On 2018-06-27 23:12:12 +0300, Michael S. Tsirkin wrote: > On Wed, Jun 27, 2018 at 02:59:01PM -0500, Venu Busireddy wrote: > > On 2018-06-27 22:47:05 +0300, Michael S. Tsirkin wrote: > > > On Wed, Jun 27, 2018 at 02:29:58PM -0500, Venu Busireddy wrote: > > > > On 2018-06-27 15:24:58 +0300, Roman Kagan 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. > > > > > > > > > > I must have missed something in those threads, but where does this > > > > > UUID > > > > > thing come about? AFAICS this identifier doesn't need to be > > > > > "universally" unique, nor persistent; it only has to be unique across > > > > > the VM and stable throughout the VM lifetime. > > > > > > > > The notion of using UUID came up in the thread > > > > > > > > https://www.spinics.net/lists/netdev/msg499011.html > > > > > > That's probably because it was expected one of standard serial number > > > capabilities > > > (VPD or PCI Express serial #) will be used, which are expected to be > > > unique. > > > > > > If you are rolling your own vendor specific one, it's just an ID and > > > does not have to be unique. > > > > > > > > FWIW Hyper-V uses a 32-bit integer for this purpose, not a UUID as > > > > > seems > > > > > to be implied in the thread you refer to. > > > > > > > > Yes, Hyper-V uses a serial number (but I think it is 64-bit value). > > > > However, what we are doing is similar to that. Instead of 32 bits, > > > > we are using 128 bits. > > > > > > That's OK. The name is confusing though. It's a failover group id, > > > not a UUID. > > > > Sure, we can name it whatever we want. I can change it to > > "failover-group-id", if that is what we want to call it. > > > > But what is more important is, what is represented by that name? I thought > > we were going to use UUID. The QEMU command line changes in this patch > > set expect the user to specify an UUID as the value for this option > > (whatever we name it). Are we still in agreement about that, or do you > > propose something else to be used? If so, what is it? A 32-bit number, a > > 64-bit number, or an arbitrary string? > > > > Regards, > > > > Venu > > If we don't really need a UUID, I'd avoid that requirement.
I don't see the need for a 128-bit UUID. I just took that approach because UUID was mentioned in "https://www.spinics.net/lists/netdev/msg499011.html";. Since it is unlikely to have more than 4 billion devices in the system, a 32-bit value would be more than enough to uniquely identify devices! I am looking for direction from you :-). Roman already opined that UUID may be an overkill. It appears that you too are leaning that way. Would it be acceptable if I change the group identifier ("failover-group-id") to a 32-bit value? If you concur, I will start reworking my patch. Could you please confirm? Thanks, Venu > > > > > > > > > > > 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". > > > > > > > > > > > > 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. > > > > > > > > > > I wonder if it makes more sense to drop the concept of failover > > > > > groups, > > > > > and just refer to the standby device by device-id, like > > > > > > > > > > -device virtio-net-pci,id=foo \ > > > > > -device pcie-downstream,failover=foo > > > > > > > > Isn't this the same as what this patch series proposes? In your > > > > suggestion, "foo" is the entity that connects the passthrough device > > > > and the failover device. In this patch set, that "foo" is the UUID, > > > > and the options "id" and "failover" are replaced by "uuid". Do you > > > > agree? > > > > > > > > Regards, > > > > > > > > Venu > > > > > > > > > The bridge device will then lookup the failover device, figure out the > > > > > common identifier to expose to the guest, and defer the visibility of > > > > > the PT device behind the bridge until the guest acknowledged the > > > > > support > > > > > for failover on the PV device. > > > > > > > > > > Roman.
