[Qemu-devel] [PATCH] virtio-pci: enable bus master for old guests

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

From: Michael S. Tsirkin <mst@redhat.com>

commit cc943c36faa192cd4b32af8fe5edb31894017d35
    pci: Use bus master address space for delivering MSI/MSI-X messages
breaks virtio-net for rhel6.[56] x86 guests because they don't
enable bus mastering for virtio PCI devices. For the same reason,
rhel6.[56] ppc64 guests cannot boot on a virtio-blk disk anymore.

Old guests forgot to enable bus mastering, enable it automatically on
DRIVER (ppc64 guests may never reach DRIVER_OK). We also need to prevent
these old guests to disable bus mastering (this happens when the driver
probes the device). And we must also re-enable bus mastering after migration.

Cc: qemu-stable@nongnu.org
Reported-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[ old guest detection on DRIVER,
  squashed patch from Michael S. Tsirkin to re-enable bus mastering,
  Greg Kurz <gkurz@linux.vnet.ibm.com> ]
Tested-by: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
---
 hw/virtio/virtio-pci.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index ddb5da1..f981841 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -317,9 +317,11 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         /* 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) &&
+        if ((val & VIRTIO_CONFIG_S_DRIVER) &&
             !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
             proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
+            memory_region_set_enabled(&proxy->pci_dev.bus_master_enable_region,
+                                      true);
         }
         break;
     case VIRTIO_MSI_CONFIG_VECTOR:
@@ -473,10 +475,14 @@ static void virtio_write_config(PCIDevice *pci_dev, uint32_t address,
     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)) {
-        virtio_pci_stop_ioeventfd(proxy);
-        virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
+        !(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
+        if (proxy->flags & VIRTIO_PCI_FLAG_BUS_MASTER_BUG) {
+            memory_region_set_enabled(&proxy->pci_dev.bus_master_enable_region,
+                                      true);
+        } else {
+            virtio_pci_stop_ioeventfd(proxy);
+            virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
+        }
     }
 }
 
@@ -890,6 +896,8 @@ static void virtio_pci_vmstate_change(DeviceState *d, bool running)
         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;
+            memory_region_set_enabled(&proxy->pci_dev.bus_master_enable_region,
+                                      true);
         }
         virtio_pci_start_ioeventfd(proxy);
     } else {