[PATCHv2 13/18] arm64: mm: add functions to walk tables in fixmap

As a prepratory step to allow us to allocate early page tables from
unmapped memory using memblock_alloc, add new p??_fixmap* functions that
can be used to walk page tables outside of the linear mapping by using
fixmap slots.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jeremy Linton <redacted>
Cc: Laura Abbott <redacted>
Cc: Will Deacon <redacted>
---
  arch/arm64/include/asm/fixmap.h  | 10 ++++++++++
  arch/arm64/include/asm/pgtable.h | 26 ++++++++++++++++++++++++++
  2 files changed, 36 insertions(+)

diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
index 3097045..1a617d4 100644
--- a/arch/arm64/include/asm/fixmap.h
+++ b/arch/arm64/include/asm/fixmap.h

@@ -62,6 +62,16 @@ enum fixed_addresses {

  	FIX_BTMAP_END = __end_of_permanent_fixed_addresses,
  	FIX_BTMAP_BEGIN = FIX_BTMAP_END + TOTAL_FIX_BTMAPS - 1,
+
+	/*
+	 * Used for kernel page table creation, so unmapped memory may be used
+	 * for tables.
+	 */
+	FIX_PTE,
+	FIX_PMD,
+	FIX_PUD,
+	FIX_PGD,
+
  	__end_of_fixed_addresses
  };

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index f5742db..824e7f0 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -57,6 +57,7 @@

  #ifndef __ASSEMBLY__

+#include <asm/fixmap.h>
  #include <linux/mmdebug.h>

  extern void __pte_error(const char *file, int line, unsigned long val);

@@ -442,6 +443,10 @@ static inline phys_addr_t pmd_page_paddr(pmd_t pmd)
  #define pte_unmap(pte)			do { } while (0)
  #define pte_unmap_nested(pte)		do { } while (0)

+#define pte_fixmap(addr)		((pte_t *)set_fixmap_offset(FIX_PTE, addr))
+#define pte_fixmap_offset(pmd, addr)	pte_fixmap(pte_offset_phys(pmd, addr))
+#define pte_fixmap_unmap()		clear_fixmap(FIX_PTE)
+
  #define pmd_page(pmd)		pfn_to_page(__phys_to_pfn(pmd_val(pmd) & PHYS_MASK))

  /*

@@ -481,12 +486,21 @@ static inline phys_addr_t pud_page_paddr(pud_t pud)
  #define pmd_offset_phys(dir, addr)	(pud_page_paddr(*(dir)) + pmd_index(addr) * sizeof(pmd_t))
  #define pmd_offset(dir, addr)		((pmd_t *)__va(pmd_offset_phys((dir), (addr))))

+#define pmd_fixmap(addr)		((pmd_t *)set_fixmap_offset(FIX_PMD, addr))
+#define pmd_fixmap_offset(pud, addr)	pmd_fixmap(pmd_offset_phys(pud, addr))
+#define pmd_fixmap_unmap()		clear_fixmap(FIX_PMD)
+
  #define pud_page(pud)		pfn_to_page(__phys_to_pfn(pud_val(pud) & PHYS_MASK))

  #else

  #define pud_page_paddr(pud)	({ BUILD_BUG(); 0; })

+/* Match pmd_offset folding in <asm/generic/pgtable-nopmd.h> */
+#define pmd_fixmap(addr)		NULL
+#define pmd_fixmap_offset(pudp, addr)	((pmd_t *)pudp)
+#define pmd_fixmap_unmap()
+
  #endif	/* CONFIG_PGTABLE_LEVELS > 2 */

  #if CONFIG_PGTABLE_LEVELS > 3

@@ -519,12 +533,21 @@ static inline phys_addr_t pgd_page_paddr(pgd_t pgd)
  #define pud_offset_phys(dir, addr)	(pgd_page_paddr(*(dir)) + pud_index(addr) * sizeof(pud_t))
  #define pud_offset(dir, addr)		((pud_t *)__va(pud_offset_phys((dir), (addr))))

+#define pud_fixmap(addr)		((pud_t *)set_fixmap_offset(FIX_PUD, addr))
+#define pud_fixmap_offset(pgd, addr)	pud_fixmap(pmd_offset_phys(pgd, addr))
+#define pud_fixmap_unmap()		clear_fixmap(FIX_PUD)
+
  #define pgd_page(pgd)		pfn_to_page(__phys_to_pfn(pgd_val(pgd) & PHYS_MASK))

  #else

  #define pgd_page_paddr(pgd)	({ BUILD_BUG(); 0;})

+/* Match pud_offset folding in <asm/generic/pgtable-nopud.h> */
+#define pud_fixmap(addr)		NULL
+#define pud_fixmap_offset(pgdp, addr)	((pud_t *)pgdp)
+#define pud_fixmap_unmap()
+
  #endif  /* CONFIG_PGTABLE_LEVELS > 3 */

  #define pgd_ERROR(pgd)		__pgd_error(__FILE__, __LINE__, pgd_val(pgd))

@@ -539,6 +562,9 @@ static inline phys_addr_t pgd_page_paddr(pgd_t pgd)
  /* to find an entry in a kernel page-table-directory */
  #define pgd_offset_k(addr)	pgd_offset(&init_mm, addr)

+#define pgd_fixmap(addr)		((pgd_t *)set_fixmap_offset(FIX_PGD, addr))
+#define pgd_fixmap_unmap()		clear_fixmap(FIX_PGD)
+
  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
  {
  	const pteval_t mask = PTE_USER | PTE_PXN | PTE_UXN | PTE_RDONLY |