PVH header + chain load a custom stage 2.5?

Hello!

As OS kernels are quite often particular about memory layout, GDT, IRQs, etc., they often choose to set these things up themselves and not rely on the bootloader. In addition, using PVH start_info [1] is now kind of an industry standard when booting guest images in VMS (e.g. [2]).

Would it be in scope of this project to support this "PVH header + chain load a realmode next stage" loading flow? I.e. load initrd, prepare PVH start_info (basically, the memory map, initrd address/len, rsdp address), and jump into the next stage? If so, I'll hack together a prototype PR... (for MBR/BIOS only, I see no use for UEFI in a VM, sorry).

Thanks,
U.L.

[1] https://github.com/Xilinx/xen/blob/master/xen/include/public/arch-x86/hvm/start_info.h
[2] https://www.cloudhypervisor.org/docs/prologue/quick-start/

Start by reviewing the PVH start_info definition linked in the issue and the project's existing MBR/BIOS loading flow. Determine the scope of preparing the memory map, initrd address and length, and RSDP address before chain-loading a real-mode next stage. Done would mean a defined and tested PVH-header loading path for the proposed VM use case.