_config(&proxy->pci_dev, PCI_COMMAND,
-                                     proxy->pci_dev.config[PCI_COMMAND] |
-                                     PCI_COMMAND_MASTER, 1);
+        /* Try to find out if the guest has bus master disabled, but is
+           in ready state. Then we have a buggy guest OS. */
+        if ((vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) &&
+            !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
+            proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
         }
         virtio_pci_start_ioeventfd(proxy);
     } else {
@@ -1042,6 +1040,7 @@ static void virtio_pci_reset(DeviceState *qdev)
     virtio_pci_stop_ioeventfd(proxy);
     virtio_bus_reset(bus);
     msix_unuse_all_vectors(&proxy->pci_dev);
+    proxy->flags &= ~VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
 }
 
 static Property virtio_pci_properties[] = {