process control and context switch

.gitignore

@@ -7,3 +7,4 @@
 *.bin
 *.log
 *.pcap
+registers

common.c

@@ -1,5 +1,41 @@
 #include "common.h"
 
+void *memcpy(void *dst, const void *src, size_t n) {
+  uint8_t *d = (uint8_t *) dst;
+  const uint8_t *s = (const uint8_t *) src;
+  while (n--)
+      *d++ = *s++;
+  return dst;
+}
+
+void *memset(void *buf, char c, size_t n) {
+  uint8_t *p = (uint8_t *) buf;
+  while (n--)
+      *p++ = c;
+  return buf;
+}
+
+char *strcpy(char *dst, const char *src) {
+  char *d = dst;
+  while (*src)
+      *d++ = *src++;
+  *d = '\0';
+  return dst;
+} // The strcpy function continues copying even if
+  // src is longer than the memory area of dst. 
+  // This can easily lead to bugs and vulnerabilities
+
+int strcmp(const char *s1, const char *s2) {
+  while (*s1 && *s2) {
+    if (*s1 != *s2)
+        break;
+    s1++;
+    s2++;
+  }
+
+  return *(unsigned char *)s1 - *(unsigned char *)s2;
+}
+
 void putchar(char ch);
 
 void printf(const char *fmt, ...) {
@@ -62,39 +98,3 @@ void printf(const char *fmt, ...) {
 end:
   va_end(vargs);
 }
-
-void *memcpy(void *dst, const void *src, size_t n) {
-  uint8_t *d = (uint8_t *) dst;
-  const uint8_t *s = (const uint8_t *) src;
-  while (n--)
-    *d++ = *s++;
-  return dst;
-}
-
-void *memset(void *buf, char c, size_t n) {
-  uint8_t *p = (uint8_t *) buf;
-  while (n--)
-    *p++ = c;
-  return buf;
-}
-
-char *strcpy(char *dst, const char *src) {
-  char *d = dst;
-  while (*src)
-    *d++ = *src++;
-  *d = '\0';
-  return dst;
-} // The strcpy function continues copying even if
-  // src is longer than the memory area of dst. 
-  // This can easily lead to bugs and vulnerabilities
-
-int strcmp(const char *s1, const char *s2) {
-  while (*s1 && *s2) {
-    if (*s1 != *s2)
-      break;
-    s1++;
-    s2++;
-  }
-
-  return *(unsigned char *)s1 - *(unsigned char *)s2;
-}

kernel.c

@@ -125,6 +125,114 @@ void handle_trap(struct trap_frame *f) {
     PANIC("Unexpected trap: scause=%x, stval=%x, sepc=%x\n", scause, stval, user_pc);
 }
 
+__attribute__((naked)) void switch_context(uint32_t *prev_sp,
+                                           uint32_t *next_sp) {
+    __asm__ __volatile__(
+        // Save calle-saved registers onto the stack of current proccess
+        "addi sp, sp, -13 * 4\n" // Allocate stack space for 13 4-byte registers
+        "sw ra,  0  * 4(sp)\n"   // Save calle-saved registers only
+        "sw s0,  1  * 4(sp)\n"
+        "sw s1,  2  * 4(sp)\n"
+        "sw s2,  3  * 4(sp)\n"
+        "sw s3,  4  * 4(sp)\n"
+        "sw s4,  5  * 4(sp)\n"
+        "sw s5,  6  * 4(sp)\n"
+        "sw s6,  7  * 4(sp)\n"
+        "sw s7,  8  * 4(sp)\n"
+        "sw s8,  9  * 4(sp)\n"
+        "sw s9,  10 * 4(sp)\n"
+        "sw s10, 11 * 4(sp)\n"
+        "sw s11, 12 * 4(sp)\n"
+
+        // Switch stack pointer
+        "sw sp, (a0)\n" // *prev_sp = a0; 
+        "lw sp, (a1)\n" // Switch stack pointer (sp) here
+        
+        // Restore calle-saved registers from the next process's stack
+        "lw ra,  0  * 4(sp)\n"   // Save calle-saved registers only
+        "lw s0,  1  * 4(sp)\n"
+        "lw s1,  2  * 4(sp)\n"
+        "lw s2,  3  * 4(sp)\n"
+        "lw s3,  4  * 4(sp)\n"
+        "lw s4,  5  * 4(sp)\n"
+        "lw s5,  6  * 4(sp)\n"
+        "lw s6,  7  * 4(sp)\n"
+        "lw s7,  8  * 4(sp)\n"
+        "lw s8,  9  * 4(sp)\n"
+        "lw s9,  10 * 4(sp)\n"
+        "lw s10, 11 * 4(sp)\n"
+        "lw s11, 12 * 4(sp)\n"
+        "addi sp, sp, 13 * 4\n" // We've popped 13 4-byte registers from the stack
+        "ret\n"
+    );
+}
+
+struct process procs[PROCS_MAX];
+
+struct process *create_process(uint32_t pc) {
+    // Find an unused proccess control structure
+    struct process *proc = NULL;
+    int i;
+    for (i = 0; i <= PROCS_MAX; i++) {
+        if (procs[i].state == PROC_UNUSED) {
+            proc = &procs[i];
+            break;
+        }
+    }
+    
+    if (!proc)
+        PANIC("No free process slots found! Fuck off!");
+
+    // Stack calle-saved registers 
+    uint32_t *sp = (uint32_t *) &proc->stack[sizeof(proc->stack)];
+    *--sp = 0;                    // s11
+    *--sp = 0;                    // s10
+    *--sp = 0;                    // s9
+    *--sp = 0;                    // s8
+    *--sp = 0;                    // s7
+    *--sp = 0;                    // s6
+    *--sp = 0;                    // s5
+    *--sp = 0;                    // s4
+    *--sp = 0;                    // s3
+    *--sp = 0;                    // s2
+    *--sp = 0;                    // s1
+    *--sp = 0;                    // s0
+    *--sp = (uint32_t) pc;        // ra
+    
+
+    // Initialize fields
+    proc->pid = i + 1;
+    proc->state = PROC_RUNNABLE;
+    proc->sp = (uint32_t) sp;
+    return proc;
+}
+
+void delay(void) {
+    for (int i = 0; i < 69696969; i++)
+        __asm__ __volatile__("nop"); // do nothing
+}
+
+struct process *proc_a;
+struct process *proc_b;
+
+void proc_a_entry(void) {
+    printf("Start process A\n");
+    while (1) {
+        putchar('6');
+        switch_context(&proc_a->sp, &proc_b->sp);
+        delay();
+    }
+}
+
+void proc_b_entry(void) {
+    printf("Start process B\n");
+    while (1) {
+        putchar('9');
+        switch_context(&proc_b->sp, &proc_a->sp);
+        delay();
+    }
+}
+
 void kernel_main(void) {
     printf("\n\nHello %s\n", "friend :3");
     printf("60 + 9 = %d, %x\n", 60 + 9, 0x1234abcd);
@@ -135,14 +243,19 @@ void kernel_main(void) {
 
     memset(__bss, 0, (size_t) __bss_end - (size_t) __bss);
     
-    paddr_t paddr0 = alloc_pages(2);
-    paddr_t paddr1 = alloc_pages(1);
-    printf("alloc_pages test paddr0=%x\n", paddr0);
-    printf("alloc_pages test paddr1=%x\n", paddr1);
+    WRITE_CSR(stvec, (uint32_t) kernel_entry);
 
-    PANIC("You booted successfully :3");
+    // paddr_t paddr0 = alloc_pages(2);
+    // paddr_t paddr1 = alloc_pages(1);
+    // printf("alloc_pages test paddr0=%x\n", paddr0);
+    // printf("alloc_pages test paddr1=%x\n", paddr1);
+
+    proc_a = create_process((uint32_t) proc_a_entry);
+    proc_b = create_process((uint32_t) proc_b_entry);
+    proc_a_entry();
+
+    PANIC("Your boot ended successfully :3");
 
-    // WRITE_CSR(stvec, (uint32_t) kernel_entry);
     // __asm__ __volatile__("unimp"); // calls a unimp which triggers kernel panic
     
     //
@@ -160,4 +273,3 @@ void boot(void) {
         : [stack_top] "r" (__stack_top) // Pass the stack top address as %[stack_top]
     );
 }
-

kernel.h

@@ -58,3 +58,15 @@ struct trap_frame {
         printf("YOU GOT A PANIC ERROR: %s:%d: " fmt "\n", __FILE__, __LINE__, ##__VA_ARGS__);       \
         while (1) {}                                                                                \
     } while (0)
+
+#define PROCS_MAX 8             // Maximum ammount of processes
+
+#define PROC_UNUSED 0           // Unused processes control structure
+#define PROC_RUNNABLE 1         // Runnable proccess
+
+struct process {
+    int pid;                    // ID of a process
+    int state;                  // State of the process: either PROC_UNUSED or PROC_RUNNABLE
+    vaddr_t sp;                 // Stack pointer
+    uint8_t stack[8192];        // Kernel stack
+};

kernel.ld

@@ -28,6 +28,6 @@ SECTIONS {
 
     . = ALIGN(4096);
     __free_ram = .;
-    . += 128 * 1024 * 1048; /* 128MB */
+    . += 64 * 1024 * 1048; /* 64MB */
     __free_ram_end = .;
 }