On Wed, Jan 19, 2011 at 11:51:58AM -0600, Anthony Liguori wrote: > On 01/19/2011 11:01 AM, Daniel P. Berrange wrote: > > > >The reason we specify 'bus' is that we wanted to be flexible wrt > >upgrades of libvirt, without needing restarts of QEMU instances > >it manages. That way we can introduce new functionality into > >libvirt that relies on it having previously set 'bus' on all > >active QEMUs. > > > >If QEMU adds PCI-to-PCI bridges, then I wouldn't expect QEMU to > >be adding the extra bridges. I'd expect that QEMU provided just > >the first bridge and then libvirt would specify how many more > >bridges to create at boot or hotplug them later. So it wouldn't > >ever need to parse topology. > > Yeah, but replacing the main chipset will certainly change the PCI > topology such that if you're specifying bus=X and addr=X and then > also using -M pc, unless you're parsing the default topology to come > up with the addressing, it will break in the future.
We never use a bare '-M pc' though, we always canonicalize to one of the versioned forms. So if we run '-M pc-0.12', then neither the main PCI chipset nor topology would have changed in newer QEMU. Of course if we deployed a new VM with '-M pc-0.20' that might have new PCI chipset, so bus=pci.0 might have different meaning that it did when used with '-M pc-0.12', but I don't think that's an immediate problem Regards, Daniel
