For SMP Boot Code: allocate physical memory instead

[sarahspberrypi]

Instead of forcefully writing the SMP boot code to physical address 0x8000, free physical memory is allocated.

This is fine because the application processor will install the boot processor's page table:

kernel/src/arch/x86_64/kernel/apic.rs

Lines 719 to 721 in 054d2d6

	// Pass the PML4 page table address to the boot code.
	*((SMP_BOOT_CODE_ADDRESS + SMP_BOOT_CODE_OFFSET_PML4).as_mut_ptr::<u32>()) =
	cr3().try_into().unwrap();

[mkroening]

Looks good to me. 👍

Thanks! :)

[mkroening]

As initially feared, this requires some modification of boot.s, which assumes to be loaded at 0x8000.

boot.s starts in 16-bit mode, which does not have paging yet.

Alternatively, we could allocate physical memory at exactly 0x8000 and deallocate it afterwards, if that helps.