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

[Nikunj A Dadhania]

"Michael S. Tsirkin" <address@hidden> writes:

> On Wed, Sep 10, 2014 at 01:44:49PM +0530, Nikunj A Dadhania wrote:
>> Greg Kurz <address@hidden> writes:
>> 
>> > On Mon, 8 Sep 2014 19:05:02 +0300
>> > "Michael S. Tsirkin" <address@hidden> 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: address@hidden
>> >> Reported-by: Greg Kurz <address@hidden>
>> >> Signed-off-by: Jan Kiszka <address@hidden>
>> >> Signed-off-by: Michael S. Tsirkin <address@hidden>
>> >> ---
>> >>  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).
>> 
>> Getting confused, above you are talking about virtio-net and here it is
>> virtio-blk.
>> 
>> Anyways, the routines still remains same for both of them.  From SLOF
>> during init we set DRIVER_OK, and after using the device during the
>> quiesce, called from linux kernel VIRTIO_CONFIG_S_FAILED is set and then
>> a VIRTIO_DEVICE_RESET is done.
>
> BTW, you really should start enabling bus mastering, avoid relying
> on the work-around we have for broken guests.

In SLOF, we do enable PCI MASTER during device scanning and then later
disable it.

Regards
Nikunj