Re: [virtio-dev] RFC: Use of bridge devices to store pairing information...

["Samudrala, Sridhar" <sridhar.samudrala@intel.com>]

On 5/31/2018 6:28 PM, Venu Busireddy wrote:
> I looked at the discussion in the threads [1] and [2], where it was
> suggested placing the passthrough device behind one bridge, and the virtio
> device behind another bridge, and storing in those bridges' configuration
> space some unique identifier that can be used to pair the two devices.
>
> After some discussions with Si-Wei Liu and others, we believe that the
> following scheme may be a viable approach. Please take a look at this
> proposal and provide your thoughts.
>
> 1. Enhance the QEMU CLI to include a "group_id" option to the bridge
>     devices for Q35 as well as i440FX models. I have already made changes
>     for the Q35 model (ioh3420 bridge).
>
> 2. When the guest is created, the operator creates two bridge devices
>     (for example, using '-device ioh3420,group_id="string"'), and specifies
>     a unique identifier string for both bridges. This identifier can be
>     the UUID generated by 'uuidgen' command.
>
> 3. QEMU places this unique identifier in the PCI 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 (with vendor ID 0x8086 and device ID 0x3420).
>
> 4. The operator places the passthrough device behind one of the bridges,
>     and the virtio device behind the other bridge.
>
> 5. Patch 4 in patch series [3] should be modified to use the unique
>     identifier string stored in the bridges' configuration space instead
>     of the MAC address for pairing the devices.

This should be an alternate option that allows failover slaves to be registered
based on MAC and ID.


> If it is desirable to create only one bridge instead of two (to conserve
> the  number of devices in the system), then the passthrough device can be
> attached to that single bridge (with the identifier), and the identifier
> for the virtio device can be stored in the virtio device's configuration
> space itself. To do that, we need to update the virtio specification,
> and I have sent a proposal [4] to the OASIS team to update the virtio
> specification. If that proposal is accepted, then we can modify QEMU to
> use the virtio device's configuration space instead of the second bridge
> to store the unique identifier.

I think one bridge solution is much cleaner than having to use 2 bridges.


> Thank you for sparing the time.
>
> Venu
>
> [1] https://www.spinics.net/lists/linux-virtualization/msg33518.html
> [2] https://www.spinics.net/lists/netdev/msg499011.html
> [3] https://patchwork.ozlabs.org/cover/920005/
> [4] https://lists.oasis-open.org/archives/virtio-dev/201805/msg00118.html
>
>
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