arm64: mm: always map fixmap at page granularity

Today the fixmap code largely maps elements at PAGE_SIZE granularity,
but we special-case the FDT mapping such that it can be mapped with 2M
block mappings when 4K pages are in use. The original rationale for this
was simplicity, but it has some unfortunate side-effects, and
complicates portions of the fixmap code (i.e. is not so simple after
all).

The FDT can be up to 2M in size but is only required to have 8-byte
alignment, and so it may straddle a 2M boundary. Thus when using 2M
block mappings we may map up to 4M of memory surrounding the FDT. This
is unfortunate as most of that memory will be unrelated to the FDT, and
any pages which happen to share a 2M block with the FDT will by mapped
with Normal Write-Back Cacheable attributes, which might not be what we
want elsewhere (e.g. for carve-outs using Non-Cacheable attributes).

The logic to handle mapping the FDT with 2M blocks requires some special
cases in the fixmap code, and ties it to the early page table
configuration by virtue of the SWAPPER_TABLE_SHIFT and
SWAPPER_BLOCK_SIZE constants used to determine the granularity used to
map the FDT.

This patch simplifies the FDT logic and removes the unnecessary mappings
of surrounding pages by always mapping the FDT at page granularity as
with all other fixmap mappings. To do so we statically reserve multiple
PTE tables to cover the fixmap VA range. Since the FDT can be at most
2M, for 4K pages we only need to allocate a single additional PTE table,
and for 16K and 64K pages the existing single PTE table is sufficient.

The PTE table allocation scales with the number of slots reserved in the
fixmap, and so this also makes it easier to add more fixmap entries if
we require those in future.

Our VA layout means that the fixmap will always fall within a single PMD
table (and consequently, within a single PUD/P4D/PGD entry), which we
can verify at compile time with a static_assert(). With that assert a
number of runtime warnings become impossible, and are removed.

I've boot-tested this patch with both 4K and 64K pages.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20230406152759.4164229-4-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/fixmap.h

@@ -17,6 +17,7 @@
 
 #ifndef __ASSEMBLY__
 #include <linux/kernel.h>
+#include <linux/math.h>
 #include <linux/sizes.h>
 #include <asm/boot.h>
 #include <asm/page.h>
@@ -36,17 +37,13 @@ enum fixed_addresses {
 	FIX_HOLE,
 
 	/*
-	 * Reserve a virtual window for the FDT that is 2 MB larger than the
-	 * maximum supported size, and put it at the top of the fixmap region.
-	 * The additional space ensures that any FDT that does not exceed
-	 * MAX_FDT_SIZE can be mapped regardless of whether it crosses any
-	 * 2 MB alignment boundaries.
-	 *
-	 * Keep this at the top so it remains 2 MB aligned.
+	 * Reserve a virtual window for the FDT that is a page bigger than the
+	 * maximum supported size. The additional space ensures that any FDT
+	 * that does not exceed MAX_FDT_SIZE can be mapped regardless of
+	 * whether it crosses any page boundary.
 	 */
-#define FIX_FDT_SIZE		(MAX_FDT_SIZE + SZ_2M)
 	FIX_FDT_END,
-	FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
+	FIX_FDT = FIX_FDT_END + DIV_ROUND_UP(MAX_FDT_SIZE, PAGE_SIZE) + 1,
 
 	FIX_EARLYCON_MEM_BASE,
 	FIX_TEXT_POKE0,

arch/arm64/include/asm/kernel-pgtable.h

@@ -59,8 +59,11 @@
 #define EARLY_KASLR	(0)
 #endif
 
+#define SPAN_NR_ENTRIES(vstart, vend, shift) \
+	((((vend) - 1) >> (shift)) - ((vstart) >> (shift)) + 1)
+
 #define EARLY_ENTRIES(vstart, vend, shift, add) \
-	((((vend) - 1) >> (shift)) - ((vstart) >> (shift)) + 1 + add)
+	(SPAN_NR_ENTRIES(vstart, vend, shift) + (add))
 
 #define EARLY_PGDS(vstart, vend, add) (EARLY_ENTRIES(vstart, vend, PGDIR_SHIFT, add))
 

arch/arm64/mm/fixmap.c

@@ -16,34 +16,77 @@
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 
-static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
+#define NR_BM_PTE_TABLES \
+	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PMD_SHIFT)
+#define NR_BM_PMD_TABLES \
+	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PUD_SHIFT)
+
+static_assert(NR_BM_PMD_TABLES == 1);
+
+#define __BM_TABLE_IDX(addr, shift) \
+	(((addr) >> (shift)) - (FIXADDR_TOT_START >> (shift)))
+
+#define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
+
+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 inline pud_t *fixmap_pud(unsigned long addr)
+static inline pte_t *fixmap_pte(unsigned long addr)
 {
-	pgd_t *pgdp = pgd_offset_k(addr);
-	p4d_t *p4dp = p4d_offset(pgdp, addr);
-	p4d_t p4d = READ_ONCE(*p4dp);
+	return &bm_pte[BM_PTE_TABLE_IDX(addr)][pte_index(addr)];
+}
 
-	BUG_ON(p4d_none(p4d) || p4d_bad(p4d));
+static void __init early_fixmap_init_pte(pmd_t *pmdp, unsigned long addr)
+{
+	pmd_t pmd = READ_ONCE(*pmdp);
+	pte_t *ptep;
 
-	return pud_offset_kimg(p4dp, addr);
+	if (pmd_none(pmd)) {
+		ptep = bm_pte[BM_PTE_TABLE_IDX(addr)];
+		__pmd_populate(pmdp, __pa_symbol(ptep), PMD_TYPE_TABLE);
+	}
 }
 
-static inline pmd_t *fixmap_pmd(unsigned long addr)
+static void __init early_fixmap_init_pmd(pud_t *pudp, unsigned long addr,
+					 unsigned long end)
 {
-	pud_t *pudp = fixmap_pud(addr);
+	unsigned long next;
 	pud_t pud = READ_ONCE(*pudp);
+	pmd_t *pmdp;
 
-	BUG_ON(pud_none(pud) || pud_bad(pud));
+	if (pud_none(pud))
+		__pud_populate(pudp, __pa_symbol(bm_pmd), PUD_TYPE_TABLE);
 
-	return pmd_offset_kimg(pudp, addr);
+	pmdp = pmd_offset_kimg(pudp, addr);
+	do {
+		next = pmd_addr_end(addr, end);
+		early_fixmap_init_pte(pmdp, addr);
+	} while (pmdp++, addr = next, addr != end);
 }
 
-static inline pte_t *fixmap_pte(unsigned long addr)
+
+static void __init early_fixmap_init_pud(p4d_t *p4dp, unsigned long addr,
+					 unsigned long end)
 {
-	return &bm_pte[pte_index(addr)];
+	p4d_t p4d = READ_ONCE(*p4dp);
+	pud_t *pudp;
+
+	if (CONFIG_PGTABLE_LEVELS > 3 && !p4d_none(p4d) &&
+	    p4d_page_paddr(p4d) != __pa_symbol(bm_pud)) {
+		/*
+		 * We only end up here if the kernel mapping and the fixmap
+		 * share the top level pgd entry, which should only happen on
+		 * 16k/4 levels configurations.
+		 */
+		BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
+	}
+
+	if (p4d_none(p4d))
+		__p4d_populate(p4dp, __pa_symbol(bm_pud), P4D_TYPE_TABLE);
+
+	pudp = pud_offset_kimg(p4dp, addr);
+	early_fixmap_init_pmd(pudp, addr, end);
 }
 
 /*
@@ -54,55 +97,13 @@ static inline pte_t *fixmap_pte(unsigned long addr)
  */
 void __init early_fixmap_init(void)
 {
-	pgd_t *pgdp;
-	p4d_t *p4dp, p4d;
-	pud_t *pudp;
-	pmd_t *pmdp;
 	unsigned long addr = FIXADDR_TOT_START;
+	unsigned long end = FIXADDR_TOP;
 
-	pgdp = pgd_offset_k(addr);
-	p4dp = p4d_offset(pgdp, addr);
-	p4d = READ_ONCE(*p4dp);
-	if (CONFIG_PGTABLE_LEVELS > 3 &&
-	    !(p4d_none(p4d) || p4d_page_paddr(p4d) == __pa_symbol(bm_pud))) {
-		/*
-		 * We only end up here if the kernel mapping and the fixmap
-		 * share the top level pgd entry, which should only happen on
-		 * 16k/4 levels configurations.
-		 */
-		BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
-		pudp = pud_offset_kimg(p4dp, addr);
-	} else {
-		if (p4d_none(p4d))
-			__p4d_populate(p4dp, __pa_symbol(bm_pud), P4D_TYPE_TABLE);
-		pudp = fixmap_pud(addr);
-	}
-	if (pud_none(READ_ONCE(*pudp)))
-		__pud_populate(pudp, __pa_symbol(bm_pmd), PUD_TYPE_TABLE);
-	pmdp = fixmap_pmd(addr);
-	__pmd_populate(pmdp, __pa_symbol(bm_pte), PMD_TYPE_TABLE);
+	pgd_t *pgdp = pgd_offset_k(addr);
+	p4d_t *p4dp = p4d_offset(pgdp, addr);
 
-	/*
-	 * The boot-ioremap range spans multiple pmds, for which
-	 * we are not prepared:
-	 */
-	BUILD_BUG_ON((__fix_to_virt(FIX_BTMAP_BEGIN) >> PMD_SHIFT)
-		     != (__fix_to_virt(FIX_BTMAP_END) >> PMD_SHIFT));
-
-	if ((pmdp != fixmap_pmd(__fix_to_virt(FIX_BTMAP_BEGIN)))
-	     || pmdp != fixmap_pmd(__fix_to_virt(FIX_BTMAP_END))) {
-		WARN_ON(1);
-		pr_warn("pmdp %p != %p, %p\n",
-			pmdp, fixmap_pmd(__fix_to_virt(FIX_BTMAP_BEGIN)),
-			fixmap_pmd(__fix_to_virt(FIX_BTMAP_END)));
-		pr_warn("fix_to_virt(FIX_BTMAP_BEGIN): %08lx\n",
-			__fix_to_virt(FIX_BTMAP_BEGIN));
-		pr_warn("fix_to_virt(FIX_BTMAP_END):   %08lx\n",
-			__fix_to_virt(FIX_BTMAP_END));
-
-		pr_warn("FIX_BTMAP_END:       %d\n", FIX_BTMAP_END);
-		pr_warn("FIX_BTMAP_BEGIN:     %d\n", FIX_BTMAP_BEGIN);
-	}
+	early_fixmap_init_pud(p4dp, addr, end);
 }
 
 /*
@@ -130,6 +131,7 @@ void __set_fixmap(enum fixed_addresses idx,
 void *__init fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 {
 	const u64 dt_virt_base = __fix_to_virt(FIX_FDT);
+	phys_addr_t dt_phys_base;
 	int offset;
 	void *dt_virt;
 
@@ -144,27 +146,12 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 	if (!dt_phys || dt_phys % MIN_FDT_ALIGN)
 		return NULL;
 
-	/*
-	 * Make sure that the FDT region can be mapped without the need to
-	 * allocate additional translation table pages, so that it is safe
-	 * to call create_mapping_noalloc() this early.
-	 *
-	 * On 64k pages, the FDT will be mapped using PTEs, so we need to
-	 * be in the same PMD as the rest of the fixmap.
-	 * On 4k pages, we'll use section mappings for the FDT so we only
-	 * have to be in the same PUD.
-	 */
-	BUILD_BUG_ON(dt_virt_base % SZ_2M);
-
-	BUILD_BUG_ON(__fix_to_virt(FIX_FDT_END) >> SWAPPER_TABLE_SHIFT !=
-		     __fix_to_virt(FIX_BTMAP_BEGIN) >> SWAPPER_TABLE_SHIFT);
-
-	offset = dt_phys % SWAPPER_BLOCK_SIZE;
+	dt_phys_base = round_down(dt_phys, PAGE_SIZE);
+	offset = dt_phys % PAGE_SIZE;
 	dt_virt = (void *)dt_virt_base + offset;
 
 	/* map the first chunk so we can read the size from the header */
-	create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE),
-			dt_virt_base, SWAPPER_BLOCK_SIZE, prot);
+	create_mapping_noalloc(dt_phys_base, dt_virt_base, PAGE_SIZE, prot);
 
 	if (fdt_magic(dt_virt) != FDT_MAGIC)
 		return NULL;
@@ -173,9 +160,10 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 	if (*size > MAX_FDT_SIZE)
 		return NULL;
 
-	if (offset + *size > SWAPPER_BLOCK_SIZE)
-		create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE), dt_virt_base,
-			       round_up(offset + *size, SWAPPER_BLOCK_SIZE), prot);
+	if (offset + *size > PAGE_SIZE) {
+		create_mapping_noalloc(dt_phys_base, dt_virt_base,
+				       offset + *size, prot);
+	}
 
 	return dt_virt;
 }