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Message-ID: <CAMj1kXFufE9UPGMsqv1ARWm6SyUCcJL+m4F4mWa0jCyhJqf2Jg@mail.gmail.com>
Date: Fri, 7 Mar 2025 08:38:28 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-arm-kernel@...ts.infradead.org,
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>, Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org,
kasan-dev@...glegroups.com
Subject: Re: [PATCH] arm64/mm: Define PTE_SHIFT
Hi Anshuman,
On Fri, 7 Mar 2025 at 06:09, Anshuman Khandual
<anshuman.khandual@....com> 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.
>
I don't disagree with this goal, but PTE_SHIFT is really not the right
name. Given that PMD_SHIFT is the log2 of the area covered by a PMD,
PTE_SHIFT should be the log2 of the area covered by a PTE, and so
defining it to anything other than PAGE_SHIFT would be a mistake IMO.
Given that we are talking about the log2 of the size of the area
occupied by a descriptor, perhaps {PT}DESC_SIZE_ORDER would be a
better name?
> 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>
> ---
> 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)
>
> #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;
> }
> --
> 2.30.2
>
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