Commit e43c0b2e authored by Michael S. Tsirkin's avatar Michael S. Tsirkin

virtio-pci: enable bus master for old guests

commit cc943c36
    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: default avatarGreg Kurz <>
Reviewed-by: default avatarGreg Kurz <>
Tested-by: default avatarGreg Kurz <>
Signed-off-by: default avatarMichael S. Tsirkin <>
parent 9e8e8c48
......@@ -314,6 +314,16 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
/* 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. */
pci_default_write_config(&proxy->pci_dev, PCI_COMMAND,
proxy->pci_dev.config[PCI_COMMAND] |
/* 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. */
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