QEMU currently abort()s if the user tries to add a second ISA VGA
device, for example:

$ ./qemu-system-x86_64 -device isa-vga -device isa-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)
$ ./qemu-system-x86_64 -device isa-cirrus-vga -device isa-cirrus-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)
$ ./qemu-system-mips64el -M pica61 -device isa-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)

Such a crash should never happen just because of giving bad parameters
at the command line. Let's return a proper error message instead.
(The idea is based on an original patch by Jose R. Ziviani for the
isa-vga device, but this now fixes it for the isa-cirrus-vga device, too)

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/44
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/display/vga.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/display/vga.c b/hw/display/vga.c
index 7fc6ab7e4f..1dca3dd7c0 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -2206,6 +2206,12 @@ void vga_common_init(VGACommonState *s, Object *obj, Error **errp)
     s->vbe_size_mask = s->vbe_size - 1;
 
     s->is_vbe_vmstate = 1;
+
+    if (s->global_vmstate && qemu_ram_block_by_name("vga.vram")) {
+        error_setg(errp, "A global VGA device has already been registered");
+        return;
+    }
+
     memory_region_init_ram_nomigrate(&s->vram, obj, "vga.vram", s->vram_size,
                                      &local_err);
     if (local_err) {
-- 
2.27.0