UEFI Processor Startup


I programmed a RTX boot-loader, which claims a CPU core at Windows-Runtime. The startup is done by accessing the Interrupt Control Register (ICR) of the local APIC.
This works fine, if Windows starts up in BIOS Mode (WINLOADER.EXE).
Now with Windows 8 (10) UEFI is supported and my approach of direct accessing the local APIC (sending IPIs) doesn’t work any more.
Now, my first idea is writing an EFI boot-loader, but since windows has to run in parallel (for sharing memory), I’m not sure, if this would work.
Also I could think about any Windows services (if available), which gives access back for the local APIC and sending IPIs.
In my understanding, EFI blocks out any APIC access after the PEI (Pre EFI Initialization) phase ?
In that case, the boot-loader of Windows (WINLOAD.EFI) and the virtual subsystem had to be started somehow at PEI phase.
I’m really stuck and would be glad for any hints.