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

[Thomas Huth <thuth@redhat.com>]

On 19/11/2021 10.49, Philippe Mathieu-Daudé wrote:
> 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

So sounds like that's the right way to go here.

> and 2/ break migration unless using HPFM which I don't master.

What's HPFM?

  Thomas