Re: [Qemu-devel] [PATCH] virtio-pci: Fix cross-version migration with older machines

[Stefan Hajnoczi]

On Wed, Dec 14, 2016 at 1:29 PM, Maxime Coquelin
<address@hidden> wrote:
> On 12/14/2016 02:21 PM, Cornelia Huck wrote:
>>
>> On Wed, 14 Dec 2016 15:16:13 +0200
>> "Michael S. Tsirkin" <address@hidden> wrote:
>>
>>> On Wed, Dec 14, 2016 at 02:12:10PM +0100, Cornelia Huck wrote:
>>>>
>>>> On Wed, 14 Dec 2016 13:52:37 +0100
>>>> Maxime Coquelin <address@hidden> wrote:
>>>>
>>>>> This patch fixes a cross-version migration regression introduced
>>>>> by commit d1b4259f ("virtio-bus: Plug devices after features are
>>>>> negotiated").
>>>>>
>>>>> The problem is encountered when host's vhost backend does not support
>>>>> VIRTIO_F_VERSION_1, and migration is initiated from a v2.7 or prior
>>>>> machine with virtio-pci modern capabilities enabled to a v2.8 machine.
>>>>
>>>>
>>>> Wait, machine versions or qemu versions?
>>>>
>>>>>
>>>>> In this case, modern capabilities get exposed to the guest by the
>>>>> source,
>>>>> whereas the target will detect version 1 is not supported so will only
>>>>> expose legacy capabilities.
>>>>>
>>>>> The problem is fixed by introducing a new "x-modern-broken" property,
>>>>> which is set in v2.7 and prior compatibility modes. Doing this, v2.7
>>>>> machine keeps its broken behaviour (enabling modern while version is
>>>>> not supported), and newer machines will behave correctly.
>>>>>
>>>>> Reported-by: Michael Roth <address@hidden>
>>>>> Suggested-by: Stefan Hajnoczi <address@hidden>
>>>>> Cc: Michael S. Tsirkin <address@hidden>
>>>>> Cc: Cornelia Huck <address@hidden>
>>>>> Cc: Marcel Apfelbaum <address@hidden>
>>>>> Cc: Dr. David Alan Gilbert <address@hidden>
>>>>> Signed-off-by: Maxime Coquelin <address@hidden>
>>>>> ---
>>>>>
>>>>> I'm not sure about the property name, let me know if you have better
>>>>> ideas.
>>>>> I didn't tested migration yet, but I wanted to share the patch while I
>>>>> test it.
>>>>> I tested booting v2.8 and v2.7 machines with !VERSION_1 and get
>>>>> expected result:
>>>>>  - v2.8: Virtio-pci probe succeed
>>>>>  - v2.7: Virtio-pci probe fails
>>>>>
>>>>> Thanks,
>>>>> Maxime
>>>>>
>>>>>  hw/virtio/virtio-pci.c | 4 +++-
>>>>>  hw/virtio/virtio-pci.h | 1 +
>>>>>  include/hw/compat.h    | 4 ++++
>>>>>  3 files changed, 8 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
>>>>> index 521ba0b..93f6b54 100644
>>>>> --- a/hw/virtio/virtio-pci.c
>>>>> +++ b/hw/virtio/virtio-pci.c
>>>>> @@ -1580,7 +1580,8 @@ static void virtio_pci_device_plugged(DeviceState
>>>>> *d, Error **errp)
>>>>>       * Virtio capabilities present without
>>>>>       * VIRTIO_F_VERSION_1 confuses guests
>>>>>       */
>>>>> -    if (!virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1))
>>>>> {
>>>>> +    if (!proxy->modern_broken &&
>>>>
>>>>
>>>> What you are de facto doing here is ignoring the features supported by
>>>> the backend. Call this ->ignore_backend_features or so?
>>>>
>>>>> +            !virtio_has_feature(vdev->host_features,
>>>>> VIRTIO_F_VERSION_1)) {
>>>>>          virtio_pci_disable_modern(proxy);
>>>>>
>>>>>          if (!legacy) {
>>>>> @@ -1852,6 +1853,7 @@ static Property virtio_pci_properties[] = {
>>>>>                      VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT, false),
>>>>>      DEFINE_PROP_BIT("page-per-vq", VirtIOPCIProxy, flags,
>>>>>                      VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT, false),
>>>>> +    DEFINE_PROP_BOOL("x-modern-broken", VirtIOPCIProxy, modern_broken,
>>>>> false),
>>>>
>>>>
>>>> What about "x-ignore-backend-features"?
>>>
>>>
>>> It's just the capability that ignores that backend is legacy.
>>>
>>> x-cap-ignore-backend-legacy
>>>
>>> ?
>>
>>
>> Sounds good. And has the benefit that nobody will be tempted to set
>> that manually :)
>
>
> Thanks Michael & Cornelia, you're definitely better than me at naming
> things :)
>
> I'll send the v2 using this nameing as soon as I get migration case
> tested.

Once the property is renamed...

Reviewed-by: Stefan Hajnoczi <address@hidden>