Re: [Qemu-devel] [PATCH v5] virtio-pci: enable bus master for old guests

[Jason Wang <jasowang@redhat.com>]

On 09/12/2014 02:20 AM, Michael S. Tsirkin wrote:
> 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>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> Sorry, v4 was broken - forgot to commit before git format-patch.
> Here's the correct version.
> Still not tested - will do next week.
> Testing reports appreciated meanwhile.
>
>  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 ddb5da1..a827cd4 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. */

So with this patch, we can also remove the detecting and handling of
VIRTIO_PCI_FLAG_BUS_MATER_BUG?