virsh: VM console does not show any output

Question

I have a VM which I can start with virsh. According to virsh dumpxml VM1, this is the allocated pty:

<serial type='pty'>
  <source path='/dev/pts/6'/>
  <target port='0'/>
  <alias name='serial0'/>
</serial>
<console type='pty' tty='/dev/pts/6'>
  <source path='/dev/pts/6'/>
  <target type='serial' port='0'/>
  <alias name='serial0'/>
</console>

The VM is running:

# virsh list
 Id Name                 State
----------------------------------
  7 VM1                  running

Inside the VM, this is the grub configuration:

kernel          /boot/vmlinuz-2.6.24-28-virtual root=UUID=7a1685b9-ecc8-4b70-932c-459a6faac07d ro quiet splash console=tty0 console=ttyS0,9600n8

And this is the command line launched by virsh to start the VM:

/usr/bin/kvm -S -M pc-0.12 -enable-kvm -m 256 -smp 1,sockets=1,cores=1,threads=1 -name VM1 -uuid 47ff6ec2-a748-4738-16b9-2ffe5780e456 -nodefaults -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/VM1.monitor,server,nowait -mon chardev=monitor,mode=readline -rtc base=utc -boot c -drive file=/var/VMs/VM1.qcow2,if=none,id=drive-ide0-0-0,boot=on,format=raw -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -device virtio-net-pci,vlan=0,id=net0,mac=52:54:00:12:34:50,bus=pci.0,addr=0x3 -net tap,fd=64,vlan=0,name=hostnet0 -chardev pty,id=serial0 -device isa-serial,chardev=serial0 -usb -vnc 127.0.0.1:0 -k de -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4

But if I connect with virsh console VM1, or I do cat /dev/pts/6, nothing is shown in the VM console.

Is there something else that I must consider?

UPDATE

I have two VMs, one started directly with kvm, the other via virsh. The one started directly has a working console. I have verified the open files in both cases:

root@host1:~# lsof | grep 25093 | grep dev
kvm       25093         root  DEL       REG                0,4                3758780 /dev/zero
kvm       25093         root  DEL       REG                0,4                3758779 /dev/zero
kvm       25093         root  DEL       REG                0,4                3758777 /dev/zero
kvm       25093         root    0u      CHR              136,3         0t0          6 /dev/pts/3
kvm       25093         root    1u      CHR              136,3         0t0          6 /dev/pts/3
kvm       25093         root    2u      CHR              136,3         0t0          6 /dev/pts/3
kvm       25093         root    3u      CHR             10,232         0t0       8025 /dev/kvm
kvm       25093         root    7u      CHR             10,200         0t0       4983 /dev/net/tun
root@host1:~# lsof | grep 8341 | grep dev
kvm        8341 libvirt-qemu  DEL       REG                0,4                9743486 /dev/zero
kvm        8341 libvirt-qemu  DEL       REG                0,4                9743485 /dev/zero
kvm        8341 libvirt-qemu  DEL       REG                0,4                9743483 /dev/zero
kvm        8341 libvirt-qemu    0r      CHR                1,3         0t0       4640 /dev/null
kvm        8341 libvirt-qemu    4u      CHR                5,2         0t0       4897 /dev/ptmx
kvm        8341 libvirt-qemu    5u      CHR             10,232         0t0       8025 /dev/kvm
kvm        8341 libvirt-qemu   64u      CHR             10,200         0t0       4983 /dev/net/tun

As you can see, in one of the VMs (the wrong one), there are no file descriptors 1 & 2, and file descriptor 0 is redirected to /dev/null. That is the problem I guess.

The question is, how do I tell virsh not to do that?

Solution 1

<serial type='pty'>
  <target port='0'/>
</serial>
<console type='pty'>
  <target type='serial' port='0'/>
</console>

This is what I normally add to the VMs definition, using virsh edit Then console=ttyS0 appended in the VM's kernel line in grub.conf

Never failed me so far

Solution 2

Working example of using Debian jessie as host and guest operating system.

1. create a VM using virt-install or virt-manager In any case you will get serial console statements added to VM.xml file

2. in guest VM run the following

   systemctl enable [email protected]
   systemctl start [email protected]
   

3. in guest VM in /etc/default/grub replace

   GRUB_CMDLINE_LINUX_DEFAULT="quiet"
   #GRUB_TERMINAL=console
   

   by

   GRUB_CMDLINE_LINUX_DEFAULT="console=tty0 console=ttyS0"
   GRUB_TERMINAL="serial console"
   

4. in guest VM run the following

   guest# update-grub
   

5. the VM console for running VM can be get by

   host# virsh console VM
   

   or start the VM with console attached

   host# virsh start VM --console
   

Sources:

• 0pointer.de systemd for Administrators, Part XVI
• keypressure.com Testing libvirt over TLS

Solution 3

I've found the most applicable answer here:

Suppose your virtual domain is myGuest, your preferred editor is vi, and your guest is installed with grub2 and uses systemd. If the last assumption is not true, you might have a look at Working with the Serial Console.

First, install libguestfs-tools on the host: sudo apt install libguestfs-tools. You will need this when working with headless guests.

Second, shut down your guest: virsh shutdown myGuest.

Next, mount the virtual disk: guestmount -d myGuest -i /mnt (or use any other existing directory as mountpoint). Now in /mnt you should be able to see the filesystem of the guest.

With grub2 and systemd, you only have to modify the grub configuration: vi /mnt/etc/default/grub, and modify like

GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,19200n8'
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --speed=19200 --unit=0 --word=8 --parity=no --stop=1"

As you will have to run update-grub on the guest, for the first start you have to also modify /mnt/boot/grub/grub.cfg. Find the default boot menu item and append the console information to the linux entry to look similar to

linux   /boot/vmlinuz-4.4.0-75-generic root=UUID=76f3e237-d791-4e9d-8ad7-fe5c9165ae55 ro console=ttyS0,19200 earlyprint=serial,ttyS0,19200

Maybe you need root privileges to mount and edit the files.

Now restart the guest and start the virtual console:

virsh start myGuest && virsh console myGuest

You should see the kernel log and then a login prompt. After logging in, don't forget to run sudo update-grub.

Solution 4

systemctl enable [email protected]
systemctl start [email protected]

has been sufficient for an Ubuntu guest (20.04), VM been initialized with virt-manager.

No need to change GRUB configuration entries, nor adding serial/console to the virsh XML file.

View more solutions

Comments

[Grég]

What is the command that you use to create the vm? I use the following: virt-install --connect qemu:///system -n VM1 -r 256 --vcpus=1 --vnc --noautoconsole --os-type linux --accelerate --network=bridge:eth0,model=virtio -m 52:54:00:12:34:50 -k de --import --nographics --disk /var/VMs/VM1.qcow2

[David Corsalini]

to tell you the truth, I don't use virt-install at all. virt-manager is my usual tool

[Grég]

The problem was somewhere else, but thanks to your hints, I found it. Using virt-manager allowed me to open a VNC connection and I saw that it is not booting at all, because I did not specify in the --disk parameter that this is a qcow2 disk image. Aparently this can not be automatically inferred - even though the extension should be a good hint?. The command must be extended with --disk path=$vm_drive,format=qcow2

[David Corsalini]

In Linux extensions are meaningless, just a part of the filename

[Grég]

Sure, but it would be very strange to suffix a python script with .sh or a bash script with .py (and would probably not work in some cases: you can not import a python module which does not end in .py for example)

[David Corsalini]

that's just done for convenience, nothing to prevent a .sh file to be executed as a python script, if the hashbang is correct

[Grég]

Not only convenience: the import mechanism in python (for example) only works with files ending in .py. If you want another suffix, you will need quite a bit of hacking around. The same is true for other tools. The OS does not enforce extensions, but some tools do.

[David Corsalini]

That is specific to python, not Linux in general.

[Grég]

Sure, which shows that extensions have sometimes meaning, even in a linux environment, which shows that the qcow2 format could be inferred from the .qcow2 extension even without specifying it manually, if the tool was so implemented, which is what I was assuming all along (and was obviously wrong), mainly because I did not know that there was a (strange?!) syntax to specify the format of the disk image. Anyways, thanks for your help.

[Chaim Eliyah]

Still working 100% on Stretch

[Lacek]

What is the purpose of tty0 and serial? It works with just GRUB_CMDLINE_LINUX_DEFAULT="console=ttyS0" and GRUB_TERMINAL="console" in my case. I couldn't find explanation in the sources.

[Serrano]

Just step 1 and 2 are sufficient. No need to update your grub configuration. I just tested this on both Debian buster and bullseye.

[Serrano]

This also works on Debian (buster, bullseye).

[rh4games]

this worked for me like a charm ! I'm using CentOS Linux release 7.9.2009 (Core) for the host and Ubuntu 18.04.5 LTS for the guest VM