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If there are multiple devices assigned to the different functions of a single PCI slot, they will not work properly if the device at function 0 doesn't have its "multi" attribute turned on, so it makes sense for libvirt to turn it on during PCI address assignment. Setting multi then assures that the new setting is stored in the config (so it will be used next time the domain is started), preventing any potential problems in the case that a future change in the configuration eliminates the devices on all non-0 functions (multi will still be set for function 0 even though it is the only function in use on the slot, which has no useful purpose, but also doesn't cause any problems). (NB: If we were to instead just decide on the setting for multifunction at runtime, a later removal of the non-0 functions of a slot would result in a silent change in the guest ABI for the remaining device on function 0 (although it may seem like an inconsequential guest ABI change, it *is* a guest ABI change to turn off the multi bit).)
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>