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

[Michael S. Tsirkin]

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 <address@hidden>
> >>> Tested-by: Greg Kurz <address@hidden>
> >>> Signed-off-by: Michael S. Tsirkin <address@hidden>
> >>> ---
> >>>
> >>> 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?

-- 
MST