?? ????? RISC-V Linux ??? ?? ??? ??? ??? ??????. RISC-V Linux ?? ??? 2M ????? ??? ???????. ??? 2M ?? ??? ???? ??? ?? ????????. ???? ??? ??????.

??: ? ??? linux5.10.111 ??? ???? ???.

??? ??? ??

? ?? ???? ??? ? ?? ???? ? ??? ??? ??? ?????. bootrom? 0x0, ? ?? ????. ??? ?? ????? ?? 0x0?? ?????.

RISC-V Linux? ???? ?? ?? opensbi? ???? ??? opensbi? 0x0處，這樣芯片上電后，就會(huì)從0x0地址處執(zhí)行opensbi。在opensbi運(yùn)行完后，會(huì)跳轉(zhuǎn)到opensbi運(yùn)行地址偏移2M的位置去執(zhí)行下一級(jí)boot（這里下一級(jí)boot是kernel），即跳轉(zhuǎn)到0x200000地址處運(yùn)行kernel，因此應(yīng)該把kernel放到內(nèi)存的0x200000 ??? ????? ???.

??? ?? ?????? ??? ????.

??? ?? ?? ? ?? ???? ??? ?????(?: 0x200000) ??? ??? ??? ???? ?????. ??? ???? ?? ??? ??? ??? ????? ??? ?? ???? ???? ? ????. ??? ?? ?? 2M ???? ?????(?: 0x0 - 0x200000)? ??? ?? ???? ? 2M ???? ?? ??? ???? ???? ????. ?, ??? ? 2M ???? ???? ? ????. 0x200000）開(kāi)始建立頁(yè)表映射，只有對(duì)物理內(nèi)存建立了頁(yè)表映射，后面才能訪問(wèn)這些內(nèi)存。而kernel加載地址前面的2M內(nèi)存（即0x0 - 0x200000）將被kernel忽略，不會(huì)對(duì)這2M內(nèi)存建立頁(yè)表，即kernel無(wú)法訪問(wèn)這2M內(nèi)存。

在QEMU上RISC-V Linux的啟動(dòng)信息：

但opensbi實(shí)際不需要使用2M這么大的范圍，默認(rèn)是512KB，opensbi的pmp會(huì)保護(hù)這512KB內(nèi)存，不讓其他程序訪問(wèn)。

因此在Kernel和opensbi之間會(huì)存在1.5M

??? opensbi? ??? 2M?? ? ??? ??? ??? ????. ???? 512KB, opensbi? pmp? ??? ?????512KB ???, ?? ?????? ???? ? ????.

??? ??

Option 1

opensbi? ??? ?? ??? ?? ?? ??? ??? 2M ??? ?????.

?, ??? ??? ??? ????, opensbi? ??? ?????.

?? ?? ??? ???? 0x0地址處，opensbi放到內(nèi)存的0x10000000地址處。這樣kernel前面就不會(huì)有預(yù)留內(nèi)存，只不過(guò)這樣需要修改bootrom的地址，將地址從0x0修改為0x0x10000000。這種方案只適合芯片還沒(méi)出廠前，因?yàn)橛脩?hù)無(wú)法修改bootrom的地址，芯片出廠后，bootrom地址是固定的，假設(shè)bootrom地址為0x0，那么芯片上電后，就會(huì)從0x0開(kāi)始運(yùn)行程序，所以opensbi必須放到0x0 ??? ????? ??? 2M?? ??????.

?? 2

?? RISC-V Linux? ?? ?? ??? ???? 2M ?? ??? ??? ?? ????. setup_vm () ??? ???? ?? ? ?? ?? ??? ???? ? ?? ?? ??? ???? ???? ?? ?? ??? 4K?? ???? ??? ??? ???? ?? ???? ?????. setup_vm()函數(shù)中，將原來(lái)的二級(jí)頁(yè)表改為三級(jí)頁(yè)表，這樣kernel入口地址只需要4K對(duì)齊，因此就能將kernel往前挪，從而利用前面的內(nèi)存。

修改代碼

路徑：arch/riscv/mm/init.c

?? ??

??: ?? / riscv/mm/init.c

?? 2M ?? ??? ??? ?????.

? ?? ?? ??? ????? ? ?? ?? ??? ???? ??? ? ?? 2M ??? ??? ??? ?????.

//新增一個(gè)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);
}

//定義三個(gè)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);
}

????????? ?????. ??? ??? ???? ?? ??? 1.5M ??? ???? opensbi??? 512KB? ??? ? ????. ?? ??? RISC-V Linux? ??? ? ?? ??? ??? ???? ?????. ??

Summary

RISC-V Linux ?? ?? 2M ?? ??? ?? ???? ???? opensbi? ?? 2M? ???? ??? ??? ?? ??? ???? ????? ?? ??? 2M? ????? ???. ???? ???? ? ??? ?? ??? ???? ??? ??? ????. ? ??? ??? ???? ?? ????? ??? ?? ?? ???? ??? ? ? ??? ????.