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

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

On Thu, 11 Sep 2014 21:20:03 +0300
"Michael S. Tsirkin" <mst@redhat.com> 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.
> 

Nice patch indeed ! :)

Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Cc'ing qemu-stable as Michael said he would queue the initial
bus master patch for 2.1.2.

>  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. */



-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.