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Commit 671f9a3e authored by Anup Patel's avatar Anup Patel Committed by Paul Walmsley
Browse files

RISC-V: Setup initial page tables in two stages 



Currently, the setup_vm() does initial page table setup in one-shot
very early before enabling MMU. Due to this, the setup_vm() has to map
all possible kernel virtual addresses since it does not know size and
location of RAM. This means we have kernel mappings for non-existent
RAM and any buggy driver (or kernel) code doing out-of-bound access
to RAM will not fault and cause underterministic behaviour.

Further, the setup_vm() creates PMD mappings (i.e. 2M mappings) for
RV64 systems. This means for PAGE_OFFSET=0xffffffe000000000 (i.e.
MAXPHYSMEM_128GB=y), the setup_vm() will require 129 pages (i.e.
516 KB) of memory for initial page tables which is never freed. The
memory required for initial page tables will further increase if
we chose a lower value of PAGE_OFFSET (e.g. 0xffffff0000000000)

This patch implements two-staged initial page table setup, as follows:
1. Early (i.e. setup_vm()): This stage maps kernel image and DTB in
a early page table (i.e. early_pg_dir). The early_pg_dir will be used
only by boot HART so it can be freed as-part of init memory free-up.
2. Final (i.e. setup_vm_final()): This stage maps all possible RAM
banks in the final page table (i.e. swapper_pg_dir). The boot HART
will start using swapper_pg_dir at the end of setup_vm_final(). All
non-boot HARTs directly use the swapper_pg_dir created by boot HART.

We have following advantages with this new approach:
1. Kernel mappings for non-existent RAM don't exists anymore.
2. Memory consumed by initial page tables is now indpendent of the
chosen PAGE_OFFSET.
3. Memory consumed by initial page tables on RV64 system is 2 pages
(i.e. 8 KB) which has significantly reduced and these pages will be
freed as-part of the init memory free-up.

The patch also provides a foundation for implementing strict kernel
mappings where we protect kernel text and rodata using PTE permissions.

Suggested-by: default avatarMike Rapoport <rppt@linux.ibm.com>
Signed-off-by: default avatarAnup Patel <anup.patel@wdc.com>
[paul.walmsley@sifive.com: updated to apply; fixed a checkpatch warning]
Signed-off-by: default avatarPaul Walmsley <paul.walmsley@sifive.com>
parent 2ebca1cb

arch/riscv/include/asm/fixmap.h

+5 −0
Original line number Diff line number Diff line
@@ -21,6 +21,11 @@ 
 */

enum fixed_addresses {

	FIX_HOLE,

#define FIX_FDT_SIZE	SZ_1M

	FIX_FDT_END,

	FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,

	FIX_PTE,

	FIX_PMD,

	FIX_EARLYCON_MEM_BASE,

	__end_of_fixed_addresses

};

arch/riscv/include/asm/pgtable-64.h

+5 −0
Original line number Diff line number Diff line
@@ -70,6 +70,11 @@ static inline pmd_t pfn_pmd(unsigned long pfn, pgprot_t prot) 
	return __pmd((pfn << _PAGE_PFN_SHIFT) | pgprot_val(prot));

}



static inline unsigned long _pmd_pfn(pmd_t pmd)

{

	return pmd_val(pmd) >> _PAGE_PFN_SHIFT;

}



#define pmd_ERROR(e) \

	pr_err("%s:%d: bad pmd %016lx.\n", __FILE__, __LINE__, pmd_val(e))

arch/riscv/include/asm/pgtable.h

+8 −0
Original line number Diff line number Diff line
@@ -59,6 +59,8 @@ 
#define PAGE_KERNEL		__pgprot(_PAGE_KERNEL)

#define PAGE_KERNEL_EXEC	__pgprot(_PAGE_KERNEL | _PAGE_EXEC)



#define PAGE_TABLE		__pgprot(_PAGE_TABLE)



extern pgd_t swapper_pg_dir[];



/* MAP_PRIVATE permissions: xwr (copy-on-write) */
@@ -118,6 +120,11 @@ static inline pgd_t pfn_pgd(unsigned long pfn, pgprot_t prot) 
	return __pgd((pfn << _PAGE_PFN_SHIFT) | pgprot_val(prot));

}



static inline unsigned long _pgd_pfn(pgd_t pgd)

{

	return pgd_val(pgd) >> _PAGE_PFN_SHIFT;

}



#define pgd_index(addr) (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1))



/* Locate an entry in the page global directory */
@@ -400,6 +407,7 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma, 
#define kern_addr_valid(addr)   (1) /* FIXME */

#endif



extern void *dtb_early_va;

extern void setup_bootmem(void);

extern void paging_init(void);

arch/riscv/kernel/head.S

+9 −8
Original line number Diff line number Diff line
@@ -55,7 +55,9 @@ clear_bss_done: 


	/* Initialize page tables and relocate to virtual addresses */

	la sp, init_thread_union + THREAD_SIZE

	mv a0, s1

	call setup_vm

	la a0, early_pg_dir

	call relocate



	/* Restore C environment */
@@ -64,25 +66,23 @@ clear_bss_done: 
	la sp, init_thread_union + THREAD_SIZE



	/* Start the kernel */

	mv a0, s1

	call parse_dtb

	tail start_kernel



relocate:

	/* Relocate return address */

	li a1, PAGE_OFFSET

	la a0, _start

	sub a1, a1, a0

	la a2, _start

	sub a1, a1, a2

	add ra, ra, a1



	/* Point stvec to virtual address of intruction after satp write */

	la a0, 1f

	add a0, a0, a1

	csrw CSR_STVEC, a0

	la a2, 1f

	add a2, a2, a1

	csrw CSR_STVEC, a2



	/* Compute satp for kernel page tables, but don't load it yet */

	la a2, swapper_pg_dir

	srl a2, a2, PAGE_SHIFT

	srl a2, a0, PAGE_SHIFT

	li a1, SATP_MODE

	or a2, a2, a1
@@ -148,6 +148,7 @@ relocate: 
	fence



	/* Enable virtual memory and relocate to virtual address */

	la a0, swapper_pg_dir

	call relocate



	tail smp_callin

arch/riscv/kernel/setup.c

+2 −4
Original line number Diff line number Diff line
@@ -39,11 +39,9 @@ struct screen_info screen_info = { 
atomic_t hart_lottery;

unsigned long boot_cpu_hartid;



void __init parse_dtb(phys_addr_t dtb_phys)

void __init parse_dtb(void)

{

	void *dtb = __va(dtb_phys);



	if (early_init_dt_scan(dtb))

	if (early_init_dt_scan(dtb_early_va))

		return;



	pr_err("No DTB passed to the kernel\n");