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Post-installation tuning

Although not strictly needed, I find it convenient to tune some settings. These are optional and independent from each other, but they are documented in case you trip on the same inconveniences as I did.

EFI32

Mac OS X 10.4 can only boot from a 32-bit EFI. For this reason, if the virtual machine is configured as Version: Mac OS X (64-bit), its bootloader must be configured as 32-bits with the command:

VBoxManage modifyvm Tiger --firmware efi32

CPU profile and CPUID settings

The kernel is unable to correctly parse some of the CPUID information of modern CPUs, leading to a hard freeze while loading modules or to a kernel panic in the kernel initialization. Luckily VirtualBox makes it possible to override most CPUID information.

In order to understand what happens, some knowledge about the CPUID instruction is required. It returns "CPU identification" (and feature information) about the processor. The contents of the EAX register (and in some cases of the ECX register) selects the leaf (and sub-leaf), i.e. what information should be returned. The EAX, EBX, ECX and EDX registers are used to store the output.

VirtualBox makes it possible to override the values returned by this instruction in two ways:

  • by requesting a different processor to be emulated
VBoxManage modifyvm Tiger --cpu-profile 'Intel Pentium 4 3.00GHz'
  • by overriding a specific leaf
VBoxManage modifyvm <VM_Name> --cpuidset <leaf> <eax> <ebx> <ecx> <edx>

The CPUID leaf EAX=0 returns the maximum value for basic CPUID leaves in EAX and a vendor identification string in EBX-EDX-ECX. In some cases (for example to perform live migration of virtual machines over different processors), it is desirable to restrict the CPU features as much as possible, but in our case we are aiming at making them available to the virtual machine. Specifically, in order to enable multiple processors, we need to enable at least the leaf EAX=4 (the vendor identification can be left as-is).

# Set CPUID EAX=0 to return 00000004 "Genu" "ntel" "ineI"
VBoxManage modifyvm Tiger --cpuidset 00000000 00000004 756e6547 6c65746e 49656e69

The sub-leaves of EAX=4 contain cache information which the kernel has trouble parsing and unfortunately there seem to be no way to override a sub-leaf in VirtualBox without overriding the whole CPU profile. Among the profiles currently shipped in VirtualBox, Intel Pentium 4 3.00GHz is the only one that seem to boot fine with the leaf limit set to 4.

Setting the CPU profile to an old Pentium 4 will likely prevent VirtualBox from exposing several of the features available on your CPU. The overrides for EAX=1 and EAX=0x80000001 are used to re-enable any feature supported by your CPU. Actually, the overrides will enable even features that are not supported by the host CPU, but VirtualBox takes care of filtering them out.

# Set CPUID EAX=1 to return the Pentium 4 version/brand information
# but override the features, enabling anything except XSAVE
VBoxManage modifyvm Tiger --cpuidset 00000001 00000f43 00020800 fbffffff ffffffff
# Set CPUID EAX=0x80000001 to return any feature as enables
VBoxManage modifyvm Tiger --cpuidset 80000001 00000000 00000000 ffffffff ffffffff

Note that XSAVE (bit 26 of ECX for EAX=1) is not enabled. This is required because VirtualBox will complain at boot if that bit is enabled on a processor without leaf EAX=0xd. These feature overrides are very aggressive and other features might have to be disabled just like XSAVE in future.

DMI data

The kernel and most of the applications do not seem to check DMI data, with the exception of System Profiler. With the default setting, it cannot show the basic hardware information, apparently because it tries to parse the BIOS version in a specific way. Namely, it tokenizes it using . as a separator and shows the first, third and fourth tokens. Therefore, in order to get the Hardware page working, the BIOS version is overridden with:

VBoxManage setextradata Tiger VBoxInternal/Devices/efi/0/Config/DmiBIOSVersion EFI32..Virtual.Box

In general, it would be possible to override it with

VBoxManage setextradata Tiger VBoxInternal/Devices/efi/0/Config/DmiBIOSVersion AAA.BBB.CCC.DDD.EEE

This results in the System Profiler → Hardware page showing "Boot ROM Version: AAA.CCC.DDD"

Additionally, the virtual machine is set up to use the default DMI data from VirtualBox instead of propagating the host data:

VBoxManage setextradata Tiger VBoxInternal/Devices/efi/0/Config/DmiUseHostInfo 0