virtio-pci: fix migration for pci bus master

Current support for bus master (clearing OK bit)
together with the need to support guests which do not
enable PCI bus mastering, leads to extra state in
VIRTIO_PCI_FLAG_BUS_MASTER_BUG bit, which isn't robust
in case of cross-version migration for the case when
guests use the device before setting DRIVER_OK.

Rip out VIRTIO_PCI_FLAG_BUS_MASTER_BUG and implement a simpler
work-around: treat clearing of PCI_COMMAND as a virtio reset.  Old
guests never touch this bit so they will work.

As reset clears device status, DRIVER and MASTER bits are
now in sync, so we can fix up cross-version migration simply
by synchronising them, without need to detect a buggy guest
explicitly.

Drop tracking VIRTIO_PCI_FLAG_BUS_MASTER_BUG completely.

As reset makes the device quiescent, in the future we'll be able to drop
checking OK bit in a bunch of places.

Cc: Jason Wang <[email protected]>
Cc: Greg Kurz <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>

hw/virtio/virtio-pci.c

@@ -86,9 +86,6 @@
  * 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);
 
@@ -323,14 +320,6 @@ 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);
@@ -480,13 +469,18 @@ 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) &&
-        !(proxy->flags & VIRTIO_PCI_FLAG_BUS_MASTER_BUG)) {
+        (cmd & PCI_COMMAND_MASTER)) {
+        /* Bus driver disables bus mastering - make it act
+         * as a kind of reset to render the device quiescent. */
         virtio_pci_stop_ioeventfd(proxy);
-        virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
+        virtio_reset(vdev);
+        msix_unuse_all_vectors(&proxy->pci_dev);
     }
 }
 
@@ -895,11 +889,19 @@ static void virtio_pci_vmstate_change(DeviceState *d, bool running)
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
 
     if (running) {
-        /* 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;
+        /* 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);
         }
         virtio_pci_start_ioeventfd(proxy);
     } else {
@@ -1040,7 +1042,6 @@ 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[] = {