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

[Alexander Graf]

On 21.10.14 14:10, Michael S. Tsirkin 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 <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?

Whoever sends it first wins I'd say ;)


Alex