Select which kernel a VM should boot

[Andre Ferreira]

Hi,

I´m having a problem to set which kernel a VM should boot.

In my case, I have a Debian install running kernel linux-image-4.19.0-9-amd64. For reasons, I want it boot with kernel linux-image-4.19.0-6-amd64, which is also installed. I've followed various manuals/tutorials/instructions in the web, but nothing worked. Grub simply does not appear and the current kernel always is the one booted.

I believe that in the paravirtualization, Xen and Debian are exchanging information that make this kernel the one to be booted. The Grub menu never shows, no matter what I try.

Now, is it really due to the paravirtualization? How do I select the kernel in a PV VM? Could somebody point me to a document with this information?

 

Tks,

Roberto

 

[Tobias Kreidl]

That's a GRUB configuration option. See for example: https://www.thegeekdiary.com/centos-rhel-7-change-default-kernel-boot-with-old-kernel/

 

-=Tobias

[Andre Ferreira]

As I've mentioned in my explanation, I've followed various guides trying to make the menu appear or set a specific kernel. The one you indicated was among of the first ones I tried, but nothing happened (no grub menu, and the default kernel loaded, ignoring the settings I tried).

 

In the mean time, I made a few more experiments and observations, and discovered something. All the VM's I have where the grub menu is working are HVM, and in all PV VM's, the menu does not appear. So, As a final test, I decided to convert a PV VM to HVM. I did this with the following commands in one of the Xen Servers:

xe vm-param-set uuid=<VM-UUID> HVM-boot-policy="BIOS order"

xe vm-param-set uuid=<VM-UUID> PV-bootloader=

xe vm-param-set uuid=<VM-UUID> PV-args=""

After this, I've set the bootable disk in properties/Boot options.

 

After this modification, the grub menu appeared as expected in the VM. So, what I suspected is now a certainty: In PV, the grub menu does not appear, in HVM it works fine. Does somebody now why this is happening, if this is expected and how do I modify this behavior?

 

Thank you,

 

Roberto Greiner

 

PS: My XenServers are running Xen 8.0 patched up to XS80E006

[Tobias Kreidl]

Roberto,

I know that PV mode for VMs is no longer supported in 8.1. Maybe your patch level has caused that to kick in, as well?

 

-=Tobias

[Andre Ferreira]

Thanks for the head-up. I was planning to upgrade to 8.2 soon. I checked my VM's and found that I only had another 2 (among a total of 20) running in PV mode. I will switch them to HVM before upgrading to avoid problems.

 

Tks,

 

Roberto

 

[Tobias Kreidl]

Glad to help! Hope that takes care of it for you, Roberto.

 

-=Tobias