Use of the 'virtio-user' exception path interface

[Patrick Mahan <mahan@mahan.org>]

This is on yocto-linux 4.19.87 on X86_64.  DPDK (stable) 18.11.11

I am working on a project where I am trying to make use of the 'virtio-user' 
exception path and it (sort of) seems to be working.  The issue I am trying to 
understand is the following:

Background -

Per the how-to documentation 
(https://doc.dpdk.org/guides-25.07/howto/virtio_user_as_exception_path.html), I 
have called rte_eal_init() passing in the PCI address of my physical interfaces. 
I am also creating a couple of these "exception" interfaces with the following code -

     char portname[32], portargs[256];
     char macaddr[6] = { 0x00, 0x0c, 0x1b, 0x29, 0x29, 0x71 };
     uint16_t portid;

     /**
      * Create the first 'iflan0'
      */
     snprintf(portargs, 256, 
"path=/dev/vhost-net,queues=2,queue_size=1024,iface=iflan0,mac=%02x:%02x:%02x:%02x:%02x:%02x",
macaddr[0], macaddr[1], macaddr[2], macaddr[3], macaddr[4], macaddr[5]);

     /**
      * Call the hotplug layer
      */
     if (rte_eal_hotplug_add("vdev", "virt-user0", portargs) < 0) {
         fprintf(stderr, "[DPDK::hotplug] failed to create iflan0 (virtio-user0)\n");
         return -1;
     }

     /**
      * Create the second 'ifwan0'
      */
     macaddr[5]++;
     snprintf(portargs, 256, 
"path=/dev/vhost-net,queues=2,queue_size=1024,iface=ifwan0,mac=%02x:%02x:%02x:%02x:%02x:%02x",
macaddr[0], macaddr[1], macaddr[2], macaddr[3], macaddr[4], macaddr[5]);

     /**
      * Call the hotplug layer
      */
     if (rte_eal_hotplug_add("vdev", "virt-user1", portargs) < 0) {
         fprintf(stderr, "[DPDK::hotplug] failed to create iflan0 (virtio-user0)\n");
         return -1;
     }

     ...

     RTE_ETH_FOREACH_DEV(portid) {
         if (port_init(portid, bufPool, NRXQUEUES, NTXQUEUES) < 0) {
             fprintf(stderr, "[DPDK::port_init] failed to init DPDK port %u",
                     portid);
             return -1;
         }
     }

     ...

     /* Retrieve the ifindex of iflan0 and ifwan0 */
     unsigned int iflan0_index = if_nametoindex("iflan0");
     unsigned int ifwan0_index = if_nametoindex("ifwan0");

     fprintf(stdout, "[DPDK] Exception interface 'iflan0': %u", iflan0_index);
     fprintf(stdout, "[DPDK] Exception interface 'ifwan0': %u", ifwan0_index);

     ... /* onto the packet processing loops */

The interfaces are instantiated and those two logs messages are seen -

. . .
[DPDK] Exception interface 'iflan0': 58
[DPDK] Exception interface 'ifwan0': 59
. . .

However, when I check the list of kernel interfaces using 'ip link show', I find:

pmahan-dpdk-v1: # ip link show
. . .
66: iflan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT 
group default qlen 1000
     link/ether 00:0c:1b:29:29:71 brd ff:ff:ff:ff:ff:ff
67: ifwan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT 
group default qlen 1000
     link/ether 00:0c:1b:29:29:72 brd ff:ff:ff:ff:ff:ff

So, some point after I created and started these ports and obtain the original 
ifindexes, they were deleted and re-created.  I have some clue from the IKE 
kernel log messages:
...
Sep 27 23:30:22 2025 pmahan-dpdk-v1 IKE: 11[KNL] interface iflan0 activated
Sep 27 23:30:22 2025 pmahan-dpdk-v1 IKE: 09[KNL] interface ifwan0 activated
...
Sep 27 23:30:22 2025 pmahan-dpdk-v1 IKE: 09[KNL] interface iflan0 deactivated
Sep 27 23:30:22 2025 pmahan-dpdk-v1 IKE: 13[KNL] interface iflan0 deleted
Sep 27 23:30:22 2025 pmahan-dpdk-v1 IKE: 12[KNL] interface ifwan0 deactivated
Sep 27 23:30:22 2025 pmahan-dpdk-v1 IKE: 05[KNL] interface ifwan0 deleted

I never see another IKE log about those interfaces being activated again 
(possibly because they show as DOWN).

Short of crawling through the guts of the virtio-user support in vdev, I thought 
I would start asking questions here.

Also, is there any specific logging I can enabled to see if I can understand why 
this is happening?

Thanks for any help,

Patrick