Upgrading the host

[Jiri Denemark]

On Wed, Feb 17, 2021 at 07:15:03 -0800, Marcin Struzak wrote:
> On 2/17/2021 12:23 AM, Michal Privoznik wrote:
> > On 2/17/21 2:45 AM, Marcin Struzak wrote:
> >> On 2/16/2021 12:51 AM, Michal Privoznik wrote:
> >>> On 2/12/21 2:10 AM, Marcin Struzak wrote:
> >>> [...]
> >>>
> >>>>  3. Anything else I should worry about or prepare ahead?
> >>>
> >>> Not strictly related to libvirt, but machine types might be problem. 
> >>> I guess qemu will be upgraded too and in general the machine type 
> >>> you're using for a domain might not be available in upgraded qemu.
> >> Correct, it looks like qemu will go from 1.6.2 to 4.2.1...
> >>>
> >>> You can view supported machine types by running 'virsh capabilities' 
> >>> with upgraded qemu.
> >> machine is pc-i440fx-1.6 in all my domains.  Is there a simple way to 
> >> figure out if that's supported in libvirt 6.1.0 (without installing it)?
> >
> > It's not libvirt who has say in machine types. Libvirt treats machine 
> > type as an opaque string. But I can see that pc-i440fx-1.6 is still 
> > available even with the latest QEMU release:
> >
> > # /usr/bin/qemu-system-x86_64 -version
> > QEMU emulator version 5.2.0
> >
> > # virsh capabilities | grep pc-i440fx-1.6
> >       <machine maxCpus='255'>pc-i440fx-1.6</machine>
> >       <machine maxCpus='255'>pc-i440fx-1.6</machine>
> >
> > So you are safe on that front.
> 
> Excellent, thank you for checking.
> 
> Out of curiosity: what would have been my course of action if that 
> particular machine were no longer defined?  I assume: substitute with a 
> close match (after figuring out how do determine what counts as "close") 
> & hope that the VMs (all Fedoras & Ubuntus) cope & adjust?

All pc-i440fs-* machine types are pretty close and based on the same
chipset. There are some differences between them (e.g., in migration
format, enabled CPU features for a given CPU model), but nothing an OS
installed in the VM couldn't cope with. Even Windows can deal with it,
but reactivation may be needed after changing the machine type.

So unless you would need to stick with a specific (or old enough)
machine type for migration compatibility with hosts that were not
upgraded yet, you would likely end up replacing "pc-i440fx-1.6" with
plain "pc", which would be expanded to the default machine type, i.e.,
the most recent pc-i440fx machine type supported by the installed
version of QEMU.

Jirka