Re: [Qemu-devel] [PATCH 2/2] virtio-pci: Disable modern interface if backend without VIRTIO_F_VERSION_1

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 09/09/2016 01:20 PM, Cornelia Huck wrote:
> On Fri, 9 Sep 2016 14:04:55 +0300
> Marcel Apfelbaum <marcel@redhat.com> wrote:
>
>> On 09/09/2016 01:40 PM, Cornelia Huck wrote:
>>> On Fri,  9 Sep 2016 12:14:32 +0200
>>> Maxime Coquelin <maxime.coquelin@redhat.com> wrote:
>>>
>>>> This patch makes pci devices plugging more robust, by not confusing
>>>> guest with modern interface when the backend doesn't support
>>>> VIRTIO_F_VERSION_1.
>>>>
>>>> Cc: Marcel Apfelbaum <marcel@redhat.com>
>>>> Cc: Michael S. Tsirkin <mst@redhat.com>
>>>> Cc: qemu-stable@nongnu.org
>>>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>>>> ---
>>>>  hw/virtio/virtio-pci.c | 15 +++++++++++++++
>>>>  hw/virtio/virtio-pci.h |  5 +++++
>>>>  2 files changed, 20 insertions(+)
>>>
>>> Note that 11380b361 ("virtio: handle non-virtio-1-capable backend for
>>> ccw") fixes this issue for ccw via the introduction of a
>>> ->post_plugged() callback. Unfortunately, we did not find a good way to
>>> make it work for pci back then.
>>
>> It seems that for ccw is enough to rewind dev->rev_max,
>> sadly for pci we need to rewind a lot of settings/resources.
>
> Yes, that what I meant with 'more flexibility for ccw'.
Maybe we could replace post_plugged with a pre_plugged approach?

In ->pre_plugged(), cww and pci would specify which features it can
support using virtio_add_feature().
Then we could call get_features() before ->device_plugged().

Doing this, both ccw and pci would have the needed information without
having to rewind any settings.

Does that make sense?

But for now, I think it would be better to merge something in the spirit
of this series (taking into account to remarks).
Indeed, I think we want this fixed in stable, but the above proposal
would be too huge for stable.

Thanks,
Maxime