Re: [Qemu-devel] [PATCH v5] virtio-pci: enable bus master for old guests

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 15, 2014 at 05:48:17PM +0800, Jason Wang wrote:
> On 09/12/2014 02:20 AM, Michael S. Tsirkin 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. For the same reason,
> > rhel6.[56] ppc64 guests cannot boot on a virtio-blk disk anymore.
> >
> > Old guests forgot to enable bus mastering, enable it automatically on
> > DRIVER (guests use some devices before DRIVER_OK).
> >
> > Reported-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > Sorry, v4 was broken - forgot to commit before git format-patch.
> > Here's the correct version.
> > Still not tested - will do next week.
> > Testing reports appreciated meanwhile.
> >
> >  hw/virtio/virtio-pci.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > index ddb5da1..a827cd4 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -314,6 +314,16 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
> >              msix_unuse_all_vectors(&proxy->pci_dev);
> >          }
> >  
> > +        /* Linux before 2.6.34 drives the device without enabling
> > +           the PCI device bus master bit. Enable it automatically
> > +           for the guest. This is a PCI spec violation but so is
> > +           initiating DMA with bus master bit clear. */
> > +        if (val == (VIRTIO_CONFIG_S_ACKNOWLEDGE | VIRTIO_CONFIG_S_DRIVER)) {
> > +            pci_default_write_config(&proxy->pci_dev, PCI_COMMAND,
> > +                                     proxy->pci_dev.config[PCI_COMMAND] |
> > +                                     PCI_COMMAND_MASTER, 1);
> > +        }
> > +
> >          /* 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. */
> 
> So with this patch, we can also remove the detecting and handling of
> VIRTIO_PCI_FLAG_BUS_MATER_BUG?

Yes - I sent a follow-up patch to do that.

-- 
MST