From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Thu, 11 Sep 2014 18:45:33 +0200
Subject: [PATCH] virtio-pci: enable bus master for old guests

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 (guests use some devices before DRIVER_OK).

Reported-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit e43c0b2ea5574efb0bedebf6a7d05916eefeba52)
---
 hw/virtio/virtio-pci.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 3007319..58ebbcf 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -314,6 +314,16 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             msix_unuse_all_vectors(&proxy->pci_dev);
         }
 
+        /* 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. */
+        if (val == (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);
+        }
+
         /* 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. */