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

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

On Mon, 20 Oct 2014 18:08:56 +0200
Alexander Graf <agraf@suse.de> 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.
> 

This is a follow-up to the following commit:

commit e43c0b2ea5574efb0bedebf6a7d05916eefeba52
Author: Michael S. Tsirkin <mst@redhat.com>
Date:   Thu Sep 11 18:45:33 2014 +0200

    virtio-pci: enable bus master for old guests

The goal was to support *again* buggy guests that do DMA without
enabling bus master, since QEMU now uses the bus master address
space for delivering MSI/MSI-x messages.

We were missing some bits to support cross version migration.
Thanks to Michael's patch, I could migrate a buggy RHEL 6.5 guest
from QEMU v2.1 to QEMU master.

> 
> Alex
> 

Cheers.

--
Greg