Since UEFI requires app to be run in physical address space,
we allocate memory that are identity mapped. Previous identity
mapping works as follows:
1. call vmm_alloc_contigious to obtain a VA1(virtual address) mapping to
PA1(physical address)
2. Use arch_mmu_query to obtain PA1 from VA1
3. arch_mmu_unmap VA1 <-> PA1
4. arch_mmu_map PA1 <-> PA1
Now we can return PA1 as a valid virtual address pointer, which is
mapped to physical memory. However, this has problems. LK allocates a
vmm_region_t struct for each call to vmm_alloc_* . This struct can
never be free'ed, as the associated virtual address VA1 is forever
lost. We can't even free PA1, as PA1 is not a valid argument to
vmm_free(PA1 does not have associated vmm_region_t struct).
The new approach uses pmm_alloc and vmm_reserve_space, allowing
vmm_free_region to be called.
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