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 11.17, Philippe Mathieu-Daudé wrote:
> On 11/19/21 10:58, Thomas Huth wrote:
>> 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?
> 
> Hocus Pocus Freakin' Magic

LOL, ok, thanks, TIL.

Anyway, IMHO I'd rather fix this issue by properly QOM'ifying the 
vga-isa-mm.c code and breaking migration here (who's migrating such old ISA 
devices anyway?) instead of introducing more kludges
that might cause other trouble (see also 
https://gitlab.com/qemu-project/qemu/-/issues/733 ).

  Thomas