[libvirt] [PATCH] kvm 32bit guest on 64bit host

[Gerd v. Egidy]

Hi Daniel,

> > I want to manage a 32 bit i686 guest on an x86_64 host with kvm and
> > libvirt. KVM is perfectly capable of doing this (look e.g. at
> > http://kvm.qumranet.com/kvmwiki/Guest_Support_Status) but libvirt did not
> > allow it.
> >
> > So I created the attached patch to allow it and make it easy to extend
> > the capabilities of kvm and kqemu later on.
>
> This is incorrect.  KVM is *not* exposing a i686 architecture CPU. It is
> exposing an x86_64 virtualized CPU which is capable of running x86_64 or
> i686 guests.  No change is required.

Thanks for looking at my patch. I understand that KVM does not offer a i686 
host cpu but allows x86_64 and i686 guests.

The reason for my patch was that I could not create i686 guests on a x86_64 
host with KVM, the only option offered was using qemu.

I'm new to libvirt so I'm not that familiar with the capabilities-xml - so 
take my findings with some salt ;)

Without my patch the capabilities look like this:

<capabilities>

  <host>
    <cpu>
      <arch>x86_64</arch>
    </cpu>
  </host>

  <guest>
    <os_type>hvm</os_type>
    <arch name='i686'>
      <wordsize>32</wordsize>
      <emulator>/usr/bin/qemu</emulator>
      <machine>pc</machine>
      <machine>isapc</machine>
      <domain type='qemu'>
      </domain>
    </arch>
    <features>
      <pae/>
      <nonpae/>
      <acpi default='on' toggle='yes'/>
      <apic default='on' toggle='no'/>
    </features>
  </guest>

  <guest>
    <os_type>hvm</os_type>
    <arch name='x86_64'>
      <wordsize>64</wordsize>
      <emulator>/usr/bin/qemu-system-x86_64</emulator>
      <machine>pc</machine>
      <machine>isapc</machine>
      <domain type='qemu'>
      </domain>
      <domain type='kvm'>
        <emulator>/usr/bin/qemu-kvm</emulator>
      </domain>
    </arch>
    <features>
      <acpi default='on' toggle='yes'/>
      <apic default='on' toggle='no'/>
    </features>
  </guest>
... (mips, sparc, etc. omitted)

The i686-guest section does not have a kvm-domain. So libvirt does not allow 
the use of kvm for i686 guests.

With my patch the capabilities are:

<capabilities>

  <host>
    <cpu>
      <arch>x86_64</arch>
    </cpu>
  </host>

  <guest>
    <os_type>hvm</os_type>
    <arch name='i686'>
      <wordsize>32</wordsize>
      <emulator>/usr/bin/qemu</emulator>
      <machine>pc</machine>
      <machine>isapc</machine>
      <domain type='qemu'>
      </domain>
      <domain type='kvm'>
        <emulator>/usr/bin/qemu-kvm</emulator>
      </domain>
    </arch>
    <features>
      <pae/>
      <nonpae/>
      <acpi default='on' toggle='yes'/>
      <apic default='on' toggle='no'/>
    </features>
  </guest>

  <guest>
    <os_type>hvm</os_type>
    <arch name='x86_64'>
      <wordsize>64</wordsize>
      <emulator>/usr/bin/qemu-system-x86_64</emulator>
      <machine>pc</machine>
      <machine>isapc</machine>
      <domain type='qemu'>
      </domain>
      <domain type='kvm'>
        <emulator>/usr/bin/qemu-kvm</emulator>
      </domain>
    </arch>
    <features>
      <acpi default='on' toggle='yes'/>
      <apic default='on' toggle='no'/>
    </features>
  </guest>
... (mips, sparc, etc. omitted)

As you can see the i686 guest section gained a kvm-domain with my patch which 
allows me to install i686 KVM-guests on x86_64 hosts.

Kind regards,

Gerd