[RFC V1 14/16] arm64/mm: Enable fixmap with 5 level page table

Anshuman Khandual
2026-02-24T05:13:35Z

Enable fixmap with 5 level page table when required. This creates table
entries at the PGD level. Add a fallback stub for pgd_page_paddr() when
(PGTBALE_LEVELS <= 4) which helps in intercepting any unintended usage.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
arch/arm64/include/asm/pgtable.h | 1 +
arch/arm64/mm/fixmap.c | 18 +++++++++++++++---
2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b39d3d3c5dfc..0f262a97e320 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1215,6 +1215,7 @@ static inline p4d_t *p4d_offset_kimg(pgd_t *pgdp, u64 addr)
#else

static inline bool pgtable_l5_enabled(void) { return false; }
+#define pgd_page_paddr(pgd) ({ BUILD_BUG(); 0; })

#define p4d_index(addr) (((addr) >> P4D_SHIFT) & (PTRS_PER_P4D - 1))

diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
index 4c2f71929777..d6209aff31d0 100644
--- a/arch/arm64/mm/fixmap.c
+++ b/arch/arm64/mm/fixmap.c
@@ -34,6 +34,7 @@ static_assert(NR_BM_PMD_TABLES == 1);
static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
+static p4d_t bm_p4d[PTRS_PER_P4D] __page_aligned_bss __maybe_unused;

static inline pte_t *fixmap_pte(unsigned long addr)
{
@@ -95,6 +96,19 @@ static void __init early_fixmap_init_pud(p4d_t *p4dp, unsigned long addr,
early_fixmap_init_pmd(pudp, addr, end);
}

+static void __init early_fixmap_init_p4d(pgd_t *pgdp, unsigned long addr,
+ unsigned long end)
+{
+ pgd_t pgd = pgdp_get(pgdp);
+ p4d_t *p4dp;
+
+ if (pgd_none(pgd))
+ __pgd_populate(pgdp, __pa_symbol(bm_p4d),
+ PGD_TYPE_TABLE | PGD_TABLE_AF);
+ p4dp = p4d_offset_kimg(pgdp, addr);
+ early_fixmap_init_pud(p4dp, addr, end);
+}
+
/*
* The p*d_populate functions call virt_to_phys implicitly so they can't be used
* directly on kernel symbols (bm_p*d). This function is called too early to use
@@ -107,9 +121,7 @@ void __init early_fixmap_init(void)
unsigned long end = FIXADDR_TOP;

pgd_t *pgdp = pgd_offset_k(addr);
- p4d_t *p4dp = p4d_offset_kimg(pgdp, addr);
-
- early_fixmap_init_pud(p4dp, addr, end);
+ early_fixmap_init_p4d(pgdp, addr, end);
}

/*
--
2.43.0