[Bug 268794] Simultaneous vcpu_lock_all() and vm_handle_rendezvous() can deadlock vmm

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268794

--- Comment #14 from Corvin Köhne <corvink@FreeBSD.org> ---
The easiest test to check if the rom works correctly would be to boot a linux
guest and checking the dmesg output. Additionally, if it fails, it'll give us
more information about the failure.

The VBIOS of older gpus don't contain an EFI driver which is required for
graphical output on boot. You can test it with the EfiRom tool included in
edk2's BaseTools. To get the tool, clone https://github.com/tianocore/edk2 and
run make in the BaseTools folder. Running `edksetup.sh` in the base folder of
the repo adds the command to your path. After that run `EfiRom -d
your-vbios.rom`. It should contain EFI ROM header like:
```
Image 1 -- Offset 0x0
  ROM header contents
    Signature              0xAA55
    PCIR offset            0x001C
    Signature               PCIR
    Vendor ID               0x8086
    Device ID               0x5912
    Length                  0x001C
    Revision                0x0003
    DeviceListOffset        0x00
    Class Code              0x000000
    Image size              0x11C00
    Code revision:          0x0000
    MaxRuntimeImageLength   0x00
    ConfigUtilityCodeHeaderOffset 0x00
    DMTFCLPEntryPointOffset 0x00
    Indicator               0x80   (last image)
    Code type               0x03   (EFI image)
  EFI ROM header contents
    EFI Signature          0x0EF1
    Compression Type       0x0000 (not compressed)
    Machine type           0x8664 (X64)
    Subsystem              0x000B (EFI boot service driver)
    EFI image offset       0x01E0 (@0x1E0)
```

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