RISC-V: Setup initial page tables in two stages

Currently, the setup_vm() does initial page table setup in one-shot
very early before enabling MMU. Due to this, the setup_vm() has to map
all possible kernel virtual addresses since it does not know size and
location of RAM. This means we have kernel mappings for non-existent
RAM and any buggy driver (or kernel) code doing out-of-bound access
to RAM will not fault and cause underterministic behaviour.

Further, the setup_vm() creates PMD mappings (i.e. 2M mappings) for
RV64 systems. This means for PAGE_OFFSET=0xffffffe000000000 (i.e.
MAXPHYSMEM_128GB=y), the setup_vm() will require 129 pages (i.e.
516 KB) of memory for initial page tables which is never freed. The
memory required for initial page tables will further increase if
we chose a lower value of PAGE_OFFSET (e.g. 0xffffff0000000000)

This patch implements two-staged initial page table setup, as follows:
1. Early (i.e. setup_vm()): This stage maps kernel image and DTB in
a early page table (i.e. early_pg_dir). The early_pg_dir will be used
only by boot HART so it can be freed as-part of init memory free-up.
2. Final (i.e. setup_vm_final()): This stage maps all possible RAM
banks in the final page table (i.e. swapper_pg_dir). The boot HART
will start using swapper_pg_dir at the end of setup_vm_final(). All
non-boot HARTs directly use the swapper_pg_dir created by boot HART.

We have following advantages with this new approach:
1. Kernel mappings for non-existent RAM don't exists anymore.
2. Memory consumed by initial page tables is now indpendent of the
chosen PAGE_OFFSET.
3. Memory consumed by initial page tables on RV64 system is 2 pages
(i.e. 8 KB) which has significantly reduced and these pages will be
freed as-part of the init memory free-up.

The patch also provides a foundation for implementing strict kernel
mappings where we protect kernel text and rodata using PTE permissions.
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
[paul.walmsley@sifive.com: updated to apply; fixed a checkpatch warning]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/include/asm/fixmap.h

@@ -21,6 +21,11 @@
  */
 enum fixed_addresses {
 	FIX_HOLE,
+#define FIX_FDT_SIZE	SZ_1M
+	FIX_FDT_END,
+	FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
+	FIX_PTE,
+	FIX_PMD,
 	FIX_EARLYCON_MEM_BASE,
 	__end_of_fixed_addresses
 };

arch/riscv/include/asm/pgtable-64.h

@@ -70,6 +70,11 @@ static inline pmd_t pfn_pmd(unsigned long pfn, pgprot_t prot)
 	return __pmd((pfn << _PAGE_PFN_SHIFT) | pgprot_val(prot));
 }
 
+static inline unsigned long _pmd_pfn(pmd_t pmd)
+{
+	return pmd_val(pmd) >> _PAGE_PFN_SHIFT;
+}
+
 #define pmd_ERROR(e) \
 	pr_err("%s:%d: bad pmd %016lx.\n", __FILE__, __LINE__, pmd_val(e))
 

arch/riscv/include/asm/pgtable.h

@@ -59,6 +59,8 @@
 #define PAGE_KERNEL		__pgprot(_PAGE_KERNEL)
 #define PAGE_KERNEL_EXEC	__pgprot(_PAGE_KERNEL | _PAGE_EXEC)
 
+#define PAGE_TABLE		__pgprot(_PAGE_TABLE)
+
 extern pgd_t swapper_pg_dir[];
 
 /* MAP_PRIVATE permissions: xwr (copy-on-write) */
@@ -118,6 +120,11 @@ static inline pgd_t pfn_pgd(unsigned long pfn, pgprot_t prot)
 	return __pgd((pfn << _PAGE_PFN_SHIFT) | pgprot_val(prot));
 }
 
+static inline unsigned long _pgd_pfn(pgd_t pgd)
+{
+	return pgd_val(pgd) >> _PAGE_PFN_SHIFT;
+}
+
 #define pgd_index(addr) (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1))
 
 /* Locate an entry in the page global directory */
@@ -400,6 +407,7 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
 #define kern_addr_valid(addr)   (1) /* FIXME */
 #endif
 
+extern void *dtb_early_va;
 extern void setup_bootmem(void);
 extern void paging_init(void);
 

arch/riscv/kernel/head.S

@@ -55,7 +55,9 @@ clear_bss_done:
 
 	/* Initialize page tables and relocate to virtual addresses */
 	la sp, init_thread_union + THREAD_SIZE
+	mv a0, s1
 	call setup_vm
+	la a0, early_pg_dir
 	call relocate
 
 	/* Restore C environment */
@@ -64,25 +66,23 @@ clear_bss_done:
 	la sp, init_thread_union + THREAD_SIZE
 
 	/* Start the kernel */
-	mv a0, s1
 	call parse_dtb
 	tail start_kernel
 
 relocate:
 	/* Relocate return address */
 	li a1, PAGE_OFFSET
-	la a0, _start
-	sub a1, a1, a0
+	la a2, _start
+	sub a1, a1, a2
 	add ra, ra, a1
 
 	/* Point stvec to virtual address of intruction after satp write */
-	la a0, 1f
-	add a0, a0, a1
-	csrw CSR_STVEC, a0
+	la a2, 1f
+	add a2, a2, a1
+	csrw CSR_STVEC, a2
 
 	/* Compute satp for kernel page tables, but don't load it yet */
-	la a2, swapper_pg_dir
-	srl a2, a2, PAGE_SHIFT
+	srl a2, a0, PAGE_SHIFT
 	li a1, SATP_MODE
 	or a2, a2, a1
 
@@ -148,6 +148,7 @@ relocate:
 	fence
 
 	/* Enable virtual memory and relocate to virtual address */
+	la a0, swapper_pg_dir
 	call relocate
 
 	tail smp_callin

arch/riscv/kernel/setup.c

@@ -39,11 +39,9 @@ struct screen_info screen_info = {
 atomic_t hart_lottery;
 unsigned long boot_cpu_hartid;
 
-void __init parse_dtb(phys_addr_t dtb_phys)
+void __init parse_dtb(void)
 {
-	void *dtb = __va(dtb_phys);
-
-	if (early_init_dt_scan(dtb))
+	if (early_init_dt_scan(dtb_early_va))
 		return;
 
 	pr_err("No DTB passed to the kernel\n");

arch/riscv/mm/init.c

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2012 Regents of the University of California
+ * Copyright (C) 2019 Western Digital Corporation or its affiliates.
  */
 
 #include <linux/init.h>
@@ -41,13 +42,6 @@ void setup_zero_page(void)
 	memset((void *)empty_zero_page, 0, PAGE_SIZE);
 }
 
-void __init paging_init(void)
-{
-	setup_zero_page();
-	local_flush_tlb_all();
-	zone_sizes_init();
-}
-
 void __init mem_init(void)
 {
 #ifdef CONFIG_FLATMEM
@@ -143,17 +137,15 @@ EXPORT_SYMBOL(va_pa_offset);
 unsigned long pfn_base;
 EXPORT_SYMBOL(pfn_base);
 
+void *dtb_early_va;
 pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
-pgd_t trampoline_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
+pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
+pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
+static bool mmu_enabled;
 
-#ifndef __PAGETABLE_PMD_FOLDED
-#define NUM_SWAPPER_PMDS ((uintptr_t)-PAGE_OFFSET >> PGDIR_SHIFT)
-pmd_t swapper_pmd[PTRS_PER_PMD*((-PAGE_OFFSET)/PGDIR_SIZE)] __page_aligned_bss;
-pmd_t trampoline_pmd[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
-pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
-#endif
+#define MAX_EARLY_MAPPING_SIZE	SZ_128M
 
-pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
+pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
 
 void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 {
@@ -172,6 +164,156 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 	}
 }
 
+static pte_t *__init get_pte_virt(phys_addr_t pa)
+{
+	if (mmu_enabled) {
+		clear_fixmap(FIX_PTE);
+		return (pte_t *)set_fixmap_offset(FIX_PTE, pa);
+	} else {
+		return (pte_t *)((uintptr_t)pa);
+	}
+}
+
+static phys_addr_t __init alloc_pte(uintptr_t va)
+{
+	/*
+	 * We only create PMD or PGD early mappings so we
+	 * should never reach here with MMU disabled.
+	 */
+	BUG_ON(!mmu_enabled);
+
+	return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
+}
+
+static void __init create_pte_mapping(pte_t *ptep,
+				      uintptr_t va, phys_addr_t pa,
+				      phys_addr_t sz, pgprot_t prot)
+{
+	uintptr_t pte_index = pte_index(va);
+
+	BUG_ON(sz != PAGE_SIZE);
+
+	if (pte_none(ptep[pte_index]))
+		ptep[pte_index] = pfn_pte(PFN_DOWN(pa), prot);
+}
+
+#ifndef __PAGETABLE_PMD_FOLDED
+
+pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
+pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
+
+#if MAX_EARLY_MAPPING_SIZE < PGDIR_SIZE
+#define NUM_EARLY_PMDS		1UL
+#else
+#define NUM_EARLY_PMDS		(1UL + MAX_EARLY_MAPPING_SIZE / PGDIR_SIZE)
+#endif
+pmd_t early_pmd[PTRS_PER_PMD * NUM_EARLY_PMDS] __initdata __aligned(PAGE_SIZE);
+
+static pmd_t *__init get_pmd_virt(phys_addr_t pa)
+{
+	if (mmu_enabled) {
+		clear_fixmap(FIX_PMD);
+		return (pmd_t *)set_fixmap_offset(FIX_PMD, pa);
+	} else {
+		return (pmd_t *)((uintptr_t)pa);
+	}
+}
+
+static phys_addr_t __init alloc_pmd(uintptr_t va)
+{
+	uintptr_t pmd_num;
+
+	if (mmu_enabled)
+		return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
+
+	pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
+	BUG_ON(pmd_num >= NUM_EARLY_PMDS);
+	return (uintptr_t)&early_pmd[pmd_num * PTRS_PER_PMD];
+}
+
+static void __init create_pmd_mapping(pmd_t *pmdp,
+				      uintptr_t va, phys_addr_t pa,
+				      phys_addr_t sz, pgprot_t prot)
+{
+	pte_t *ptep;
+	phys_addr_t pte_phys;
+	uintptr_t pmd_index = pmd_index(va);
+
+	if (sz == PMD_SIZE) {
+		if (pmd_none(pmdp[pmd_index]))
+			pmdp[pmd_index] = pfn_pmd(PFN_DOWN(pa), prot);
+		return;
+	}
+
+	if (pmd_none(pmdp[pmd_index])) {
+		pte_phys = alloc_pte(va);
+		pmdp[pmd_index] = pfn_pmd(PFN_DOWN(pte_phys), PAGE_TABLE);
+		ptep = get_pte_virt(pte_phys);
+		memset(ptep, 0, PAGE_SIZE);
+	} else {
+		pte_phys = PFN_PHYS(_pmd_pfn(pmdp[pmd_index]));
+		ptep = get_pte_virt(pte_phys);
+	}
+
+	create_pte_mapping(ptep, va, pa, sz, prot);
+}
+
+#define pgd_next_t		pmd_t
+#define alloc_pgd_next(__va)	alloc_pmd(__va)
+#define get_pgd_next_virt(__pa)	get_pmd_virt(__pa)
+#define create_pgd_next_mapping(__nextp, __va, __pa, __sz, __prot)	\
+	create_pmd_mapping(__nextp, __va, __pa, __sz, __prot)
+#define PTE_PARENT_SIZE		PMD_SIZE
+#define fixmap_pgd_next		fixmap_pmd
+#else
+#define pgd_next_t		pte_t
+#define alloc_pgd_next(__va)	alloc_pte(__va)
+#define get_pgd_next_virt(__pa)	get_pte_virt(__pa)
+#define create_pgd_next_mapping(__nextp, __va, __pa, __sz, __prot)	\
+	create_pte_mapping(__nextp, __va, __pa, __sz, __prot)
+#define PTE_PARENT_SIZE		PGDIR_SIZE
+#define fixmap_pgd_next		fixmap_pte
+#endif
+
+static void __init create_pgd_mapping(pgd_t *pgdp,
+				      uintptr_t va, phys_addr_t pa,
+				      phys_addr_t sz, pgprot_t prot)
+{
+	pgd_next_t *nextp;
+	phys_addr_t next_phys;
+	uintptr_t pgd_index = pgd_index(va);
+
+	if (sz == PGDIR_SIZE) {
+		if (pgd_val(pgdp[pgd_index]) == 0)
+			pgdp[pgd_index] = pfn_pgd(PFN_DOWN(pa), prot);
+		return;
+	}
+
+	if (pgd_val(pgdp[pgd_index]) == 0) {
+		next_phys = alloc_pgd_next(va);
+		pgdp[pgd_index] = pfn_pgd(PFN_DOWN(next_phys), PAGE_TABLE);
+		nextp = get_pgd_next_virt(next_phys);
+		memset(nextp, 0, PAGE_SIZE);
+	} else {
+		next_phys = PFN_PHYS(_pgd_pfn(pgdp[pgd_index]));
+		nextp = get_pgd_next_virt(next_phys);
+	}
+
+	create_pgd_next_mapping(nextp, va, pa, sz, prot);
+}
+
+static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
+{
+	uintptr_t map_size = PAGE_SIZE;
+
+	/* Upgrade to PMD/PGDIR mappings whenever possible */
+	if (!(base & (PTE_PARENT_SIZE - 1)) &&
+	    !(size & (PTE_PARENT_SIZE - 1)))
+		map_size = PTE_PARENT_SIZE;
+
+	return map_size;
+}
+
 /*
  * setup_vm() is called from head.S with MMU-off.
  *
@@ -191,54 +333,115 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 	"not use absolute addressing."
 #endif
 
-asmlinkage void __init setup_vm(void)
+asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 {
-	uintptr_t i;
-	uintptr_t pa = (uintptr_t) &_start;
-	pgprot_t prot = __pgprot(pgprot_val(PAGE_KERNEL) | _PAGE_EXEC);
+	uintptr_t va, end_va;
+	uintptr_t load_pa = (uintptr_t)(&_start);
+	uintptr_t load_sz = (uintptr_t)(&_end) - load_pa;
+	uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
+
+	va_pa_offset = PAGE_OFFSET - load_pa;
+	pfn_base = PFN_DOWN(load_pa);
 
-	va_pa_offset = PAGE_OFFSET - pa;
-	pfn_base = PFN_DOWN(pa);
+	/*
+	 * Enforce boot alignment requirements of RV32 and
+	 * RV64 by only allowing PMD or PGD mappings.
+	 */
+	BUG_ON(map_size == PAGE_SIZE);
 
 	/* Sanity check alignment and size */
 	BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
-	BUG_ON((pa % (PAGE_SIZE * PTRS_PER_PTE)) != 0);
+	BUG_ON((load_pa % map_size) != 0);
+	BUG_ON(load_sz > MAX_EARLY_MAPPING_SIZE);
+
+	/* Setup early PGD for fixmap */
+	create_pgd_mapping(early_pg_dir, FIXADDR_START,
+			   (uintptr_t)fixmap_pgd_next, PGDIR_SIZE, PAGE_TABLE);
 
 #ifndef __PAGETABLE_PMD_FOLDED
-	trampoline_pg_dir[(PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD] =
-		pfn_pgd(PFN_DOWN((uintptr_t)trampoline_pmd),
-			__pgprot(_PAGE_TABLE));
-	trampoline_pmd[0] = pfn_pmd(PFN_DOWN(pa), prot);
+	/* Setup fixmap PMD */
+	create_pmd_mapping(fixmap_pmd, FIXADDR_START,
+			   (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
+	/* Setup trampoline PGD and PMD */
+	create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
+			   (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
+	create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
+			   load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
+#else
+	/* Setup trampoline PGD */
+	create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
+			   load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
+#endif
 
-	for (i = 0; i < (-PAGE_OFFSET)/PGDIR_SIZE; ++i) {
-		size_t o = (PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD + i;
+	/*
+	 * Setup early PGD covering entire kernel which will allows
+	 * us to reach paging_init(). We map all memory banks later
+	 * in setup_vm_final() below.
+	 */
+	end_va = PAGE_OFFSET + load_sz;
+	for (va = PAGE_OFFSET; va < end_va; va += map_size)
+		create_pgd_mapping(early_pg_dir, va,
+				   load_pa + (va - PAGE_OFFSET),
+				   map_size, PAGE_KERNEL_EXEC);
+
+	/* Create fixed mapping for early FDT parsing */
+	end_va = __fix_to_virt(FIX_FDT) + FIX_FDT_SIZE;
+	for (va = __fix_to_virt(FIX_FDT); va < end_va; va += PAGE_SIZE)
+		create_pte_mapping(fixmap_pte, va,
+				   dtb_pa + (va - __fix_to_virt(FIX_FDT)),
+				   PAGE_SIZE, PAGE_KERNEL);
+
+	/* Save pointer to DTB for early FDT parsing */
+	dtb_early_va = (void *)fix_to_virt(FIX_FDT) + (dtb_pa & ~PAGE_MASK);
+}
 
-		swapper_pg_dir[o] =
-			pfn_pgd(PFN_DOWN((uintptr_t)swapper_pmd) + i,
-				__pgprot(_PAGE_TABLE));
-	}
-	for (i = 0; i < ARRAY_SIZE(swapper_pmd); i++)
-		swapper_pmd[i] = pfn_pmd(PFN_DOWN(pa + i * PMD_SIZE), prot);
-
-	swapper_pg_dir[(FIXADDR_START >> PGDIR_SHIFT) % PTRS_PER_PGD] =
-		pfn_pgd(PFN_DOWN((uintptr_t)fixmap_pmd),
-				__pgprot(_PAGE_TABLE));
-	fixmap_pmd[(FIXADDR_START >> PMD_SHIFT) % PTRS_PER_PMD] =
-		pfn_pmd(PFN_DOWN((uintptr_t)fixmap_pte),
-				__pgprot(_PAGE_TABLE));
-#else
-	trampoline_pg_dir[(PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD] =
-		pfn_pgd(PFN_DOWN(pa), prot);
+static void __init setup_vm_final(void)
+{
+	uintptr_t va, map_size;
+	phys_addr_t pa, start, end;
+	struct memblock_region *reg;
+
+	/* Set mmu_enabled flag */
+	mmu_enabled = true;
 
-	for (i = 0; i < (-PAGE_OFFSET)/PGDIR_SIZE; ++i) {
-		size_t o = (PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD + i;
+	/* Setup swapper PGD for fixmap */
+	create_pgd_mapping(swapper_pg_dir, FIXADDR_START,
+			   __pa(fixmap_pgd_next),
+			   PGDIR_SIZE, PAGE_TABLE);
 
-		swapper_pg_dir[o] =
-			pfn_pgd(PFN_DOWN(pa + i * PGDIR_SIZE), prot);
+	/* Map all memory banks */
+	for_each_memblock(memory, reg) {
+		start = reg->base;
+		end = start + reg->size;
+
+		if (start >= end)
+			break;
+		if (memblock_is_nomap(reg))
+			continue;
+		if (start <= __pa(PAGE_OFFSET) &&
+		    __pa(PAGE_OFFSET) < end)
+			start = __pa(PAGE_OFFSET);
+
+		map_size = best_map_size(start, end - start);
+		for (pa = start; pa < end; pa += map_size) {
+			va = (uintptr_t)__va(pa);
+			create_pgd_mapping(swapper_pg_dir, va, pa,
+					   map_size, PAGE_KERNEL_EXEC);
+		}
 	}
 
-	swapper_pg_dir[(FIXADDR_START >> PGDIR_SHIFT) % PTRS_PER_PGD] =
-		pfn_pgd(PFN_DOWN((uintptr_t)fixmap_pte),
-				__pgprot(_PAGE_TABLE));
-#endif
+	/* Clear fixmap PTE and PMD mappings */
+	clear_fixmap(FIX_PTE);
+	clear_fixmap(FIX_PMD);
+
+	/* Move to swapper page table */
+	csr_write(sptbr, PFN_DOWN(__pa(swapper_pg_dir)) | SATP_MODE);
+	local_flush_tlb_all();
+}
+
+void __init paging_init(void)
+{
+	setup_vm_final();
+	setup_zero_page();
+	zone_sizes_init();
 }