Kernel should handle, QEMU highmem=off

[manaskamal]

Turning off highmem in qemu virt, causes kernel to crash, because of 4GiB physical memory limit. Kernel should rely on dtb for hardware mmio too.

[kheersagarpatel]

Hi @manaskamal,

Adding PR #57 as a reference.

During the ARM64/GCC bring-up, I was testing primarily with QEMU using -machine virt,gic-version=2,highmem=off, so this issue became a major blocker.

PR #57 includes architectural fixes to the ARM64 physical memory manager, including dynamic HighestAddress tracking for bitmap sizing, along with related MMIO and early initialization fixes required for the highmem=off configuration.

These PMM changes are architecture-level changes rather than GCC-specific work, so the implementation is toolchain-independent. However, I've only validated the complete boot flow on the GCC/Linux bring-up so far.

The DTB-based MMIO discovery mentioned in this issue is still a separate enhancement and isn't addressed by this PR.

It would be great to retest this issue after PR #57 is merged to confirm whether the reported crash is resolved.