On Wed, Jul 20, 2022 at 02:00:16PM +0300, Roman Kagan wrote: > On Wed, Jul 20, 2022 at 11:44:26AM +0100, Daniel P. Berrangé wrote: > > On Wed, Jul 20, 2022 at 01:25:55PM +0300, Roman Kagan wrote: > > > It's possible to create non-working configurations by attaching a device > > > to a derivative of PCIe slot (pcie-root-port, ioh3420, etc) and > > > specifying a slot number other that zero, e.g.: > > > > > > -device pcie-root-port,id=s0,... \ > > > -device virtio-blk-pci,bus=s0,addr=4,... > > > > > > Make QEMU reject such configurations and only allow addr=0 on the > > > secondary bus of a PCIe slot. > > > > What do you mean by 'non-working' in this case. The guest OS boots > > OK, but I indeed don't see the device in the guest, but IIUC it was > > said that was just because Linux doesn't scan for a non-zero slot. > > Right. I don't remember if it was Linux or firmware or both but indeed > at least Linux guests don't see devices if attached to a PCIe slot at > addr != 0. (Which is kinda natural for a thing called "slot", isn't it?)
I vaguely recall there was an option to tell linux to scan all slots, not just slot 0, not sure if that's applicable here. > > > That wouldn't be a broken config from QEMU's POV though, merely a > > guest OS limitation ? > > Strictly speaking it wouldn't, indeed. But we've had created such a > configuration (due to a bug in our management layer) and spent > non-negligible time trying to figure out why the attached device didn't > appear in the guest. So I thought it made sense to reject a > configuration which is known to confuse guests. Doesn't it? If a configuration is a permissible per the hardware design / spec, then QEMU should generally allow it. We don't want to constrain host side configs based on the current limitations of guest OS whose behaviour can change over time, or where a different guest OS may have a different POV. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
