Re: RFC: Use of bridge devices to store pairing information...

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 31, 2018 at 08:28:32PM -0500, 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.

Thanks for writing this up!  I mostly agree which isn't surprising since
I proposed this scheme originally ;)
https://patchwork.ozlabs.org/patch/901596/#1901665

Some minor comments though:

> 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.

I suggest we limit this to a bridge with QEMU vendor/device ID.

We have pci-bridge and pci-bridge-seat.

This could be similar: pci-bridge-group.
Less of a chance there's a conflict.

Accordingly bridge is the group, not the id :)

> 3. QEMU places this unique identifier in the PCI configuration space of
>    the bridge as Vendor-Specific capability (0x09).

Why not use the standard UUID capability? Accordingly name property
uuid?

> 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).

So that has a small chance of conflicting with a vendor
specific capability used by some driver of this bridge.
We are better off with an actual vendor/device ID IMHO.

> 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.
> 
> 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,

I think we can use the standard PCI UUID. No need to update the spec
for this.

> 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.
> 
> 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