On Wed, Dec 14, 2016 at 01:52:37PM +0100, Maxime Coquelin 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.
>
> 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 <mdr...@linux.vnet.ibm.com>
> Suggested-by: Stefan Hajnoczi <stefa...@redhat.com>
> Cc: Michael S. Tsirkin <m...@redhat.com>
> Cc: Cornelia Huck <cornelia.h...@de.ibm.com>
> Cc: Marcel Apfelbaum <mar...@redhat.com>
> Cc: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
I wouldn't call it modern broken though. Modern works fine.
What this does is allow modern capability in legacy mode.
x-virtio-legacy-with-modern-cap
?
> ---
>
> 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 &&
> + !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),
> DEFINE_PROP_END_OF_LIST(),
> };
>
> diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
> index b2a996f..1dca223 100644
> --- a/hw/virtio/virtio-pci.h
> +++ b/hw/virtio/virtio-pci.h
> @@ -153,6 +153,7 @@ struct VirtIOPCIProxy {
> int config_cap;
> uint32_t flags;
> bool disable_modern;
> + bool modern_broken;
> OnOffAuto disable_legacy;
> uint32_t class_code;
> uint32_t nvectors;
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 0f06e11..fe11723 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -18,6 +18,10 @@
> .driver = "intel-iommu",\
> .property = "x-buggy-eim",\
> .value = "true",\
> + },{\
> + .driver = "virtio-pci",\
> + .property = "x-modern-broken",\
> + .value = "on",\
> },
>
> #define HW_COMPAT_2_6 \
> --
> 2.9.3
