[Qemu-devel] Re: [FOR 0.12 PATCH v3 05/21] default devices: core code & serial lines.

[Gerd Hoffmann <kraxel@redhat.com>]

   Hi,

>> Doesn't make sense.  The list is not about *creating* default devices, but about *not* creating them.
>
> Why do we need a negative list? Wouldn't it be a lot more useful to have positive lists? Maybe I'm just missing the whole point of your patchset though.

Probably the latter ;)

The situation we have right is (with the serial lines as example, the 
same applies to some other devices as well):

(1) If you start qemu without any arguments, it will automagically
     create a chardev for the serial line (serial_hds[]), which then
     will be used by machine->init() to create the (board-specific)
     serial lines.
(2) If you start qemu with -serial <something> the specified chardev
     will be created and assigned to serial_hds[], which again will be
     used by machine->init() ...

Worked fine before qdev appeared.  Now we also can create serial lines 
like this:

(3) Start qemu with '-device isa-serial,...'.

Problem is this clashes with (1) and qemu will attempt to create the 
serial port twice.  So this patchset introduces default_serial 
(+friends) to fix this issue.  The new workflow is:

  * default_serial defaults to enabled, when qemu finds it still enabled
    after processing all command line the automagic default serial line
    is actually created.
  * If qemu finds '-serial <something>' it clears default_serial.
  * If qemu finds '-device isa-serial,...' it clears default_serial.
    - this fixes the conflict mentioned above.
    - other serial drivers can be added to the list if needed to solve
      the same conflict for them.
  * If qemu finds '-nodefaults' it clears default_serial (and the
    others).
    - new feature, figured this could be useful.

What we could do on top of the above is:

   * If the board has the machine->no_serial flag set, clear
     default_serial.

s390 could set that flag then and qemu would stop creating a chardev for 
a serial line nobody will ever use because s390 has no serial ports.

/me hopes the whole picture is more clear now.

cheers,
   Gerd