Re: [virtio-dev] Re: [PATCH v2 0/4] Use of unique identifier for pairing virtio and passthrough devices...

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 27 Jun 2018 07:06:42 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Jun 26, 2018 at 10:49:30PM -0500, Venu Busireddy wrote:
> > The patch set "Enable virtio_net to act as a standby for a passthru
> > device" [1] deals with live migration of guests that use passthrough
> > devices. However, that scheme uses the MAC address for pairing
> > the virtio device and the passthrough device. The thread "netvsc:
> > refactor notifier/event handling code to use the failover framework"
> > [2] discusses an alternate mechanism, such as using an UUID, for pairing
> > the devices. Based on that discussion, proposals "Add "Group Identifier"
> > to virtio PCI capabilities." [3] and "RFC: Use of bridge devices to
> > store pairing information..." [4] were made.
> > 
> > The current patch set includes all the feedback received for proposals [3]
> > and [4]. For the sake of completeness, patch for the virtio specification
> > is also included here. Following is the updated proposal.
> > 
> > 1. Extend the virtio specification to include a new virtio PCI capability
> >    "VIRTIO_PCI_CAP_GROUP_ID_CFG".  
> 
> There's still discussion around whether it should be
> a virtio pci capability, a virtio net config field or
> a new kind of capability.

Moreover, I hate to be that person again, but...

> 
> > 2. Enhance the QEMU CLI to include a "uuid" option to the virtio device.
> >    The "uuid" is a string in UUID format.
> > 
> > 3. Enhance the QEMU CLI to include a "uuid" option to the bridge device.
> >    The "uuid" is a string in UUID format. Currently, PCIe bridge for
> >    the Q35 model is supported.
> > 
> > 4. The operator creates a unique identifier string using 'uuidgen'.
> > 
> > 5. When the virtio device is created, the operator uses the "uuid" option
> >    (for example, '-device virtio-net-pci,uuid="string"') and specifies
> >    the UUID created in step 4.
> > 
> >    QEMU stores the UUID in the virtio device's configuration space
> >    in the capability "VIRTIO_PCI_CAP_GROUP_ID_CFG".
> > 
> > 6. When assigning a PCI device to the guest in passthrough mode, the
> >    operator first creates a bridge using the "uuid" option (for example,
> >    '-device pcie-downstream,uuid="string"') to specify the UUID created
> >    in step 4, and then attaches the passthrough device to the bridge.
> > 
> >    QEMU stores the UUID in the configuration space of the bridge as
> >    Vendor-Specific capability (0x09). The "Vendor" here is not to be
> >    confused with a specific organization. Instead, the vendor of the
> >    bridge is QEMU. To avoid mixing up with other bridges, the bridge
> >    will be created with vendor ID 0x1b36 (PCI_VENDOR_ID_REDHAT) and
> >    device ID 0x000e (PCI_DEVICE_ID_REDHAT_PCIE_BRIDGE) if the "uuid"
> >    option is specified. Otherwise, current defaults are used.

...if the bridge is the means of getting the uuid to the guest for the
passthrough device, I don't see how this can work on s390. Unless I'm
mistaken, we don't see the pci bridge created in QEMU from the guest
(zPCI uses instructions to list functions, and does not have any
topology view). Adding some folks who have worked on zPCI for their
comments.

> > 
> > 7. Patch 4 in patch series "Enable virtio_net to act as a standby for
> >    a passthru device" [1] needs to be modified to use the UUID values
> >    present in the bridge's configuration space and the virtio device's
> >    configuration space instead of the MAC address for pairing the devices.
> > 
> > Thanks!
> > 
> > Venu  
> 
> The part where the visibility of a vfio device is controlled by the
> virtio driver acknowledging the backup feature is missing here.

Also, shouldn't libvirt be involved at some point in time?

>  
> 
> > [1] https://lists.oasis-open.org/archives/virtio-dev/201805/msg00156.html
> > [2] https://www.spinics.net/lists/netdev/msg499011.html
> > [3] https://lists.oasis-open.org/archives/virtio-dev/201805/msg00118.html
> > [4] https://lists.oasis-open.org/archives/virtio-dev/201805/msg00204.html
> > 
> > Changes in v2:
> >   - As Michael Tsirkin suggested, changed the virtio specification
> >     to restrict the group identifier to be a 16-byte field, presented
> >     entirely in the virtio device's configuration space.
> >   - As Michael Tsirkin suggested, instead of tweaking the ioh3420
> >     device with Red Hat vendor ID, create a new PCIe bridge device
> >     named "pcie-downstream" with Red Hat Vendor ID, and include the
> >     group identifier in this device.
> >   - Added a new patch to enhance the "pci-bridge" device to support
> >     the group identifier (for the i440FX model).
> > 
> > Venu Busireddy (4):
> >   Add a true or false option to the DEFINE_PROP_UUID macro.
> >   Add "Group Identifier" support to virtio devices.
> >   Add "Group Identifier" support to Red Hat PCI bridge.
> >   Add "Group Identifier" support to Red Hat PCI Express bridge.
> > 
> >  default-configs/arm-softmmu.mak             |   1 +
> >  default-configs/i386-softmmu.mak            |   1 +
> >  default-configs/x86_64-softmmu.mak          |   1 +
> >  hw/acpi/vmgenid.c                           |   2 +-
> >  hw/pci-bridge/Makefile.objs                 |   1 +
> >  hw/pci-bridge/pci_bridge_dev.c              |   8 +
> >  hw/pci-bridge/pcie_downstream.c             | 215 ++++++++++++++++++++
> >  hw/pci-bridge/pcie_downstream.h             |  10 +
> >  hw/pci/pci_bridge.c                         |  26 +++
> >  hw/virtio/virtio-pci.c                      |  15 ++
> >  hw/virtio/virtio-pci.h                      |   3 +-
> >  include/hw/pci/pci.h                        |   3 +
> >  include/hw/pci/pcie.h                       |   1 +
> >  include/hw/qdev-properties.h                |   4 +-
> >  include/standard-headers/linux/virtio_pci.h |   8 +
> >  15 files changed, 295 insertions(+), 4 deletions(-)
> >  create mode 100644 hw/pci-bridge/pcie_downstream.c
> >  create mode 100644 hw/pci-bridge/pcie_downstream.h  
> 
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