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Message-ID: <c3dddb6f-dce1-45a6-b5f1-1fd247c510ab@arm.com>
Date: Fri, 7 Mar 2025 14:50:56 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>, linux-arm-kernel@...ts.infradead.org
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
Mark Rutland <mark.rutland@....com>, Andrey Ryabinin
<ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
kasan-dev@...glegroups.com
Subject: Re: [PATCH] arm64/mm: Define PTE_SHIFT
On 3/7/25 14:37, Ryan Roberts wrote:
> On 07/03/2025 05:08, Anshuman Khandual wrote:
>> Address bytes shifted with a single 64 bit page table entry (any page table
>> level) has been always hard coded as 3 (aka 2^3 = 8). Although intuitive it
>> is not very readable or easy to reason about. Besides it is going to change
>> with D128, where each 128 bit page table entry will shift address bytes by
>> 4 (aka 2^4 = 16) instead.
>>
>> Let's just formalise this address bytes shift value into a new macro called
>> PTE_SHIFT establishing a logical abstraction, thus improving readability as
>> well. This does not cause any functional change.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>
>> Cc: Alexander Potapenko <glider@...gle.com>
>> Cc: Andrey Konovalov <andreyknvl@...il.com>
>> Cc: Dmitry Vyukov <dvyukov@...gle.com>
>> Cc: Ard Biesheuvel <ardb@...nel.org>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: kasan-dev@...glegroups.com
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>
>
> +1 for PTDESC_ORDER
Alright.
>
> Implementation looks good to me so:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
Thanks !
>
> one nit below.
>
>> ---
>> This patch applies on v6.14-rc5
>>
>> arch/arm64/Kconfig | 2 +-
>> arch/arm64/include/asm/kernel-pgtable.h | 3 ++-
>> arch/arm64/include/asm/pgtable-hwdef.h | 26 +++++++++++++------------
>> arch/arm64/kernel/pi/map_range.c | 2 +-
>> arch/arm64/mm/kasan_init.c | 6 +++---
>> 5 files changed, 21 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 940343beb3d4..fd3303f2ccda 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -323,7 +323,7 @@ config ARCH_MMAP_RND_BITS_MIN
>> default 18
>>
>> # max bits determined by the following formula:
>> -# VA_BITS - PAGE_SHIFT - 3
>> +# VA_BITS - PAGE_SHIFT - PTE_SHIFT
>> config ARCH_MMAP_RND_BITS_MAX
>> default 19 if ARM64_VA_BITS=36
>> default 24 if ARM64_VA_BITS=39
>> diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
>> index fd5a08450b12..7150a7a10f00 100644
>> --- a/arch/arm64/include/asm/kernel-pgtable.h
>> +++ b/arch/arm64/include/asm/kernel-pgtable.h
>> @@ -49,7 +49,8 @@
>> (SPAN_NR_ENTRIES(vstart, vend, shift) + (add))
>>
>> #define EARLY_LEVEL(lvl, lvls, vstart, vend, add) \
>> - (lvls > lvl ? EARLY_ENTRIES(vstart, vend, SWAPPER_BLOCK_SHIFT + lvl * (PAGE_SHIFT - 3), add) : 0)
>> + (lvls > lvl ? EARLY_ENTRIES(vstart, vend, SWAPPER_BLOCK_SHIFT + \
>> + lvl * (PAGE_SHIFT - PTE_SHIFT), add) : 0)
>
> nit: not sure what style guide says, but I would indent this continuation an
> extra level.
IIUC - An indentation is not normally required with a line continuation although
the starting letter should match the starting letter in the line above but after
the '(' (if any).
>
> Thanks,
> Ryan
>
>>
>> #define EARLY_PAGES(lvls, vstart, vend, add) (1 /* PGDIR page */ \
>> + EARLY_LEVEL(3, (lvls), (vstart), (vend), add) /* each entry needs a next level page table */ \
>> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
>> index a9136cc551cc..43f98eac7653 100644
>> --- a/arch/arm64/include/asm/pgtable-hwdef.h
>> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
>> @@ -7,40 +7,42 @@
>>
>> #include <asm/memory.h>
>>
>> +#define PTE_SHIFT 3
>> +
>> /*
>> * Number of page-table levels required to address 'va_bits' wide
>> * address, without section mapping. We resolve the top (va_bits - PAGE_SHIFT)
>> - * bits with (PAGE_SHIFT - 3) bits at each page table level. Hence:
>> + * bits with (PAGE_SHIFT - PTE_SHIFT) bits at each page table level. Hence:
>> *
>> - * levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
>> + * levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - PTE_SHIFT))
>> *
>> * where DIV_ROUND_UP(n, d) => (((n) + (d) - 1) / (d))
>> *
>> * We cannot include linux/kernel.h which defines DIV_ROUND_UP here
>> * due to build issues. So we open code DIV_ROUND_UP here:
>> *
>> - * ((((va_bits) - PAGE_SHIFT) + (PAGE_SHIFT - 3) - 1) / (PAGE_SHIFT - 3))
>> + * ((((va_bits) - PAGE_SHIFT) + (PAGE_SHIFT - PTE_SHIFT) - 1) / (PAGE_SHIFT - PTE_SHIFT))
>> *
>> * which gets simplified as :
>> */
>> -#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3))
>> +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - PTE_SHIFT - 1) / (PAGE_SHIFT - PTE_SHIFT))
>>
>> /*
>> * Size mapped by an entry at level n ( -1 <= n <= 3)
>> - * We map (PAGE_SHIFT - 3) at all translation levels and PAGE_SHIFT bits
>> + * We map (PAGE_SHIFT - PTE_SHIFT) at all translation levels and PAGE_SHIFT bits
>> * in the final page. The maximum number of translation levels supported by
>> * the architecture is 5. Hence, starting at level n, we have further
>> * ((4 - n) - 1) levels of translation excluding the offset within the page.
>> * So, the total number of bits mapped by an entry at level n is :
>> *
>> - * ((4 - n) - 1) * (PAGE_SHIFT - 3) + PAGE_SHIFT
>> + * ((4 - n) - 1) * (PAGE_SHIFT - PTE_SHIFT) + PAGE_SHIFT
>> *
>> * Rearranging it a bit we get :
>> - * (4 - n) * (PAGE_SHIFT - 3) + 3
>> + * (4 - n) * (PAGE_SHIFT - PTE_SHIFT) + PTE_SHIFT
>> */
>> -#define ARM64_HW_PGTABLE_LEVEL_SHIFT(n) ((PAGE_SHIFT - 3) * (4 - (n)) + 3)
>> +#define ARM64_HW_PGTABLE_LEVEL_SHIFT(n) ((PAGE_SHIFT - PTE_SHIFT) * (4 - (n)) + PTE_SHIFT)
>>
>> -#define PTRS_PER_PTE (1 << (PAGE_SHIFT - 3))
>> +#define PTRS_PER_PTE (1 << (PAGE_SHIFT - PTE_SHIFT))
>>
>> /*
>> * PMD_SHIFT determines the size a level 2 page table entry can map.
>> @@ -49,7 +51,7 @@
>> #define PMD_SHIFT ARM64_HW_PGTABLE_LEVEL_SHIFT(2)
>> #define PMD_SIZE (_AC(1, UL) << PMD_SHIFT)
>> #define PMD_MASK (~(PMD_SIZE-1))
>> -#define PTRS_PER_PMD (1 << (PAGE_SHIFT - 3))
>> +#define PTRS_PER_PMD (1 << (PAGE_SHIFT - PTE_SHIFT))
>> #endif
>>
>> /*
>> @@ -59,14 +61,14 @@
>> #define PUD_SHIFT ARM64_HW_PGTABLE_LEVEL_SHIFT(1)
>> #define PUD_SIZE (_AC(1, UL) << PUD_SHIFT)
>> #define PUD_MASK (~(PUD_SIZE-1))
>> -#define PTRS_PER_PUD (1 << (PAGE_SHIFT - 3))
>> +#define PTRS_PER_PUD (1 << (PAGE_SHIFT - PTE_SHIFT))
>> #endif
>>
>> #if CONFIG_PGTABLE_LEVELS > 4
>> #define P4D_SHIFT ARM64_HW_PGTABLE_LEVEL_SHIFT(0)
>> #define P4D_SIZE (_AC(1, UL) << P4D_SHIFT)
>> #define P4D_MASK (~(P4D_SIZE-1))
>> -#define PTRS_PER_P4D (1 << (PAGE_SHIFT - 3))
>> +#define PTRS_PER_P4D (1 << (PAGE_SHIFT - PTE_SHIFT))
>> #endif
>>
>> /*
>> diff --git a/arch/arm64/kernel/pi/map_range.c b/arch/arm64/kernel/pi/map_range.c
>> index 2b69e3beeef8..3530a5427f57 100644
>> --- a/arch/arm64/kernel/pi/map_range.c
>> +++ b/arch/arm64/kernel/pi/map_range.c
>> @@ -31,7 +31,7 @@ void __init map_range(u64 *pte, u64 start, u64 end, u64 pa, pgprot_t prot,
>> {
>> u64 cmask = (level == 3) ? CONT_PTE_SIZE - 1 : U64_MAX;
>> pteval_t protval = pgprot_val(prot) & ~PTE_TYPE_MASK;
>> - int lshift = (3 - level) * (PAGE_SHIFT - 3);
>> + int lshift = (3 - level) * (PAGE_SHIFT - PTE_SHIFT);
>> u64 lmask = (PAGE_SIZE << lshift) - 1;
>>
>> start &= PAGE_MASK;
>> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
>> index b65a29440a0c..90548079b42e 100644
>> --- a/arch/arm64/mm/kasan_init.c
>> +++ b/arch/arm64/mm/kasan_init.c
>> @@ -190,7 +190,7 @@ static void __init kasan_pgd_populate(unsigned long addr, unsigned long end,
>> */
>> static bool __init root_level_aligned(u64 addr)
>> {
>> - int shift = (ARM64_HW_PGTABLE_LEVELS(vabits_actual) - 1) * (PAGE_SHIFT - 3);
>> + int shift = (ARM64_HW_PGTABLE_LEVELS(vabits_actual) - 1) * (PAGE_SHIFT - PTE_SHIFT);
>>
>> return (addr % (PAGE_SIZE << shift)) == 0;
>> }
>> @@ -245,7 +245,7 @@ static int __init root_level_idx(u64 addr)
>> */
>> u64 vabits = IS_ENABLED(CONFIG_ARM64_64K_PAGES) ? VA_BITS
>> : vabits_actual;
>> - int shift = (ARM64_HW_PGTABLE_LEVELS(vabits) - 1) * (PAGE_SHIFT - 3);
>> + int shift = (ARM64_HW_PGTABLE_LEVELS(vabits) - 1) * (PAGE_SHIFT - PTE_SHIFT);
>>
>> return (addr & ~_PAGE_OFFSET(vabits)) >> (shift + PAGE_SHIFT);
>> }
>> @@ -269,7 +269,7 @@ static void __init clone_next_level(u64 addr, pgd_t *tmp_pg_dir, pud_t *pud)
>> */
>> static int __init next_level_idx(u64 addr)
>> {
>> - int shift = (ARM64_HW_PGTABLE_LEVELS(vabits_actual) - 2) * (PAGE_SHIFT - 3);
>> + int shift = (ARM64_HW_PGTABLE_LEVELS(vabits_actual) - 2) * (PAGE_SHIFT - PTE_SHIFT);
>>
>> return (addr >> (shift + PAGE_SHIFT)) % PTRS_PER_PTE;
>> }
>
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