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Re: [PATCH-for-6.2 0/2] hw/display: Do not allow multiple (identical) VG
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH-for-6.2 0/2] hw/display: Do not allow multiple (identical) VGA devices |
Date: |
Fri, 19 Nov 2021 10:49:30 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 |
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.