pc-bios/s390-ccw: Fix boot problem with virtio-net devices

pc-bios/s390-ccw: Fix boot problem with virtio-net devices

When we are trying to boot from virtio-net devices, the
s390-ccw bios currently leaves the virtio-net device enabled
after using it. That means that the receiving virt queues will
continue to happily write incoming network packets into memory.
This can corrupt data of the following boot process. For example,
if you set up a second guest on a virtual network and create a
lot of broadcast traffic there, e.g. with:

 ping -i 0.02 -s 1400  -b 192.168.1.255

and then you try to boot a guest with two boot devices, a network
device first (which should not be bootable) and e.g. a bootable SCSI
CD second, then this guest will fail to load the kernel from the CD
image:

 $ qemu-system-s390x -m 2G -nographic -device virtio-scsi-ccw \
    -netdev tap,id=net0 -device virtio-net-ccw,netdev=net0,bootindex=1 \
    -drive if=none,file=test.iso,format=raw,id=cd1 \
    -device scsi-cd,drive=cd1,bootindex=2
 LOADPARM=[        ]

 Network boot device detected
 Network boot starting...
   Using MAC address: 52:54:00:12:34:56
   Requesting information via DHCP: done
   Using IPv4 address: 192.168.1.76
   Using TFTP server: 192.168.1.1
 Trying pxelinux.cfg files...
   TFTP error: ICMP ERROR "port unreachable"
   Receiving data:  0 KBytes
 Repeating TFTP read request...
   TFTP error: ICMP ERROR "port unreachable"
 Failed to load OS from network.
 Failed to IPL from this network!
 LOADPARM=[        ]

 Using virtio-scsi.

 ! virtio-scsi:setup:inquiry: response VS RESP=ff !
 ERROR: No suitable device for IPL. Halting...

We really have to shut up the virtio-net devices after we're not
using it anymore. The easiest way to do this is to simply reset
the device, so let's do that now.

Reviewed-by: Jared Rossi <jrossi@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Tested-by: Jared Rossi <jrossi@linux.ibm.com>
Message-ID: <20250116115826.192047-3-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>