very basic paging

3 files changed, 121 insertions, 0 deletions

diff --git a/src/kernel/arch/generic.h b/src/kernel/arch/generic.h
index 46782f8..22efe58 100644
--- a/src/kernel/arch/generic.h
+++ b/src/kernel/arch/generic.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <kernel/arch/log.h>
+#include <stdbool.h>
 
 // i have no idea where else to put it
 // some code assumes that it's a power of 2
@@ -15,3 +16,9 @@ void halt_cpu();
 // src/arch/i386/sysenter.s
 void sysexit(void (*fun)(), void *stack_top);
 void sysenter_setup();
+
+// all of those can allocate memory
+struct pagedir *pagedir_new();
+void pagedir_map(struct pagedir *dir, void *virt, void *phys,
+                 bool user, bool writeable);
+void pagedir_use(struct pagedir *);
diff --git a/src/kernel/arch/i386/pagedir.c b/src/kernel/arch/i386/pagedir.c
new file mode 100644
index 0000000..264508b
--- /dev/null
+++ b/src/kernel/arch/i386/pagedir.c
@@ -0,0 +1,98 @@
+#include <kernel/arch/generic.h>
+#include <kernel/mem.h>
+#include <stdint.h>
+
+/* <heat> nitpick: I highly recommend you dont use bitfields for paging
+ *        structures
+ * <heat> you can't change them atomically and the way they're layed out
+ *        in memory is implementation defined iirc
+ */
+struct pagetable_entry {
+	uint32_t present   : 1;
+	uint32_t writeable : 1;
+	uint32_t user      : 1;
+	uint32_t writethru : 1;
+	uint32_t uncached  : 1;
+	uint32_t dirty     : 1;
+	uint32_t always0   : 1; // memory type thing?
+	uint32_t global    : 1;
+	uint32_t _unused   : 3;
+	uint32_t address   : 21;
+};
+
+struct pagedir_entry {
+	uint32_t present   : 1;
+	uint32_t writeable : 1;
+	uint32_t user      : 1;
+	uint32_t writethru : 1;
+	uint32_t uncached  : 1;
+	uint32_t accessed  : 1;
+	uint32_t always0   : 1;
+	uint32_t large     : 1; // 4 MiB instead of 4 KiB
+	uint32_t _unused   : 3;
+	uint32_t address   : 21;
+} __attribute__((packed));
+
+struct pagedir {
+	struct pagedir_entry e[1024];
+} __attribute__((packed));
+
+
+struct pagedir *pagedir_new() {
+	struct pagedir *dir = page_alloc(1);
+	for (int i = 0; i < 1024; i++)
+		dir->e[i].present = 0;
+	return dir;
+}
+
+void pagedir_map(struct pagedir *dir, void *virt, void *phys,
+                 bool user, bool writeable)
+{
+	uintptr_t virt_casted = (uintptr_t) virt;
+	uint32_t pd_idx = virt_casted >> 22;
+	uint32_t pt_idx = virt_casted >> 12 & 0x03FF;
+	struct pagetable_entry *pagetable;
+
+	if (dir->e[pd_idx].present) {
+		pagetable = (void*) (dir->e[pd_idx].address << 11);
+	} else {
+		pagetable = page_alloc(1);
+		for (int i = 0; i < 1024; i++)
+			pagetable[i].present = 0;
+
+		dir->e[pd_idx] = (struct pagedir_entry) {
+			.present   = 1,
+			.writeable = 1,
+			.user      = 1,
+			.writethru = 1,
+			.uncached  = 0,
+			.accessed  = 0,
+			.always0   = 0,
+			.large     = 0,
+			._unused   = 0,
+			.address   = (uintptr_t) pagetable >> 11
+		};
+	}
+
+	pagetable[pt_idx] = (struct pagetable_entry) {
+		.present   = 1,
+		.writeable = writeable,
+		.user      = user,
+		.writethru = 1,
+		.uncached  = 0,
+		.dirty     = 0,
+		.always0   = 0,
+		.global    = 0,
+		._unused   = 0,
+		.address   = (uintptr_t) phys >> 11
+	};
+}
+
+void pagedir_use(struct pagedir *dir) {
+	asm volatile("mov %0, %%cr3;"
+
+	    "mov %%cr0, %%eax;"
+	    "or $0x80000001, %%eax;"
+	    "mov %%eax, %%cr0;"
+	    : : "r" (dir) : "eax");
+}
diff --git a/src/kernel/main.c b/src/kernel/main.c
index 7f49fed..1b5e504 100644
--- a/src/kernel/main.c
+++ b/src/kernel/main.c
@@ -8,10 +8,26 @@
 
 void r3_test();
 
+static void setup_paging() {
+	struct pagedir *dir = pagedir_new();
+
+	// map VGA
+	pagedir_map(dir, 0xB8000, 0xB8000, true, true);
+
+	// map the kernel
+	for (size_t p = 0x100000; p < &_bss_end; p += PAGE_SIZE)
+		pagedir_map(dir, p, p, false, true); // yes, .text is writeable too
+
+	pagedir_use(dir);
+}
+
 void kmain(struct kmain_info info) {
 	log_const("mem...");
 	mem_init(&info);
 
+	log_const("paging...");
+	setup_paging();
+
 	log_const("creating process...");
 
 	void *init_addr = (void*)0x200000;