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

[Alexander Graf]

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