[Pkg-libvirt-maintainers] Bug#1120807: Bug#1120807: virt-install: fails to create ppc64el machines due to page size choice

[Andrea Bolognani]

On Sun, Nov 16, 2025 at 08:20:31AM +0100, Helmut Grohne wrote:
> Package: virt-install
> Version: 1:5.0.0-5+deb13u1
> X-Debbugs-Cc: debian-powerpc at lists.debian.org, mjt at debian.org
> User: debian-powerpc at lists.debian.org
> Usertags: ppc64el
> 
> Hello,
> 
> I tried using virt-install to create a ppc64el kvm virtual machine. This 
> fails with an error relayed from qemu:
> 
> | Can't support 64 kiB guest pages with 4 kiB host pages with this KVM implementation
> 
> The default machine used by virt-install for ppc64el is "pseries" and it 
> appears to default to 64 kiB guest pages. Debian has changed (#1074217) 
> the kernel's page size to 4 kiB for trixie though, so that's what Debian 
> hosts typically use.
> 
> On the qemu side, we may change this by passing:
> 
>     -machine pseries,cap-hpt-max-page-size=4096
> 
> Now getting this option through virt-install is not obvious. Trying to 
> pass a machine with an option to virt-install's --machine option results 
> in a failure as it interprets the combination of machine and option as 
> the machine and has no clue about this machine.
> 
> A workaround here is passing it as a --qemu-commandline.
> 
>     --machine=pseries "--qemu-commandline=-machine pseries,cap-hpt-max-page-size=4096"
> 
> Would it be possible for virt-install to support this use case in a more 
> convenient way? For instance, it could detect the host's page size and 
> adjust it itself or it could enable passing such options via --machine.
> 
> Thanks for considering

So, a few things.

First of all, libvirt already exposes this capability: use

  <domain>
    <features>
      <hpt>
        <maxpagesize unit='KiB'>4</maxpagesize>
  ...

to configure it.

virt-install doesn't seem to support manipulating this value
directly though, at least based on the output of

  $ virt-install --features help

so an upstream RFE will need to be filed to make that happen.

Taking a step back though, shouldn't QEMU default to a 4k page size
when 64k cannot work on the host?

Maybe there's something in the sPAPR spec that mandates 64k as the
default, I don't know. That would be very unfortunate. Users should
really not need to be aware of this kind of detail just to get a
basic VM going.

-- 
Andrea Bolognani <eof at kiyuko.org>
Resistance is futile, you will be garbage collected.
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