Page fault handler now allocates new page tables

Rather than contantly checking if a page table exists before access, the fault handler will automatically allocate them as needed.

src/x86/isr.c

@@ -2,6 +2,9 @@
 #include "stdio.h"
 #include "x86/apic.h"
 #include "platform/interrupts.h"
+#include "mmgr.h"
+#include "string.h"
+#include "kernel.h"
 
 #include <stdint.h>
 
@@ -14,6 +17,18 @@ struct interrupt_frame_t
     uint32_t ss;
 };
 
+typedef struct page_fault_code_t
+{
+    uint32_t present : 1;
+    uint32_t write : 1;
+    uint32_t usermode : 1;
+    uint32_t reserved : 1;
+    uint32_t inst_fetch : 1;
+    uint32_t pk : 1;
+    uint32_t shadow_stack : 1;
+    uint32_t padding : 25;
+} page_fault_code_t;
+
 void isr_generic(struct interrupt_frame_t *frame)
 {
     printf("Generic interrupt.\n");
@@ -46,6 +61,7 @@ void isr_gp_fault(struct interrupt_frame_t *frame, unsigned int error)
 
 void isr_page_fault(struct interrupt_frame_t *frame, unsigned int error)
 {
+    page_fault_code_t *code = &error;
     uint32_t addr;
     asm("mov %%cr2, %0"
         : "=r"(addr));
@@ -55,8 +71,28 @@ void isr_page_fault(struct interrupt_frame_t *frame, unsigned int error)
         "mov %%ax, %%fs; "
         "mov %%ax, %%gs; " ::
             : "ax");
-    printf("Exception: Page fault, code %08x, linear address %08x\n", error, addr);
-    asm("hlt");
+    if(code->usermode == 0
+        && code->present == 0
+        && addr >= 0xFFC00000
+        && addr < 0xFFFFF000)
+    {
+        printf("Allocating new page table %08x within fault handler.\n", addr);
+        physaddr_t new_table = reserve_page();
+        if(new_table == ENOMEM)
+        {
+            kernel_panic("Out of memory while allocating page table.\n");
+        }
+        set_pte((void*)addr, 1, PAGE_PRESENT | PAGE_USERMODE | PAGE_RW, new_table);
+        asm volatile("mov %%cr3, %%eax;"
+                     "mov %%eax, %%cr3" ::
+                        : "eax", "memory");
+        memset((void*)(addr & ~0xFFF), 0, page_size);
+    }
+    else
+    {
+        printf("Exception: Page fault, code %08x, linear address %08x\n", error, addr);
+        kernel_panic("Unhandled page fault.\n");
+    }
 }
 
 void isr_double_fault(struct interrupt_frame_t *frame, unsigned int error)