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

[Michael S. Tsirkin]

On Wed, Sep 10, 2014 at 11:01:48AM +0200, Greg Kurz wrote:
> On Wed, 10 Sep 2014 12:32:30 +0300
> "Michael S. Tsirkin" <address@hidden> wrote:
> 
> > 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.
> > 
> 
> FWIW during my debug session, I see that SLOF enables bus mastering...
> unfortunately, it gets disabled at some point after the guest kernel
> is started (around the ppc64 prom_init() call).


OK I'm not sure I have all the details but does the patch I sent help
you?

-- 
MST