Re: [PATCH v3 6/7] arm64: mm: Implement 4 levels of translation tables

From: Steve Capper
Date: Wed Apr 23 2014 - 12:02:18 EST

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In reply to: Jungseok Lee: "[PATCH v3 6/7] arm64: mm: Implement 4 levels of translation tables"
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On Fri, Apr 18, 2014 at 04:59:20PM +0900, Jungseok Lee wrote:
> This patch implements 4 levels of translation tables since 3 levels
> of page tables with 4KB pages cannot support 40-bit physical address
> space described in [1] due to the following issue.
>
> It is a restriction that kernel logical memory map with 4KB + 3 levels
> (0xffffffc000000000-0xffffffffffffffff) cannot cover RAM region from
> 544GB to 1024GB in [1]. Specifically, ARM64 kernel fails to create
> mapping for this region in map_mem function since __phys_to_virt for
> this region reaches to address overflow.
>
> If SoC design follows the document, [1], over 32GB RAM would be placed
> from 544GB. Even 64GB system is supposed to use the region from 544GB
> to 576GB for only 32GB RAM. Naturally, it would reach to enable 4 levels
> of page tables to avoid hacking __virt_to_phys and __phys_to_virt.
>
> However, it is recommended 4 levels of page table should be only enabled
> if memory map is too sparse or there is about 512GB RAM.

Hello Jungseok,
A few comments can be found inline...

>
> References
> ----------
> [1]: Principles of ARM Memory Maps, White Paper, Issue C
>
> Signed-off-by: Jungseok Lee <jays.lee@xxxxxxxxxxx>
> Reviewed-by: Sungjinn Chung <sungjinn.chung@xxxxxxxxxxx>
> ---
> arch/arm64/Kconfig | 7 +++++
> arch/arm64/include/asm/memblock.h | 6 +++++
> arch/arm64/include/asm/page.h | 4 ++-
> arch/arm64/include/asm/pgalloc.h | 20 +++++++++++++++
> arch/arm64/include/asm/pgtable-hwdef.h | 6 +++--
> arch/arm64/include/asm/pgtable.h | 44 ++++++++++++++++++++++++++++++--
> arch/arm64/include/asm/tlb.h | 8 ++++++
> arch/arm64/kernel/head.S | 40 ++++++++++++++++++++---------
> arch/arm64/kernel/traps.c | 5 ++++
> arch/arm64/mm/fault.c | 1 +
> arch/arm64/mm/mmu.c | 16 +++++++++---
> 11 files changed, 136 insertions(+), 21 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 431acbc..7f5270b 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -184,12 +184,19 @@ config ARM64_3_LEVELS
> help
> This feature enables 3 levels of translation tables.
>
> +config ARM64_4_LEVELS
> + bool "4 level"
> + depends on ARM64_4K_PAGES
> + help
> + This feature enables 4 levels of translation tables.
> +
> endchoice
>
> config ARM64_VA_BITS
> int "Virtual address space size"
> range 39 39 if ARM64_4K_PAGES && ARM64_3_LEVELS
> range 42 42 if ARM64_64K_PAGES && ARM64_2_LEVELS
> + range 48 48 if ARM64_4K_PAGES && ARM64_4_LEVELS
> help
> This feature is determined by a combination of page size and
> level of translation tables.
> diff --git a/arch/arm64/include/asm/memblock.h b/arch/arm64/include/asm/memblock.h
> index 6afeed2..e4ac8bf 100644
> --- a/arch/arm64/include/asm/memblock.h
> +++ b/arch/arm64/include/asm/memblock.h
> @@ -16,6 +16,12 @@
> #ifndef __ASM_MEMBLOCK_H
> #define __ASM_MEMBLOCK_H
>
> +#ifndef CONFIG_ARM64_4_LEVELS
> +#define MEMBLOCK_INITIAL_LIMIT PGDIR_SIZE
> +#else
> +#define MEMBLOCK_INITIAL_LIMIT PUD_SIZE
> +#endif
> +
> extern void arm64_memblock_init(void);
>
> #endif
> diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> index 268e53d..83b5289 100644
> --- a/arch/arm64/include/asm/page.h
> +++ b/arch/arm64/include/asm/page.h
> @@ -35,8 +35,10 @@
>
> #ifdef CONFIG_ARM64_2_LEVELS
> #include <asm/pgtable-2level-types.h>
> -#else
> +#elif defined(CONFIG_ARM64_3_LEVELS)
> #include <asm/pgtable-3level-types.h>
> +#else
> +#include <asm/pgtable-4level-types.h>
> #endif
>
> extern void __cpu_clear_user_page(void *p, unsigned long user);
> diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
> index 4829837..8d745fa 100644
> --- a/arch/arm64/include/asm/pgalloc.h
> +++ b/arch/arm64/include/asm/pgalloc.h
> @@ -26,6 +26,26 @@
>
> #define check_pgt_cache() do { } while (0)
>
> +#ifdef CONFIG_ARM64_4_LEVELS
> +
> +static inline pud_t *pud_alloc_one(struct mm_struct *mm, unsigned long addr)
> +{
> + return (pud_t *)get_zeroed_page(GFP_KERNEL | __GFP_REPEAT);
> +}
> +
> +static inline void pud_free(struct mm_struct *mm, pud_t *pud)
> +{
> + BUG_ON((unsigned long)pud & (PAGE_SIZE-1));
> + free_page((unsigned long)pud);
> +}
> +
> +static inline void pgd_populate(struct mm_struct *mm, pgd_t *pgd, pud_t *pud)
> +{
> + set_pgd(pgd, __pgd(__pa(pud) | PUD_TYPE_TABLE));
> +}
> +
> +#endif /* CONFIG_ARM64_4_LEVELS */
> +
> #ifndef CONFIG_ARM64_2_LEVELS
>
> static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
> index 9cd86c6..ba30053 100644
> --- a/arch/arm64/include/asm/pgtable-hwdef.h
> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
> @@ -18,8 +18,10 @@
>
> #ifdef CONFIG_ARM64_2_LEVELS
> #include <asm/pgtable-2level-hwdef.h>
> -#else
> +#elif defined(CONFIG_ARM64_3_LEVELS)
> #include <asm/pgtable-3level-hwdef.h>
> +#else
> +#include <asm/pgtable-4level-hwdef.h>
> #endif
>
> /*
> @@ -27,7 +29,7 @@
> *
> * Level 1 descriptor (PUD).
> */
> -
> +#define PUD_TYPE_TABLE (_AT(pudval_t, 3) << 0)
> #define PUD_TABLE_BIT (_AT(pgdval_t, 1) << 1)
>
> /*
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index a64ce5e..efc40d1 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -35,7 +35,11 @@
> * VMALLOC and SPARSEMEM_VMEMMAP ranges.
> */
> #define VMALLOC_START (UL(0xffffffffffffffff) << VA_BITS)
> +#ifndef CONFIG_ARM64_4_LEVELS
> #define VMALLOC_END (PAGE_OFFSET - UL(0x400000000) - SZ_64K)
> +#else
> +#define VMALLOC_END (PAGE_OFFSET - UL(0x40000000000) - SZ_64K)
> +#endif
>
> #define vmemmap ((struct page *)(VMALLOC_END + SZ_64K))
>
> @@ -44,12 +48,16 @@
> #ifndef __ASSEMBLY__
> extern void __pte_error(const char *file, int line, unsigned long val);
> extern void __pmd_error(const char *file, int line, unsigned long val);
> +extern void __pud_error(const char *file, int line, unsigned long val);
> extern void __pgd_error(const char *file, int line, unsigned long val);
>
> #define pte_ERROR(pte) __pte_error(__FILE__, __LINE__, pte_val(pte))
> #ifndef CONFIG_ARM64_2_LEVELS
> #define pmd_ERROR(pmd) __pmd_error(__FILE__, __LINE__, pmd_val(pmd))
> #endif
> +#ifdef CONFIG_ARM64_4_LEVELS
> +#define pud_ERROR(pud) __pud_error(__FILE__, __LINE__, pud_val(pud))
> +#endif
> #define pgd_ERROR(pgd) __pgd_error(__FILE__, __LINE__, pgd_val(pgd))
>
> /*
> @@ -344,6 +352,30 @@ static inline pmd_t *pud_page_vaddr(pud_t pud)
>
> #endif /* CONFIG_ARM64_2_LEVELS */
>
> +#ifdef CONFIG_ARM64_4_LEVELS
> +
> +#define pgd_none(pgd) (!pgd_val(pgd))
> +#define pgd_bad(pgd) (!(pgd_val(pgd) & 2))
> +#define pgd_present(pgd) (pgd_val(pgd))
> +
> +static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
> +{
> + *pgdp = pgd;
> + dsb();
> +}
> +
> +static inline void pgd_clear(pgd_t *pgdp)
> +{
> + set_pgd(pgdp, __pgd(0));
> +}
> +
> +static inline pud_t *pgd_page_vaddr(pgd_t pgd)
> +{
> + return __va(pgd_val(pgd) & PHYS_MASK & (s32)PAGE_MASK);
> +}
> +
> +#endif /* CONFIG_ARM64_4_LEVELS */
> +
> /* to find an entry in a page-table-directory */
> #define pgd_index(addr) (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1))
>
> @@ -352,6 +384,14 @@ static inline pmd_t *pud_page_vaddr(pud_t pud)
> /* to find an entry in a kernel page-table-directory */
> #define pgd_offset_k(addr) pgd_offset(&init_mm, addr)
>
> +#ifdef CONFIG_ARM64_4_LEVELS
> +#define pud_index(addr) (((addr) >> PUD_SHIFT) & (PTRS_PER_PUD - 1))
> +static inline pud_t *pud_offset(pgd_t *pgd, unsigned long addr)
> +{
> + return (pud_t *)pgd_page_vaddr(*pgd) + pud_index(addr);
> +}
> +#endif
> +
> /* Find an entry in the second-level page table.. */
> #ifndef CONFIG_ARM64_2_LEVELS
> #define pmd_index(addr) (((addr) >> PMD_SHIFT) & (PTRS_PER_PMD - 1))
> @@ -380,8 +420,8 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
> extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
>
> -#define SWAPPER_DIR_SIZE (3 * PAGE_SIZE)
> -#define IDMAP_DIR_SIZE (2 * PAGE_SIZE)
> +#define SWAPPER_DIR_SIZE (4 * PAGE_SIZE)
> +#define IDMAP_DIR_SIZE (3 * PAGE_SIZE)
>
> /*
> * Encode and decode a swap entry:
> diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
> index df378b2..dedfb04 100644
> --- a/arch/arm64/include/asm/tlb.h
> +++ b/arch/arm64/include/asm/tlb.h
> @@ -99,5 +99,13 @@ static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmdp,
> }
> #endif
>
> +#ifdef CONFIG_ARM64_4_LEVELS
> +static inline void __pud_free_tlb(struct mmu_gather *tlb, pmd_t *pudp,
> + unsigned long addr)
> +{
> + tlb_add_flush(tlb, addr);
> + tlb_remove_page(tlb, virt_to_page(pudp));
> +}
> +#endif
>
> #endif
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 0fd5650..f313a7a 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -37,8 +37,8 @@
>
> /*
> * swapper_pg_dir is the virtual address of the initial page table. We place
> - * the page tables 3 * PAGE_SIZE below KERNEL_RAM_VADDR. The idmap_pg_dir has
> - * 2 pages and is placed below swapper_pg_dir.
> + * the page tables 4 * PAGE_SIZE below KERNEL_RAM_VADDR. The idmap_pg_dir has
> + * 3 pages and is placed below swapper_pg_dir.
> */
> #define KERNEL_RAM_VADDR (PAGE_OFFSET + TEXT_OFFSET)
>
> @@ -46,8 +46,8 @@
> #error KERNEL_RAM_VADDR must start at 0xXXX80000
> #endif
>
> -#define SWAPPER_DIR_SIZE (3 * PAGE_SIZE)
> -#define IDMAP_DIR_SIZE (2 * PAGE_SIZE)
> +#define SWAPPER_DIR_SIZE (4 * PAGE_SIZE)
> +#define IDMAP_DIR_SIZE (3 * PAGE_SIZE)
>
> .globl swapper_pg_dir
> .equ swapper_pg_dir, KERNEL_RAM_VADDR - SWAPPER_DIR_SIZE
> @@ -371,16 +371,29 @@ ENDPROC(__calc_phys_offset)
>
> /*
> * Macro to populate the PGD for the corresponding block entry in the next
> - * level (tbl) for the given virtual address.
> + * levels (tbl1 and tbl2) for the given virtual address.
> *
> - * Preserves: pgd, tbl, virt
> + * Preserves: pgd, tbl1, tbl2, virt

tbl1 and tbl2 are *not* preserved for 4 level. tbl1 is bumped up one
page to make space for the pud, then fed into create_block_mapping
later.

> * Corrupts: tmp1, tmp2
> */
> - .macro create_pgd_entry, pgd, tbl, virt, tmp1, tmp2
> + .macro create_pgd_entry, pgd, tbl1, tbl2, virt, tmp1, tmp2
> lsr \tmp1, \virt, #PGDIR_SHIFT
> and \tmp1, \tmp1, #PTRS_PER_PGD - 1 // PGD index
> - orr \tmp2, \tbl, #3 // PGD entry table type
> + orr \tmp2, \tbl1, #3 // PGD entry table type
> str \tmp2, [\pgd, \tmp1, lsl #3]
> +#ifdef CONFIG_ARM64_4_LEVELS
> + ldr \tbl2, =FIXADDR_TOP
> + cmp \tbl2, \virt

Do we need this extra logic? See my other comment below where the fixed
mapping is placed down.

> + add \tbl2, \tbl1, #PAGE_SIZE
> + b.ne 1f
> + add \tbl2, \tbl2, #PAGE_SIZE
> +1:
> + lsr \tmp1, \virt, #PUD_SHIFT
> + and \tmp1, \tmp1, #PTRS_PER_PUD - 1 // PUD index
> + orr \tmp2, \tbl2, #3 // PUD entry table type
> + str \tmp2, [\tbl1, \tmp1, lsl #3]
> + mov \tbl1, \tbl2
> +#endif

It may be easier to read to have a create_pud_entry macro too?

> .endm
>
> /*
> @@ -444,7 +457,7 @@ __create_page_tables:
> add x0, x25, #PAGE_SIZE // section table address
> ldr x3, =KERNEL_START
> add x3, x3, x28 // __pa(KERNEL_START)
> - create_pgd_entry x25, x0, x3, x5, x6
> + create_pgd_entry x25, x0, x1, x3, x5, x6
> ldr x6, =KERNEL_END
> mov x5, x3 // __pa(KERNEL_START)
> add x6, x6, x28 // __pa(KERNEL_END)
> @@ -455,7 +468,7 @@ __create_page_tables:
> */
> add x0, x26, #PAGE_SIZE // section table address
> mov x5, #PAGE_OFFSET
> - create_pgd_entry x26, x0, x5, x3, x6
> + create_pgd_entry x26, x0, x1, x5, x3, x6
> ldr x6, =KERNEL_END
> mov x3, x24 // phys offset
> create_block_map x0, x7, x3, x5, x6
> @@ -480,8 +493,11 @@ __create_page_tables:
> * Create the pgd entry for the fixed mappings.
> */
> ldr x5, =FIXADDR_TOP // Fixed mapping virtual address
> - add x0, x26, #2 * PAGE_SIZE // section table address
> - create_pgd_entry x26, x0, x5, x6, x7
> + add x0, x26, #PAGE_SIZE
> +#ifndef CONFIG_ARM64_4_LEVELS
> + add x0, x0, #PAGE_SIZE
> +#endif

This is overly complicated. For <4 levels we set x0 to be:
ttbr1 + 2*PAGE_SIZE. For 4-levels, we set x0 to be ttbr1 + PAGE_SIZE,
then inside the create_pgd_entry macro, we check the VA for FIXADDR_TOP
then add another PAGE_SIZE. This is presumably done so the same PUD is
used for the swapper block map and the FIXADDR map.

If you assume that the PUD always follows the PGD for 4-levels, then
you can remove this #ifdef and the conditional VA logic in
set_pgd_entry. To make the logic simpler for <4 levels, you could call
create_pud_entry in the middle of create_pgd_entry, then put down the
actual pgd after.

> + create_pgd_entry x26, x0, x1, x5, x6, x7
>

So before this patch we have the following created by
__create_page_tables:

+========================+ <--- TEXT_OFFSET + PHYS_OFFSET
| FIXADDR (pmd or pte) |
+------------------------+
| block map (pmd or pte) |
+------------------------+
| PGDs for swapper |
+========================+ <--- TTBR1 swapper_pg_dir
| block map for idmap |
+------------------------+
| PGDs for idmap |
+------------------------+ <--- TTBR0 idmap_pg_dir

After the patch, for 4 levels activated we have:
+========================+ <--- TEXT_OFFSET + PHYS_OFFSET
| FIXADDR (ptes) |
+------------------------+
| block map (ptes) |
+------------------------+
| PUDs for swapper |
+------------------------+
| PGDs for swapper |
+========================+ <--- TTBR1 swapper_pg_dir
| block map for idmap |
+------------------------+
| PUDs for idmap |
+------------------------+
| PGDs for idmap |
+------------------------+ <--- TTBR0 idmap_pg_dir

and without 4 levels activated we have:
+========================+ <--- TEXT_OFFSET + PHYS_OFFSET
| ZERO BYTES |
+------------------------+
| FIXADDR (pmd or pte) |
+------------------------+
| block map (pmd or pte) |
+------------------------+
| PGDs for swapper |
+========================+ <--- TTBR1 swapper_pg_dir
| ZERO BYTES |
+------------------------+
| block map for idmap |
+------------------------+
| PGDs for idmap |
+------------------------+ <--- TTBR0 idmap_pg_dir

This is a pity as we are potentially throwing away 128KB.
I would recommend only extending the sizes of IDMAP_DIR_SIZE and
SWAPPER_DIR_SIZE if necessary.

> /*
> * Since the page tables have been populated with non-cacheable
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 0484e81..16d5ee5 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -336,6 +336,11 @@ void __pmd_error(const char *file, int line, unsigned long val)
> pr_crit("%s:%d: bad pmd %016lx.\n", file, line, val);
> }
>
> +void __pud_error(const char *file, int line, unsigned long val)
> +{
> + pr_crit("%s:%d: bad pud %016lx.\n", file, line, val);
> +}
> +
> void __pgd_error(const char *file, int line, unsigned long val)
> {
> pr_crit("%s:%d: bad pgd %016lx.\n", file, line, val);
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index c23751b..ed4a343 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -61,6 +61,7 @@ void show_pte(struct mm_struct *mm, unsigned long addr)
> break;
>
> pud = pud_offset(pgd, addr);
> + printk(", *pud=%016llx", pud_val(*pud));
> if (pud_none(*pud) || pud_bad(*pud))
> break;
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 6b7e895..4d29332 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -32,6 +32,7 @@
> #include <asm/setup.h>
> #include <asm/sizes.h>
> #include <asm/tlb.h>
> +#include <asm/memblock.h>
> #include <asm/mmu_context.h>
>
> #include "mm.h"
> @@ -222,9 +223,15 @@ static void __init alloc_init_pmd(pud_t *pud, unsigned long addr,
> static void __init alloc_init_pud(pgd_t *pgd, unsigned long addr,
> unsigned long end, unsigned long phys)
> {
> - pud_t *pud = pud_offset(pgd, addr);
> + pud_t *pud;
> unsigned long next;
>
> + if (pgd_none(*pgd) || pgd_bad(*pgd)) {
> + pud = early_alloc(PTRS_PER_PUD * sizeof(pud_t));
> + pgd_populate(&init_mm, pgd, pud);
> + }
> +
> + pud = pud_offset(pgd, addr);
> do {
> next = pud_addr_end(addr, end);
> alloc_init_pmd(pud, addr, next, phys);
> @@ -271,10 +278,11 @@ static void __init map_mem(void)
> * memory addressable from the initial direct kernel mapping.
> *
> * The initial direct kernel mapping, located at swapper_pg_dir,
> - * gives us PGDIR_SIZE memory starting from PHYS_OFFSET (which must be
> - * aligned to 2MB as per Documentation/arm64/booting.txt).
> + * gives us PGDIR_SIZE (2 and 3 levels) or PUD_SIZE (4 levels) memory
> + * starting from PHYS_OFFSET (which must be aligned to 2MB as per
> + * Documentation/arm64/booting.txt).
> */
> - limit = PHYS_OFFSET + PGDIR_SIZE;
> + limit = PHYS_OFFSET + MEMBLOCK_INITIAL_LIMIT;
> memblock_set_current_limit(limit);
>
> /* map all the memory banks */
> --
> 1.7.10.4
>
>
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