Use virtio-serial on newer kernels

[bmwiedemann]

Assuming that all 4.x target kernels have virtio-serial
This avoids such ugly+noisy messages in output:
serial8250: too much work for irq4

It makes diffing of build logs easier to help reproducible builds.

https://bugzilla.redhat.com/show_bug.cgi?id=986761
https://bugs.launchpad.net/qemu/+bug/1719339

[adrianschroeter]

hm, all kernels since version 4 can have virtio-serial, but there is no guarantee that it is compiled in or that the matching module is available in initrd. Not sure about this...

[bmwiedemann]

if the associated kernel config was available somewhere, we could grep there.

rpm -qf /boot/config-4.12.14-lp150.12.16-default
kernel-default-4.12.14-lp150.12.16.1.x86_64

[bmwiedemann]

or maybe we can use lsinitrd | grep virtio_console.ko ... will give that a try.

[bmwiedemann]

I tried

if xzcat $vm_initrd | cpio -t | grep -q 'kernel/drivers/char/virtio_console.ko$' ; then

but the driver is not part of the initrd. But virtio_serial still works, because the relevant work happens after initrd in the rootfs.

[adrianschroeter]

This has been implemented here now: #566