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

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Tue, 21 Oct 2014 15:10:22 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Oct 21, 2014 at 01:35:39PM +0200, Alexander Graf wrote:
> > 
> > 
> > On 21.10.14 12:16, Michael S. Tsirkin wrote:
> > > 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.
> > 
> > Ok, works for me then.
> > 
> > 
> > Alex
> 
> cool.
> Gerd tells me there's a conflict with your next tree:
> would you like for me to wait until it's merged,
> or go ahead and send pull request and rebase yours on top?
> 

This patch also needs to be rebased on current master actually
because PC_COMPAT_2_1 in include/hw/i386/pc.h got changed.

Cheers.

--
Greg