Re: [PATCH-for-6.2 0/2] hw/display: Do not allow multiple (identical) VGA devices

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 11/19/21 09:21, Mark Cave-Ayland wrote:
> On 18/11/2021 19:20, Philippe Mathieu-Daudé wrote:
> 
>> Commit 7852a77f598 fixed creating multiple TYPE_ISA_VGA devices,
>> generalize the fix to all VGA devices.
>>
>> See https://gitlab.com/qemu-project/qemu/-/issues/44
>>
>> Philippe Mathieu-Daudé (2):
>>    hw/display: Add Error* handle to vga_common_init()
>>    hw/display: Do not allow multiple identical VGA devices
>>
>>   hw/display/vga_int.h        |  2 +-
>>   hw/display/ati.c            |  4 +++-
>>   hw/display/cirrus_vga.c     |  4 +++-
>>   hw/display/cirrus_vga_isa.c |  4 +++-
>>   hw/display/qxl.c            |  4 +++-
>>   hw/display/vga-isa-mm.c     |  3 ++-
>>   hw/display/vga-isa.c        | 11 ++---------
>>   hw/display/vga-pci.c        |  8 ++++++--
>>   hw/display/vga.c            | 17 ++++++++++++++++-
>>   hw/display/virtio-vga.c     |  4 +++-
>>   hw/display/vmware_vga.c     |  2 +-
>>   11 files changed, 43 insertions(+), 20 deletions(-)
> 
> Hi Phil,
> 
> I don't think this is correct for non-ISA devices: for example years ago
> I had a PC running Windows 98SE with 2 identical PCI graphics cards
> configured in dual-head mode.
> 
> IIRC the BIOS would bring up the first graphics card and configure it to
> use the legacy ISA VGA ioports for compatibility, and then once the main
> OS drivers loaded both cards were switched to PCI mode and configured
> using the BARs as normal.

The problem here is QEMU technical debt, not the hardware.

When vga_common_init() calls memory_region_init_ram_nomigrate()
with obj=NULL, "vga.vram" is registered as a QOM singleton.

Updating it would 1/ require non-QOM devices to be QOM'ified
and 2/ break migration unless using HPFM which I don't master.