virtualbox.org

Right now our SBS2003 server is virtualized with VB 3.1.2 on a two-core un-hyperthreaded Xeon. The SBS VM was installed without IO APIC on and has only one processor in it. But we're getting a new host for our company server: Dual 6-core hyperthreaded Xeons & 32GB memory, to be ready for SBS2011 upgrade next year, and I did some tests on my Core-I7 workstation to see if I could add processors to the SBS VM. I was successful using Halu. I had to turn IO APIC on in Virtualbox to get multiple processors, then run Halu inside the VM to change the HAL so the multiple processors would show in Task Manager. I switched up to 4 processors and the VM remained highly responsive while full-blasting 3 of the 4 processors with a litlle VBS script I ran (its GUI is sluggish when under heavy use with its normal one processor now). So it seems the VM is using the extra processors.

The on-line user's manual (as of 1-2-2012) says this in section 12.3.1:

The ACPI and I/O APIC settings should never be changed after installing Windows....(Enabling them for a Windows VM which was installed without
them does not cause any harm. However, Windows will not use these features in this case.)

The underlined part is my question. Does the manual just not account for use of Halu, or is there more to IO APIC than just getting more processors, and should I be on the lookout for lurking monsters if I bump up the processor count?

You can switch it on and use Halu or have it on when the original install was created giving you SMP support. The warning is really meant to tell you to *NOT* turn it off as this will give you a non-functional guest and since it is Windows it will try to remove the SMP (multi-processor kernel) and leave you in a disaster. You can recover but it is slightly on the difficult side and not recommended for most users.