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");