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

[Siwei Liu <loseweigh@gmail.com>]

On Fri, Jun 1, 2018 at 8:48 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Fri, Jun 01, 2018 at 08:26:03AM -0700, Samudrala, Sridhar wrote:
>> 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.
>
> I wonder whether we ever want to pair based solely on ID and not on MAC.
> There used to be devices which randomized VF MAC on each reset but I
> think most of them have been changed to get MAC from the PF. Not sure
> whether any are left.
>
> If we want the flexibility, we can use a separate feature bit for
> matching by ID. If not we can just extend the meaning of the
> existing one.
>
>>
>> >
>> > 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.
>>
>
> So for one bridge, we'd have a bridge with a special device/vendor ID,
> if virtio is behind that, we use that for pairing.

What does that mean? The virtio deivce needs to be in a same bridge
for pairing? I think Venu's 1-bridge proposal will be able to support
the case when virtio and passthrough devices are placed in separate
bridges.

-Siwei

>
>
>> >
>> > 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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