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

[Michael S. Tsirkin]

On Wed, Sep 10, 2014 at 02:45:51PM +0530, Nikunj A Dadhania wrote:
> "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

But device is then reset, right Greg?
You get as far as reset?

If yes I doubt something that happens before reset
matters, unless we are leaking some state
across reset which would be a bug in itself.