Re: KVM call minutes 2013-01-29 - Port I/O

["Andreas Färber" <afaerber@suse.de>]

Am 30.01.2013 17:33, schrieb Anthony Liguori:
> Gerd Hoffmann <kraxel@redhat.com> writes:
> 
>>> hw/qxl.c:    portio_list_add(qxl_vga_port_list,
>>> pci_address_space_io(dev), 0x3b0);
>>> hw/vga.c:        portio_list_add(vga_port_list, address_space_io, 0x3b0);
>>
>> That reminds me I should solve this in a more elegant way.
>>
>> qxl takes over the vga io ports.  The reason it does this is because qxl
>> switches into vga mode in case the vga ports are accessed while not in
>> vga mode.  After doing the check (and possibly switching mode) the vga
>> handler is called to actually handle it.
> 
> The best way to handle this would be to remodel how we do VGA.
> 
> Make VGACommonState a proper QOM object and use it as the base class for
> QXL, CirrusVGA, QEMUVGA (std-vga), and VMwareVGA.

That would require polymorphism since we already need to derive from
PCIDevice or ISADevice respectively for interfacing with the bus...
Modern object-oriented languages have tried to avoid multi-inheritence
due to arising complications, I thought. Wouldn't object if someone
wanted to do the dirty implementation work though. ;)

Another such example is EHCI, with PCIDevice and SysBusDevice frontends,
sharing an EHCIState struct and having helper functions operating on
that core state only. Quite a few device share such a pattern today
actually (serial, m48t59, ...).

> The VGA accessors should be exposed as a memory region but the sub class
> ought to be responsible for actually adding it to a subregion.
> 
>>
>> That twist makes it a bit hard to convert vga ...
>>
>> Anyone knows how one would do that with the memory api instead? I think
>> taking over the ports is easy as the memory regions have priorities so I
>> can simply register a region with higher priority. I have no clue how to
>> forward the access to the vga code though.
>>
> 
> That should be possible with priorities, but I think it's wrong.  There
> aren't two VGA devices.  QXL is-a VGA device and the best way to
> override behavior of base VGA device is through polymorphism.

In this particular case QXL is-a PCI VGA device though, so we can
decouple it from core VGA modeling. Placing the MemoryRegionOps inside
the Class (rather than static const) might be a short-term solution for
overriding read/write handlers of a particular VGA MemoryRegion. :)

Cheers,
Andreas

> This isn't really a memory API issue, it's a modeling issue.
> 
> Regards,
> 
> Anthony Liguori
> 
>> Anyone has clues / suggestions?
>>
>> thanks,
>>   Gerd

-- 
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg