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Subject: Re: [Qemu-devel] [PATCH v3 0/3] Use of unique identifier for pairing virtio and passthrough devices...

On 7/3/2018 2:58 AM, Roman Kagan wrote:
On Mon, Jul 02, 2018 at 02:14:52PM -0700, si-wei liu wrote:
On 7/2/2018 9:14 AM, Roman Kagan wrote:
On Fri, Jun 29, 2018 at 05:19:03PM -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

2. Enhance the QEMU CLI to include a "failover-group-id" option to the
     virtio device. The "failover-group-id" is a 64 bit value.

3. Enhance the QEMU CLI to include a "failover-group-id" option to the
     Red Hat PCI bridge device (support for the i440FX model).

4. Add a new "pcie-downstream" device, with the option
     "failover-group-id" (support for the Q35 model).

5. The operator creates a 64 bit unique identifier, failover-group-id.

6. When the virtio device is created, the operator uses the
     "failover-group-id" option (for example, '-device
     virtio-net-pci,failover-group-id=<identifier>') and specifies the
     failover-group-id created in step 4.

     QEMU stores the failover-group-id in the virtio device's configuration
     space in the capability "VIRTIO_PCI_CAP_GROUP_ID_CFG".

7. When assigning a PCI device to the guest in passthrough mode, the
     operator first creates a bridge using the "failover-group-id" option
     (for example, '-device pcie-downstream,failover-group-id=<identifier>')
     to specify the failover-group-id created in step 4, and then attaches
     the passthrough device to the bridge.

     QEMU stores the failover-group-id 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.

8. 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.
I'm still missing a few bits in the overall scheme.

Is the guest supposed to acknowledge the support for PT-PV failover?
Yes. We are leveraging virtio's feature negotiation mechanism for that.
Guest which does not acknowledge the support will not have PT plugged in.

Should the PT device be visibile to the guest before it acknowledges the
support for failover?
No. QEMU will only expose PT device after guest acknowledges the support
through virtio's feature negotiation.

    How is this supposed to work with legacy guests that don't support it?
Only PV device will be exposed on legacy guest.
So how is this coordination going to work?  One possibility is that the
PV device emits a QMP event upon the guest driver confirming the support
for failover, the management layer intercepts the event and performs
device_add of the PT device.  Another is that the PT device is added
from the very beginning (e.g. on the QEMU command line) but its parent
PCI bridge subscribes a callback with the PV device to "activate" the PT
device upon negotiating the failover feature.

I think this needs to be decided within the scope of this patchset.
As what had been discussed in previous thread below, we would go with the approach that QEMU manages the visibility of the PT device automatically. Management layer supplies PT device to QEMU from the very beginning. This PT device won't be exposed to guest immediately, unless or until the guest virtio driver acknowledges the backup feature already. Once virtio driver in the guest initiates a device reset, the corresponding PT device must be taken out from guest. Then add it back later on after guest virtio completes negotiation for the backup feature.


Is the guest supposed to signal the datapath switch to the host?
No, guest doesn't need to be initiating datapath switch at all.
What happens if the guest supports failover in its PV driver, but lacks
the driver for the PT device?
The assumption of failover driver is that the primary (PT device) will be able to get a datapath once it shows up in the guest . If adding a PT device to an unsupported guest, the result will be same as that without a standby PV driver - basically got no networking as you don't get a working driver.

Then perhaps don't add the PT device in the first place if guest lacks driver support?

However, QMP
events may be generated when exposing or hiding the PT device through hot
plug/unplug to facilitate host to switch datapath.
The PT device hot plug/unplug are initiated by the host, aren't they?  Why
would it also need QMP events for them?
As indicated above, the hot plug/unplug are initiated by QEMU not the management layer. Hence the QMP hot plug event is used as an indicator to switch host datapath. Unlike Windows Hyper-V SR-IOV driver model, the Linux host network stack does not offer a fine grained PF driver API to move MAC/VLAN filter, and the VF driver has to start with some initial MAC address filter programmed in when present in the guest. The QMP event is served as a checkpoint to switch MAC filter and/or VLAN filter between the PV and the VF.

Is the scheme going to be applied/extended to other transports (vmbus,
virtio-ccw, etc.)?
Well, it depends on the use case, and how feasible it can be extended to
other transport due to constraints and transport specifics.

Is the failover group concept going to be used beyond PT-PV network
device failover?
Although the concept of failover group is generic, the implementation itself
may vary.
My point with these two questions is that since this patchset is
defining external interfaces -- with guest OS, with management layer --
which are not easy to change later, it might make sense to try and see
if the interfaces map to other usecases.  E.g. I think we can get enough
information on how Hyper-V handles PT-PV network device failover from
the current Linux implementation; it may be a good idea to share some
concepts and workflows with virtio-pci.
As you may see from above, the handshake of virtio failover depends on hot plug (PCI or ACPI) and virtio specifics (feature negotiation). So far as I see the Hyper-V uses a completely different handshake protocol of its own (guest initiated datapath switch, Serial number in VMBus PCI bridge) than that of virtio. I can barely imagine how code could be implemented in a shared manner, although I agree conceptually failover group between these two is similar or the same.



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