Re: [Qemu-devel] [PATCH v2] virtio-pci: enable bus master for old guests

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

On Wed, 10 Sep 2014 20:30:47 +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
> 
> Old guests forgot to enable bus mastering, enable it
> automatically when driver discovers device.
> 
> Cc: qemu-stable@nongnu.org
> Reported-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 
> OK, this should have better luck.

Unfortunately not... it is even worse than V1 as
both x86 and ppc64 guests fail. I've added some
debug messages in virtio_ioport_write() and
virtio_write_config() to track:
- VIRTIO_PCI_STATUS changes
- bus master disablement

This is what I get for x86 with a virtio-net device:

virtio_write_config: PCI_COMMAND = 0x3
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x0
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x1 ==> v2 patch enables BM
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x3
virtio_write_config: PCI_COMMAND = 0x3       ==> guest disables BM
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x7
virtio_ioport_write: detected bus master bug ==> v1 patch would have
                                                 enabled BM here

and for ppc64 when booting from a virtio-blk disk:

virtio_write_config: PCI_COMMAND = 0x3
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x0
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x1 ==> v2 patch enables BM
virtio_ioport_write: VIRTIO_PCI_STATUS = 0x3
virtio_write_config: PCI_COMMAND = 0x3       ==> guest disables BM

I think we should:
- detect the "old driver" when reaching DRIVER as it works for
  both architectures
- always re-enable BM when an "old driver" disables it

That is basically your v1 plus its follow-up patch:

https://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg01918.html

plus:

--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -317,7 +317,7 @@ 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,

I could verify it fixes the issues. I will send a patch shortly.


Cheers.

--
Greg

> This also makes it possible to simplify code,
> will do that in a follow-up patch.
> 
> 
>  hw/virtio/virtio-pci.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index ddb5da1..a29d94f 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -303,6 +303,14 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
>              virtio_pci_stop_ioeventfd(proxy);
>          }
> 
> +        /* Linux before 2.6.34 uses the device without enabling
> +           the PCI device bus master bit. As a work-around, enable it
> +           automatically when driver detects the device. */
> +        if (val == VIRTIO_CONFIG_S_ACKNOWLEDGE) {
> +            memory_region_set_enabled(&proxy->pci_dev.bus_master_enable_region,
> +                                      true);
> +        }
> +
>          virtio_set_status(vdev, val & 0xFF);
> 
>          if (val & VIRTIO_CONFIG_S_DRIVER_OK) {