On Tue, Nov 20, 2018 at 01:27:05PM +0100, Andrea Bolognani wrote: > On Mon, 2018-11-19 at 14:14 -0500, Michael S. Tsirkin wrote: > > On Mon, Nov 19, 2018 at 07:56:38PM +0100, Cornelia Huck wrote: > > > On Mon, 19 Nov 2018 13:42:58 -0500 "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > We have this assumption that if we force a choice then people will > > > > choose the right thing but in practice they will do what we all do, play > > > > with it until it kind of works and leave well alone afterwards. > > > > That's at best - at worst give up and use an easier tool. > > > > > > That implies that we (the developers) need to care and make sure that > > > "model=virtio" gets them the best possible transport (i.e. on s390x, > > > that would be ccw unless the user explicitly requests pci; I'm not sure > > > what the situation with mmio is -- probably "use pci whenever > > > possible"?) I think that's what libvirt already gives us today (I hope.) > > The interface at the libvirt level is exactly "model=virtio", with > that ultimately translating to virtio-*-pci or virtio-*-ccw or > virtio-*-device or whatever else based on the architecture, machine > type and other information about the guest. > > > > What makes it messy on the pci side is that the "best option" actually > > > depends on what kind of guest the user wants to run (if the guest is > > > too old, you're stuck with transitional; if you want to reap the > > > benefits of PCIe, you need non-transitional...) > > > > Well it works now - connect it to a bus and it figures out whether it > > should do transitional or not. You can force transitional in PCIe anyway > > but then you are limited to about 15 devices - probably sufficient for > > most people ... > > That's not how it works, though: current virtio-*-pci devices will > be transitional (and thus support older guest OS) or not based on > the kind of slot you plug them into. > > >From the management point of view that's problematic, because libvirt > (which takes care of the virtual hardware, including assigning PCI > addresses to devices) has no knowledge of the guest OS running on > said hardware, and management apps (which know about the guest OS and > can figure out its capabilities using libosinfo) don't want to be in > the business of assigning PCI addresses themselves. > > Having separate transitional and non-transitional variants solves the > issue because now management apps can query libosinfo to figure out > whether the guest OS supports non-transitional virtio devices, and > based on that they can ask libvirt to use either the transitional or > non-transitional variant; from that, libvirt will be able to choose > the correct slot for the device. > > None of the above quite works if we have a single variant that > morphs based on the slot, as we have today. So can we get an ack on the patchset then? > -- > Andrea Bolognani / Red Hat / Virtualization -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
