Don't check for bus master for old guests

Older Linux guests don't activate the bus master enable bit. So for those we
can just try to be clever and track if they set the DEVICE_OK bit even though
bus mastering is still disabled.

Under that condition we can disable the windows safety check. With that logic
in place both guests should work just fine. Without PCI hotplug breaks
virtio-net in Linux < 2.6.34 guests.
Signed-off-by: NAlexander Graf <agraf@suse.de>
CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-pci.c

@@ -79,6 +79,10 @@
  * 12 is historical, and due to x86 page size. */
 #define VIRTIO_PCI_QUEUE_ADDR_SHIFT    12
 
+/* We can catch some guest bugs inside here so we continue supporting older
+   guests. */
+#define VIRTIO_PCI_BUG_BUS_MASTER	(1 << 0)
+
 /* QEMU doesn't strictly need write barriers since everything runs in
  * lock-step.  We'll leave the calls to wmb() in though to make it obvious for
  * KVM or if kqemu gets SMP support.
@@ -90,6 +94,7 @@
 typedef struct {
     PCIDevice pci_dev;
     VirtIODevice *vdev;
+    uint32_t bugs;
     uint32_t addr;
     uint32_t class_code;
     uint32_t nvectors;
@@ -144,6 +149,13 @@ static int virtio_pci_load_config(void * opaque, QEMUFile *f)
     if (proxy->vdev->config_vector != VIRTIO_NO_VECTOR) {
         return msix_vector_use(&proxy->pci_dev, proxy->vdev->config_vector);
     }
+
+    /* Try to find out if the guest has bus master disabled, but is
+       in ready state. Then we have a buggy guest OS. */
+    if (!(proxy->vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) &&
+        !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
+        proxy->bugs |= VIRTIO_PCI_BUG_BUS_MASTER;
+    }
     return 0;
 }
 
@@ -168,6 +180,7 @@ static void virtio_pci_reset(DeviceState *d)
     VirtIOPCIProxy *proxy = container_of(d, VirtIOPCIProxy, pci_dev.qdev);
     virtio_reset(proxy->vdev);
     msix_reset(&proxy->pci_dev);
+    proxy->bugs = 0;
 }
 
 static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
@@ -211,6 +224,14 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             virtio_reset(proxy->vdev);
             msix_unuse_all_vectors(&proxy->pci_dev);
         }
+
+        /* 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->bugs |= VIRTIO_PCI_BUG_BUS_MASTER;
+        }
         break;
     case VIRTIO_MSI_CONFIG_VECTOR:
         msix_vector_unuse(&proxy->pci_dev, vdev->config_vector);
@@ -377,7 +398,9 @@ static void virtio_write_config(PCIDevice *pci_dev, uint32_t address,
 
     if (PCI_COMMAND == address) {
         if (!(val & PCI_COMMAND_MASTER)) {
-            proxy->vdev->status &= ~VIRTIO_CONFIG_S_DRIVER_OK;
+            if (!(proxy->bugs & VIRTIO_PCI_BUG_BUS_MASTER)) {
+                proxy->vdev->status &= ~VIRTIO_CONFIG_S_DRIVER_OK;
+            }
         }
     }