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

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

On Mon, 8 Sep 2014 19:05:02 +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 on DRIVER_OK.
> 
> Note: we should either back out the original patch from
> stable or apply this one on top.
> 
> Cc: qemu-stable@nongnu.org
> Reported-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  hw/virtio/virtio-pci.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index ddb5da1..af937d2 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -320,6 +320,8 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
>          if ((val & VIRTIO_CONFIG_S_DRIVER_OK) &&
>              !(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,
> +                                      true);
>          }
>          break;
>      case VIRTIO_MSI_CONFIG_VECTOR:

Cc'ing Alexey for some SLOF and early boot of the ppc64 kernel expertise.

Michael,

This was enough to fix virtio-net in the rhel6.5 x86 guest case. Unfortunately,
this fails for rhel6.5 ppc64 because it is never called... I did some debugging:
it looks like the guest kernel calls the OF quisece call to flush pending DMA
and disables bus master on the virtio-blk device (PCI_COMMAND == 0x3). The
guest then continues to boot and hangs... It appears that waiting for the
guest to issue VIRTIO_CONFIG_S_DRIVER_OK is not enough. Since we need this for
MSI to work, I tried the following and it fixes the issue:

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index af937d2..3d72aa8 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -111,9 +111,14 @@ static void virtio_pci_notify(DeviceState *d, uint16_t vector)
 {
     VirtIOPCIProxy *proxy = to_virtio_pci_proxy_fast(d);
 
-    if (msix_enabled(&proxy->pci_dev))
+    if (msix_enabled(&proxy->pci_dev)) {
+        if (!(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,
+                                      true);
+        }
         msix_notify(&proxy->pci_dev, vector);
-    else {
+    } else {
         VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
         pci_set_irq(&proxy->pci_dev, vdev->isr & 1);
     }

If this is acceptable, I'll make it a helper and squash it into your patch.

Thoughts ?

-- 
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.