Bootloader MMU & HHDM

Document Revision: 26h1.0
Source: arch/s390x/init/zxfl/common/mmu.c

1. Purpose

Before transferring control to the kernel, Stage 1 must enable DAT (Dynamic Address Translation) and establish the virtual address space the kernel expects. This involves building a 5-level page table hierarchy with two mappings:

HHDM_BASE = 0xFFFF800000000000 (CONFIG_KERNEL_VIRT_OFFSET).

2. Page Table Allocation

The bootloader allocates page tables from a bump allocator backed by a contiguous physical region immediately after the kernel image. The region base is the first 1 MB-aligned address after kernel_phys_end, floored at 32 MB. The end of this region is recorded in proto->pgtbl_pool_end.

The kernel PMM must mark [pool_base, pgtbl_pool_end) as reserved during initialization.

3. Build Sequence

zxfl_mmu_setup_and_jump(proto, entry_point)
  │
  ├─ Allocate R1 table (16 KB, zero-filled)
  ├─ For each 4 KB page in [0, RAM):
  │    ├─ Map VA = PA         (identity)
  │    └─ Map VA = PA + HHDM  (HHDM)
  ├─ Build ASCE: R1_phys | DT=11 | TL=2048
  ├─ Load ASCE into CR1 (LCTL)
  ├─ Translate all proto pointer fields to HHDM virtual
  ├─ Set PSW.DAT = 1 in the new PSW
  └─ LPSWE → entry_point (DAT on, interrupts masked)

Large pages (EDAT-1 / EDAT-2) are used if the corresponding STFLE facility is present, reducing the number of page table entries required.

4. Pointer Translation

All pointer fields in zxfl_boot_protocol_t that reference physical memory are translated to HHDM virtual addresses before the jump:

$$va = pa + \texttt{CONFIG_KERNEL_VIRT_OFFSET}$$

This includes mem_map_addr, kernel_entry, kernel_stack_top, cmdline_addr, and lowcore_phys. The kernel must not attempt to dereference any protocol pointer as a physical address.

5. State at Kernel Entry