Virtual memory

src/kernel.c

@@ -35,6 +35,24 @@ paddr_t alloc_pages(uint32_t n) {
     return paddr;
 }
 
+void map_page(uint32_t *table1, uint32_t vaddr, paddr_t paddr, uint32_t flags) {
+    if (!is_aligned(vaddr, PAGE_SIZE))
+        PANIC("unaligned vaddr %x", vaddr);
+
+    if (!is_aligned(paddr, PAGE_SIZE))
+        PANIC("unaligned paddr %x", paddr);
+
+    uint32_t vpn1 = (vaddr >> 22) & 0x3ff;
+    if ((table1[vpn1] & PAGE_V) == 0) {
+        uint32_t pt_paddr = alloc_pages(1);
+        table1[vpn1] = ((pt_paddr / PAGE_SIZE) << 10) | PAGE_V;
+    }
+
+    uint32_t vpn0 = (vaddr >> 12) & 0x3ff;
+    uint32_t *table0 = (uint32_t *)((table1[vpn1] >> 10) * PAGE_SIZE);
+    table0[vpn0] = ((paddr / PAGE_SIZE) << 10) | flags | PAGE_V;
+}
+
 // ===== Exception handler =====
 void kernel_entry(void) {
     __asm__ __volatile__("csrrw sp, sscratch, sp\n"
@@ -192,9 +210,15 @@ struct process *create_process(uint32_t pc) {
     *--sp = 0;            // s0
     *--sp = (uint32_t)pc; // ra
 
+    uint32_t *page_table = (uint32_t *)alloc_pages(1);
+    for (paddr_t paddr = (paddr_t)__kernel_base;
+         paddr < (paddr_t)__free_ram_end; paddr += PAGE_SIZE)
+        map_page(page_table, paddr, paddr, PAGE_R | PAGE_W | PAGE_X);
+
     proc->pid = i + 1;
     proc->state = PROC_RUNNABLE;
     proc->sp = (uint32_t)sp;
+    proc->page_table = page_table;
     return proc;
 }
 void yield(void) {
@@ -211,9 +235,13 @@ void yield(void) {
         return;
 
     __asm__ __volatile__(
+        "sfence.vma\n"
+        "csrw satp, %[satp]\n"
+        "sfence.vma\n"
         "csrw sscratch, %[sscratch]\n"
         :
-        : [sscratch] "r"((uint32_t)&next->stack[sizeof(next->stack)]));
+        : [satp] "r"(SATP_SV32 | ((uint32_t)next->page_table / PAGE_SIZE)),
+          [sscratch] "r"((uint32_t)&next->stack[sizeof(next->stack)]));
 
     struct process *prev = current_proc;
     current_proc = next;
@@ -221,17 +249,35 @@ void yield(void) {
 }
 
 // ===== ENTRY POINT =====
+struct process *proc_a;
+struct process *proc_b;
+
+void proc_a_entry(void) {
+    while (1) {
+        putchar('A');
+        yield();
+    }
+}
+
+void proc_b_entry(void) {
+    while (1) {
+        putchar('B');
+        yield();
+    }
+}
+
 void kernel_main(void) {
     memset(__bss, 0, (size_t)__bss_end - (size_t)__bss);
 
-    printf("\n\n");
-
     WRITE_CSR(stvec, (uint32_t)kernel_entry);
 
     idle_proc = create_process((uint32_t)NULL);
     idle_proc->pid = -1;
     current_proc = idle_proc;
 
+    proc_a = create_process((uint32_t)proc_a_entry);
+    proc_b = create_process((uint32_t)proc_b_entry);
+
     yield();
     PANIC("switched to idle process");
 }

src/kernel.h

@@ -6,6 +6,7 @@
 extern char __bss[], __bss_end[];
 extern char __stack_top[];
 extern char __free_ram[], __free_ram_end[];
+extern char __kernel_base[];
 
 // ===== SBI data ======
 struct sbiret {
@@ -58,12 +59,21 @@ struct process {
     int pid;
     int state;
     vaddr_t sp;
+    uint32_t *page_table;
     uint8_t stack[8192];
 };
 struct process procs[PROCS_MAX];
 struct process *current_proc;
 struct process *idle_proc;
 
+// ===== Memory allocation =====
+#define SATP_SV32 (1u << 31)
+#define PAGE_V (1 << 0)
+#define PAGE_R (1 << 1)
+#define PAGE_W (1 << 2)
+#define PAGE_X (1 << 3)
+#define PAGE_U (1 << 4)
+
 // ===== Macros =====
 #define READ_CSR(reg)                                                          \
     ({                                                                         \

src/kernel.ld

@@ -2,6 +2,7 @@ ENTRY(boot)
 
 SECTIONS {
     . = 0x80200000;
+    __kernel_base = .;
 
     .text :{
         KEEP(*(.text.boot));

src/run.sh

@@ -1,5 +1,4 @@
 #!/bin/bash
 set -xue
 
-make kernel
-qemu-system-riscv32 -machine virt -bios default -nographic -serial mon:stdio --no-reboot -kernel kernel.elf
+make kernel && qemu-system-riscv32 -machine virt -bios default -nographic -serial mon:stdio --no-reboot -kernel kernel.elf