Re: [Qemu-devel] [PATCH v3] virtio-pci: fix migration for pci bus master

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

On Mon, Oct 20, 2014 at 06:08:56PM +0200, Alexander Graf wrote:
> 
> 
> On 20.10.14 08:58, Michael S. Tsirkin wrote:
> > Current support for bus master (clearing OK bit) together with the need to
> > support guests which do not enable PCI bus mastering, leads to extra state in
> > VIRTIO_PCI_FLAG_BUS_MASTER_BUG bit, which isn't robust in case of cross-version
> > migration for the case when guests use the device before setting DRIVER_OK.
> > 
> > Rip out this code, and replace it:
> > -   Modern QEMU doesn't need VIRTIO_PCI_FLAG_BUS_MASTER_BUG
> >     so just drop it for latest machine type.
> > -   For compat machine types, set PCI_COMMAND if DRIVER_OK
> >     is set.
> > 
> > As this is needed for 2.1 for both pc and ppc, move PC_COMPAT macros from pc.h
> > to a new common header.
> > 
> > Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > 
> > Alexander, could you pls ack me merging this?
> > Thanks!
> 
> Have you tried whether this works with old kernel versions? We
> introduced the broken flag for very specific old kernel versions that
> got their flags wrong IIRC.
> 
> 
> Alex

Yes. The point is that since 2.1 the broken flag isn't effective
anymore: when bus master is off, PCI blocks outgoing transactions and
setting internal flags isn't going to help.

Instead, we detect a buggy guest and immediately enable bus master for
it - no need for a separate flag.



-- 
MST