FreeBSD guest on Linux KVM, some vtnet(4) packets don't arrive

Lexi Winter <[email protected]> Wed, 8 Apr 2026 15:38:40 +0100
Newsgroups	gmane.os.freebsd.devel.net
Message-ID	<[email protected]>
hello,

i'm using FreeBSD main:

FreeBSD 16.0-CURRENT #0 main-n284768-e6eba5076929: Mon Mar 30 16:07:28 UTC 2026
[email protected]:/usr/obj/usr/src/powerpc.powerpc64/sys/GENERIC64

as a qemu (KVM) guest on a Linux host:

Linux 6.18.21-0-lts #1-Alpine SMP 2026-04-07 10:34:29 ppc64le

on the Linux side, qemu is configured to provide a tap interface to the
guest:

qemu-system-ppc64 \
	-smp cpus=176,sockets=2,cores=22,threads=4 \
	-accel kvm -M pseries -m 8g -nographic -serial mon:stdio \
	-drive file=/vm/ppc64-main/root.img,format=raw,if=virtio \
	-netdev tap,id=net0,ifname=vm-ppc64-main,script=no,downscript=no \
	-device virtio-net-pci,netdev=net0

the tap interface looks fine:

10: vm-ppc64-main: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether ae:dc:d8:39:be:fa brd ff:ff:ff:ff:ff:ff
    inet6 fe80::acdc:d8ff:fe39:befa/64 scope link proto kernel_ll
       valid_lft forever preferred_lft forever
    inet6 fe80::1/64 scope link
       valid_lft forever preferred_lft forever

on the FreeBSD side, the vtnet interface also looks fine:

vtnet0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
        options=cc00ba<TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,VLAN_HWTSO,LINKSTATE,TXCSUM_IPV6,HWSTATS>
        ether 52:54:00:12:34:56
        inet6 fe80::2%vtnet0 prefixlen 64 scopeid 0x1
        media: Ethernet autoselect (10Gbase-T <full-duplex>)
        status: active
        nd6 options=821<PERFORMNUD,AUTO_LINKLOCAL,STABLEADDR>

on the host, i can ping fe80::2%vm-ppc64-main:

PING fe80::2%vm-ppc64-main (fe80::2%10): 56 data bytes
64 bytes from fe80::2: seq=0 ttl=64 time=14.833 ms
64 bytes from fe80::2: seq=1 ttl=64 time=12.524 ms

then i configured an IP address on lo0 on the guest:

lo0: flags=1008049<UP,LOOPBACK,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 16384
        options=680003<RXCSUM,TXCSUM,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
        inet 127.0.0.1 netmask 0xff000000
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2
        inet6 fd00:0:0:1::b prefixlen 128
        groups: lo
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

a route exists for this on the host:

fd00:0:0:1::b via fe80::2 dev vm-ppc64-main proto bird metric 32 pref medium

when trying to ping this address, on the host, i can see ICMP packets leaving
the tap interface:

15:34:39.106898 IP6 fd00:0:0:1::12 > fd00:0:0:1::b: ICMP6, echo request, id 49664, seq 3, length 64
15:34:40.106951 IP6 fd00:0:0:1::12 > fd00:0:0:1::b: ICMP6, echo request, id 49664, seq 4, length 64

but these packets never arrive on the guest; tcpdump shows nothing.

however, 'netstat -in' shows that the packets *are* arriving on the guest,
since Ipkts increments for each ICMP packet.  it seems like they're just
getting lost somewhere in the network stack. (Ierrs/Oerrs is zero.)

does anyone have any ideas what might be causing this?
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