[Qemu-devel] tap networking - how?

[Alexey Kardashevskiy <aik@ozlabs.ru>]

Hi!

I am debugging spapr-vlan and hit the following issue.

When I run QEMU as below, the kernel's DHCP client does not continue till I
hit any key in console. If I replace spapr-vlan with
e1000/rtl8139/virtio-net, everything is just fine. If I use "user" network
- everything is fine too. So the problem is with combination of spapr-vlan
+ tap.

The issue looks like - the guest kernel boots and then prints:
Sending DHCP requests ..
and it keeps printing dots till I press key or timeout expires. tcpdump
(running on the tap interface) shows one DHCP request and one DHCP response.

What normally happens is that QEMU calls os_host_main_loop_wait() which
calls qemu_poll_ns() and it is sitting there till eventfd signals.
This eventfd is registered via qemu_init_main_loop() -> aio_context_new()
-> aio_set_event_notifier() but I cannot find where it gets passed to the
kernel (otherwise why would we need eventfd?).  When eventfd signals, QEMU
calls qemu_iohandler_poll() which checks if TAP device has something to
read and eventually calls tap_send().

However in my bad example QEMU does not exit qemu_poll_ns() on eventfd,
only on stdin event.

I can see AIO eventfd created and event_notifier_test_and_clear() is called
on it before the kernel starts using spapr-vlan.

So. h_send_logical_lan() is called to sent a DHCP request packet. Now I
expect eventfd to signal but this does not happen. Have I missed some reset
or notification request or "bottom half" (virtio-net uses them but
e1000/rtl8139 do not)?


Please, help. Thanks!


./qemu-system-ppc64 \
	-enable-kvm \
	-m 2048 \
	-L qemu-ppc64-bios/ \
	-machine pseries \
	-trace events=qemu_trace_events \
	-nographic \
	-vga none \
	-netdev tap,id=id0,ifname=tap-id0,script=ifup.sh,downscript=ifdown.sh \
	-device spapr-vlan,id=id1,netdev=id0,mac=C0:41:49:4b:00:00 \
	-kernel vml313 \
	-append "root=/dev/nfs ip=dhcp selinux=0
nfsroot=10.61.145.11:/scratch/alexey/fc19nfs_/"




-- 
Alexey