[Libguestfs] FYI: virtio-pci will be the default with libvirt 3.0.0, aarch64 & -M virt

[Richard W.M. Jones]

(Thanks to Andrea Bolognani for bringing this to my attention)

from https://libvirt.org/news.html:

* v3.0.0 (unreleased) 
  - qemu: Use virtio-pci by default for aarch64 mach-virt guests
  - virtio-pci provides several advantages over virtio-mmio, such as
    the ability to hotplug devices and improved performance. While
    opting in to virtio-pci has been possible for a while,
    newly-defined guests will now use it automatically.

This is a good change because virtio-mmio is really slow, and because
all other platforms (except S/390!) use virtio-pci and we want to
minimize differences.

I previously wrote a patch to make this happen in libguestfs:

  https://www.redhat.com/archives/libguestfs/2016-October/thread.html#00078

However the change was still being finalized in qemu & libvirt at that
time, so I think that patch is probably not quite right for the final
qemu/libvirt.  In any case nothing is upstream.

This is just a note that things will probably be broken for
LIBGUESTFS_BACKEND=libvirt + libvirt 3.0.0 + aarch64 until we fix this
in libguestfs.

There is also a related RHEL bug:

  https://bugzilla.redhat.com/show_bug.cgi?id=1237250

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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