Re: [PATCH v3] hw/pci/pci_bridge: ensure PCIe slots have only one slot

[Daniel P . Berrangé]

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 :|