我們也可以修改RISC-V Linux的內核源碼，解除2M對齊的限制。我們只需要在setup_vm()函數中，將原來的 二級頁表改為三級頁表 ，這樣kernel入口地址只需要4K對齊，因此就能將kernel往前挪，從而利用前面的內存。

修改代碼

路徑：arch/riscv/mm/init.c

注釋原來的2M對齊檢查：

對kernel的前2M頁表映射由二級頁表改為三級頁表：

//新增一個PTE
pte_t trampoline_pte[PTRS_PER_PTE] __page_aligned_bss;

create_pgd_mapping(trampoline_pg_dir,PAGE_OFFSET,
                   (uintptr_t)trampoline_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(trampoline_pmd,PAGE_OFFSET,
                   (uintptr_t)trampoline_pte,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + PMD_SIZE;
for (va = PAGE_OFFSET; va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(trampoline_pte,PAGE_OFFSET,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

對整個kernel的頁表映射由二級頁表改為三級頁表：

假設kernel大小為4M+

//定義三個PTE
pte_t load_sz_pte[PTRS_PER_PTE] __page_aligned_bss;
pte_t load_sz_pte1[PTRS_PER_PTE] __page_aligned_bss;
pte_t load_sz_pte2[PTRS_PER_PTE] __page_aligned_bss;

//=======0-2M======
create_pgd_mapping(early_pg_dir,PAGE_OFFSET,
                   (uintptr_t)early_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(early_pmd,PAGE_OFFSET,
                   (uintptr_t)load_sz_pte,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + PMD_SIZE;
for (va = PAGE_OFFSET; va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(load_sz_pte,PAGE_OFFSET,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

//=======2-4M==========
create_pgd_mapping(early_pg_dir,PAGE_OFFSET + PMD_SIZE,
                   (uintptr_t)early_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(early_pmd,PAGE_OFFSET,
                   (uintptr_t)load_sz_pte1,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + (PMD_SIZE * 2);
for (va = PAGE_OFFSET + PMD_SIZE; va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(load_sz_pte1,va,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

//=======4-6M==========
create_pgd_mapping(early_pg_dir,PAGE_OFFSET + (PMD_SIZE*2),
                   (uintptr_t)early_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(early_pmd,PAGE_OFFSET,
                   (uintptr_t)load_sz_pte2,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + (PMD_SIZE * 3);
for (va = PAGE_OFFSET + (PMD_SIZE*2); va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(load_sz_pte2,va,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

通過以上的代碼修改，就能將Kernel入口地址往前挪1.5M，只給opensbi預留512KB，這樣RISC-V Linux啟動之后，可用物理內存就會增加。