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

[Amos Kong]

On Mon, Sep 15, 2014 at 12:59:44PM +0300, Michael S. Tsirkin wrote:
> 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 <address@hidden>
> > > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > > ---
> > > 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.

I will check if this patch fixed this bug (it seems WIN8r2 doesn't set
BUS_MASTER bit correctly): https://bugzilla.redhat.com/show_bug.cgi?id=1090237

-- 
                        Amos.