OVMF split image support

Gerd Hoffmann's Fedora OVMF builds have been updated to support installing the split CODE/VARS binaries.  Wherever you get your OVMF binaries, the advantage of this is that the EFI variables, ex. bootloader information, is stored separately from the executable code of the firmware allowing it to be updated without blasting the variable store.  The libvirt update mentioned the other day already supports this quite nicely.  Rather than having a loader entry with a single read-write image, we switch that to read-only entry and add nvram storage.  The XML looks like this:

<domain type='kvm'>
  ...
  <os>
    <loader readonly='yes' type='pflash'>/usr/share/edk2.git/ovmf-x64/OVMF_CODE-pure-efi.fd</loader>
    <nvram template='/usr/share/edk2.git/ovmf-x64/OVMF_VARS-pure-efi.fd'>
    ...
  </os>
</domain>

Once the guest is started, a copy of the NVRAM templace is made an placed under /var/lib/libvirt/qemu/nvram/$DOMAIN_VARS.fd.  This then becomes part of the state of the VM.

On the QEMU commandline, you'll need to manually create a copy of the VARS file for each VM and specify the CODE and VARS as:

/usr/libexec/qemu-kvm ... \

    -drive if=pflash,format=raw,readonly,file=/path/to/OVMF_CODE.fd \

    -drive if=pflash,format=raw,file=/copy/of/OVMF_VARS.fd

I'm also told that virt-install and virt-manager support for OVMF are coming real soon and the interface will be similar to the XML, allowing selection of both a CODE and template VARS files.  The libvirt config file, /etc/libvirt/qemu.conf, also allows a default VARS template image to be specified per code image, so that the <nvram> entry gets filled in automatically based on the file used for the <loader> entry.

Finally, how do you tell whether you have a split or unified image for OVMF?  Lacking some sort of parser, apparently the best way to tell is by file size.  A unified image will be exactly 2MB while the split CODE image will be 2MB-128KB and the VARS image will be 128KB.  Unsurprisingly then, you can also create a split image with dd, taking the first 128K as VARS and the rest as CODE.

Good luck.

libvirt now supports OVMF

Thanks to the work of Michal Privoznik and support of Laszlo Ersek and others, libvirt can now manage VMs using OVMF natively.  If you're on Fedora and using Gerd's OVMF RPMs, you simply need to create a copy of /usr/share/edk2.git/ovmf-x64/OVMF-pure-efi.fd for each VM (put it somewhere like /var/lib/libvirt/images/), and make it writable (support is still new and it doesn't seem to change file permissions for the VM yet).  Then, edit the domain XML to include this:

<domain type='kvm'>
  ...
  <os>
    ...
    <loader type='pflash'>/var/lib/libvirt/images/VM1-OVMF.fd</loader>
  </os>
</domain>

Since the OVMF image we're using is a "unified" image, it contains both the UEFI code itself as well as variable storage space, so the above adds it as writable by the VM.  There are also ways to have a split image so you can maintain the UEFI code separate from the variables, but I'll wait for builds from Gerd that support that before I attempt to document it.

With support for both the kvm=off cpu option and OVMF in libvirt, we're now able to run completely native libvirt VMs with GeForce and Radeon GPU assignment.  Support is already underway for virt-manager and virt-install of OVMF.

Also, a VM CPU selection tip, since we don't care about migration with an assigned GPU, there are few reasons left not to want to use the -cpu host option for QEMU.  To enable that through libvirt, change the CPU definition in the XML to this:

<domain type='kvm'>
  ...
  <cpu mode='host-passthrough'/>
  ...
</domain>

Automatic vCPU pinning is also available:

<domain type='kvm'>
  ...
  <cputune>
    <vcpupin vcpu='0' cpuset='0'/>
    <vcpupin vcpu='1' cpuset='1'/>
  </cputune>
  ...
</domain>

And yes, hugepage support is also available, see libvirt documentation for details.  Enjoy.

Upstream updates for August 26th 2014

A couple updates relevant to Nvidia GeForce assignment:

QEMU

fe08275d is now in qemu.git, decoupling the primary Nvidia GPU device quirk from the x-vga=on option.  This means that an Nvidia GPU assigned to a legacy-free OVMF VM will now enable this quirk automatically.

libvirt

d0711642 is now in libvirt.git enabling libvirt support for the kvm=off QEMU cpu option.  To enable this in your XML, add this to your VM definition:

<domain type='kvm'...>
  <features>
    <kvm>
      <hidden state='on'/>
    </kvm>
  </features>
  ...
</domain>