Re: [libvirt] [PATCH for-4.0 v2] virtio: Provide version-specific variants of virtio PCI devices

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: > > 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?