Re: [RFC QEMU PATCH 02/10] qdev/qbus: Allow hidden devices to be busless on QEMU startup

[Joshua Lant <joshualant@gmail.com>]

> > On Wed, 29 Apr 2026 14:48:36 +0100
> > Joshua Lant <joshualant@gmail.com> wrote:
> > 
> > > Extend the capability for hiding devices, introduced for virtio-net
> > > device in:
> > > 
> > > commit f3a8505656935cde32e28c1c6317f725084da1e0
> > > Author: Jens Freimann <jfreimann@redhat.com>
> > > Date:   Tue Oct 29 12:48:55 2019 +0100
> > > qdev/qbus: add hidden device support
> > > 
> > > Currently only endpoint devices can be hidden with a primary device
> > > and failover (known static configuration). However, looking at future
> > > composable systems, we see a need for hidden devices which have no associated
> > > bus upon boot. Move the check for hidden devices to before the bus
> > > search, and if it is hidden ignore the case where the device was
> > > described on the CLI without the "bus=" field.
> > > 
> > > This is motivated by a specific use-case: implementing the VCS
> > > command set, part of the CXL specification (CXL r3.2 Section 7.1.3). In
> > > this scenario a switch controlled by a Fabric Manager is able to change the
> > > virtual hierarchy of devices seen by a guest within a fixed physical system
> > > topology. The connecting bus is not known until runtime when
> > > a bind command is issued by the Fabric Manager.
> > 
> > So this comes back to the earlier comment on whether they are completely
> > hidden from a guest.  They aren't quite - we need to be able to
> > get to them via the MCTP or cxl-switch-cci (in band mailbox) paths.
> > 
> > Maybe we can solve that later without any compatibility problems
> > or by cheating and just searching hidden devices. I'm not sure.
> 
> Funnily enough I had something like this when I was originally writing
> this series. I scrapped it becasue I ran into issues. I was storing
> as a tree the QDicts of hidden busses and devices below them, but then realised
> I couldnt hotplug into the USP and abandoned it. I was doing this with the
> aim of eventual support for cascaded switches.
> 
> There was another issue I cant fully remember... That the searching was easy
> when it was referring to unrealized devices with QDicts from the CLI, but it
> became much more difficult to get proper reference to some structures I needed
> from the devices inside the qdev/qbus code when they were realized...
> 

I just remembered... I was having to introduce cxl specific code into the qdev
code... I'm sure it could be separated but would require a fair bit more work.

> > > 
> > > Signed-off-by: Joshua Lant <joshualant@gmail.com>
> > > ---
> > >  system/qdev-monitor.c | 10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
> > > index f2aa400a77..b51dfe0645 100644
> > > --- a/system/qdev-monitor.c
> > > +++ b/system/qdev-monitor.c
> > > @@ -650,6 +650,7 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
> > >      DeviceState *dev;
> > >      BusState *bus = NULL;
> > >      QDict *properties;
> > > +    bool hide_device;
> > >  
> > >      driver = qdict_get_try_str(opts, "driver");
> > >      if (!driver) {
> > > @@ -663,6 +664,11 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
> > >          return NULL;
> > >      }
> > >  
> > > +    /* Is the device hidden from the guest?
> > > +     * If yes, no need to find a default bus if none given...
> > > +     * Bus could be provided at runtime (i.e. in a switch)*/
> > > +    hide_device = qdev_should_hide_device(opts, from_json, errp);
> > > +
> > >      /* find bus */
> > >      path = qdict_get_try_str(opts, "bus");
> > >      if (path != NULL) {
> > > @@ -675,14 +681,14 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
> > >                         driver, object_get_typename(OBJECT(bus)));
> > >              return NULL;
> > >          }
> > > -    } else if (dc->bus_type != NULL) {
> > > +    } else if (dc->bus_type != NULL && !hide_device) {
> > >          bus = qdev_find_default_bus(dc, errp);
> > >          if (!bus) {
> > >              return NULL;
> > >          }
> > >      }
> > >  
> > > -    if (qdev_should_hide_device(opts, from_json, errp)) {
> > > +    if (hide_device) {
> > >          if (bus && !qbus_is_hotpluggable(bus)) {
> > >              error_setg(errp, "Bus '%s' does not support hotplugging",
> > >                         bus->name);
> >