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This patch was made in response to: https://bugzilla.redhat.com/show_bug.cgi?id=738095 In short, qemu's default for the rombar setting (which makes the firmware ROM of a PCI device visible/not on the guest) was previously 0 (not visible), but they recently changed the default to 1 (visible). Unfortunately, there are some PCI devices that fail in the guest when rombar is 1, so the setting must be exposed in libvirt to prevent a regression in behavior (it will still require explicitly setting <rom bar='off'/> in the guest XML). rombar is forced on/off by adding: <rom bar='on|off'/> inside a <hostdev> element that defines a PCI device. It is currently ignored for all other types of devices. At the moment there is no clean method to determine whether or not the rombar option is supported by QEMU - this patch uses the advice of a QEMU developer to assume support for qemu-0.12+. There is currently a patch in the works to put this information in the output of "qemu-kvm -device pci-assign,?", but of course if we switch to keying off that, we would lose support for setting rombar on all the versions of qemu between 0.12 and whatever version gets that patch.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>