Re: [Bug 1923663] [NEW] Can't(?) disable default floppy drive any more in qemu 6.0

[Markus Armbruster <armbru@redhat.com>]

Adam Williamson <1923663@bugs.launchpad.net> writes:

> Public bug reported:
>
> There's a documented change in qemu 6.0:
>
> https://qemu-project.gitlab.io/qemu/system/removed-features.html#floppy-
> controllers-drive-properties-removed-in-6-0
>
> where you can't configure floppy controller device properties with
> -global any more. However, there's a thing you could do with the old
> parameter which I can't figure out a way to do with the documented
> replacement. openQA passed exactly this argument:
>
> -global isa-fdc.driveA=
>
> and that has the effect of removing/disabling the default floppy
> drive/controller. If you just run `qemu-system-i686` (no other args)
> you'll see the VM briefly try to boot from a floppy drive; if you run
> `qemu-system-i686 -global isa-fdc.driveA=` (with an earlier version of
> qemu, obviously) you'll see it does not do so.
>
> I can't see a way to do this with `-device floppy`. Going by the docs,
> the equivalent should be:
>
> -device floppy,unit=0,drive=
>
> but that does not seem to have the same effect. If you run `qemu-
> system-i686 -device floppy,unit=0,drive=`, it still tries to boot from a
> floppy drive.
>
> I see there's a -nodefaults option that disables *all* default devices,
> but I don't think that's what we want here either. We might want the
> other default devices, we just don't want the floppy drive.

= Short answer =

In my opinion, management applications are better off with -nodefaults.
It's easier to understand than the complicated mess I'm going to
describe under "Long answer" below.

If you'd prefer not to, try -global isa-fdc.fdtypeA=none.


= Long answer =

-global isa-fdc.driveA= worked.  Whether it was supported usage or
accidental dirt effect is unclear.  Doesn't matter now.

-nodefaults suppresses a number of backends:

* Character device backend for a serial device

  Also suppressed when -serial ... or -device isa-serial,... or -global
  isa-serial.PROP=VAL is given, or the machine type opts out of this
  backend.

  Backend configuration depends on other options; too complicated to
  explain here.

* Character device backend for a parallel device

  Also suppressed when -parallel ... or -device isa-parallel,... or
  -global isa-parallel.PROP=VAL is given,  or the machine type opts out
  of this backend.

  Backend configuration depends on other options; too complicated to
  explain here.

* Block device backend a floppy device

  Also suppressed when -device isa-fdc,... or -global isa-fdc.PROP=VAL
  or -device floppy or -global floppy.PROP=VAL is given, or the machine
  type opts out of this backend.

* Block device backend a CD-ROM device

  Also suppressed when -device {ide,scsi}-{cd,hd},... or -global
  {ide,scsi}-{cd,hd}.PROP=VAL is given, or the machine type opts out of
  this backend.

* SD card

  Also suppressed when the machine type opts out of this backend.

When a backend exists, the machine type may

* Create a frontend (a.k.a. device model) connected to the backend

* Ignore the backend silently

* Complain about the useless backend

-nodefaults additionally suppresses:

* Default HMP monitor

  Also suppressed when -monitor or -qmp or -qmp-pretty or -mon or
  -serial mon:... or -parallel mon:... is given.

  Monitor configuration depends on other options; too complicated to
  explain here.

* Default network frontend (-net nic) and backend (-net user)

  Also suppressed when -netdev or -nic or -net is given.

  Default backend is only done when we have SLIRP.

* Default VGA type, if any

  Actual type depends on the machine machine type.  Set to "none" when
  -vga or -device DRV,... or -global DRV.PROP=VAL is given, where DRV is
  a VGA device model.

  When the type is not "none", the machine type may:

  * Create a device of that type

  * Ignore the type silently

  * Complain about the type

* Additional stuff depending on the machine type


Questions?