On Thu, Sep 08, 2016 at 03:50:40PM +0200, Michal Privoznik wrote: > > After going through patch 3, I'm beginning to think it wouldn't be such > > a good idea to keep nvdimm's and dimm's in the same nmems list. I think > > they might just be too different. I also recall some algorithm > > somewhere which made sure addresses and sizes used fit "properly"... > > Searching on DIMM in cscope brings up a few more references and questions. > > So, I've just discussed this with Peter who implemented the DIMM (aka > mem hotplug) feature. In this case, libvirt tries to not duplicate code > that already lives in qemu. So how this works is: on domain startup > libvirt just puts the dimms (and after my patches nvdimms as well) onto > the qemu cmd line just in the order as in the XML. Then, when qemu is > started its magic algorithm assigns addresses to all the modules (so > that they don't overlap) and libvirt just queries this information on > the monitor. And it is only in case of migration when we explicitly put > the addresses onto qemu's cmd line (to preserve ABI stability). > On the next run of domain (I mean hard stop & start => new qemu > process), the process repeats itself again. So addresses can be possibly > different, but I'd say this is okay. For two reasons: even in real > hardware if you switch two devices (e.g. DIMM modules, disks, ...) > you'll boot into different environment (e.g. disks gets renamed, what > used to be sda is now sdb). Secondly, this NVDIMM is meant to be used as > a sync mechanism between host & guest.
This is very different to how we deal with addressing for all other types of device in libvirt, where we always assign addresses immediately at time the guest is defined. By only assigning addresses transiently when QEMU starts up, then you're liable to get different addressing even if the DIMM config isn't isn't changed. eg addition of other unrelated devices may alter the address QEMU ends up assigning to the DIMMS. Your arguments in favour of not having persistent addresses aren't really in any way specific to DIMMs, so if we wanted to take that approach, then it would apply to all types of device. So I think we need to fix DIMM address assignment so that it works exactly the same way as we do for all other device addressing needs and assign it up front. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :| -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
