Hotconfig is not supported with DPDK

[abooteg]

Hi,

I’ve encountered an issue where I’m unable to use hotconfig with DPDK.
Each time I try to load a new configuration, I get an error:
520-Write handler 'hotconfig' error:
520-config:71: While initializing 'FromDPDKDevice@16 :: FromDPDKDevice':
520- Trying to configure DPDK device after initialization
520 Router could not be initialized!

After reviewing the code, I found that fastclick does not support hotconfig in /elements/userlevel/fromdpdkdevice.hh:
bool can_live_reconfigure() const override { return false; }

Is there any way to fix or work around this limitation?

[tbarbette]

The rewrite to allow this is more complicated than anticipated. The FromDPDKElement negotiate queues and threading at configuration time. The way hotswap works is to launch the configuration, initialize the new element, and then enable a special path to "take the state" of the old element.
Here, the initialization clashes because you have two element opening the same device at the same time. We might delay the activation until the previous one is open but that requires more work.

[abooteg]

Is it possible to do this without any traffic loss? So that packets aren’t dropped at the moment FromDPDKElement is restarted.

[tbarbette]

Yes, I mean if you don't change any parameter this is just internal mumbo jumbo. Now if you do changes like the number of queues, that depends on the NIC.Changing RSS with Mellanox leads to packet drops for instance. Changing the MTU, probably not. But if the goal is just to change the rest of the router but the old FromDPDKDevice is the same, then for sure we can avoid packet losses. We just need the new FromDPDKDevice to understand it should take a running device instead of configuring a new one. Currently Click is not built like that. Tom Barbette

…

On Fri, Oct 10, 2025 at 12:30 PM Aleksey Shcherbakov < ***@***.***> wrote: *abooteg* left a comment (tbarbette/fastclick#429) <#429 (comment)> Is it possible to do this without any traffic loss? So that packets aren’t dropped at the moment FromDPDKElement is restarted. — Reply to this email directly, view it on GitHub <#429 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAB4ZAPR342MAKZJRHXOUAL3W6DFRAVCNFSM6AAAAACIXY5ULOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTGOBZGI3DSMRRGQ> . You are receiving this because you commented.Message ID: ***@***.***>

[abooteg]

I understand. I don’t want to change the NIC parameters — I plan to set those as variables at startup.
My goal with hotconfig is to modify the rest of the router without making any changes to FromDPDKDevice.