#include "userprog/exception.h" #include #include #include "userprog/gdt.h" #include "threads/interrupt.h" #include "threads/thread.h" /* Number of page faults processed. */ static long long page_fault_cnt; static void kill (struct intr_frame *); static void page_fault (struct intr_frame *); /* Registers handlers for interrupts that can be caused by user programs. In a real Unix-like OS, most of these interrupts would be passed along to the user process in the form of signals, as described in [SV-386] 3-24 and 3-25, but we don't implement signals. Instead, we'll make them simply kill the user process. Page faults are an exception. Here they are treated the same way as other exceptions, but this will need to change to implement virtual memory. Refer to [IA32-v3a] section 5.15 "Exception and Interrupt Reference" for a description of each of these exceptions. */ void exception_init (void) { /* These exceptions can be raised explicitly by a user program, e.g. via the INT, INT3, INTO, and BOUND instructions. Thus, we set DPL==3, meaning that user programs are allowed to invoke them via these instructions. */ intr_register_int (3, 3, INTR_ON, kill, "#BP Breakpoint Exception"); intr_register_int (4, 3, INTR_ON, kill, "#OF Overflow Exception"); intr_register_int (5, 3, INTR_ON, kill, "#BR BOUND Range Exceeded Exception"); /* These exceptions have DPL==0, preventing user processes from invoking them via the INT instruction. They can still be caused indirectly, e.g. #DE can be caused by dividing by 0. */ intr_register_int (0, 0, INTR_ON, kill, "#DE Divide Error"); intr_register_int (1, 0, INTR_ON, kill, "#DB Debug Exception"); intr_register_int (6, 0, INTR_ON, kill, "#UD Invalid Opcode Exception"); intr_register_int (7, 0, INTR_ON, kill, "#NM Device Not Available Exception"); intr_register_int (11, 0, INTR_ON, kill, "#NP Segment Not Present"); intr_register_int (12, 0, INTR_ON, kill, "#SS Stack Fault Exception"); intr_register_int (13, 0, INTR_ON, kill, "#GP General Protection Exception"); intr_register_int (16, 0, INTR_ON, kill, "#MF x87 FPU Floating-Point Error"); intr_register_int (19, 0, INTR_ON, kill, "#XF SIMD Floating-Point Exception"); /* Most exceptions can be handled with interrupts turned on. We need to disable interrupts for page faults because the fault address is stored in CR2 and needs to be preserved. */ intr_register_int (14, 0, INTR_OFF, page_fault, "#PF Page-Fault Exception"); } /* Prints exception statistics. */ void exception_print_stats (void) { printf ("Exception: %lld page faults\n", page_fault_cnt); } /* Handler for an exception (probably) caused by a user process. */ static void kill (struct intr_frame *f) { /* This interrupt is one (probably) caused by a user process. For example, the process might have tried to access unmapped virtual memory (a page fault). For now, we simply kill the user process. Later, we'll want to handle page faults in the kernel. Real Unix-like operating systems pass most exceptions back to the process via signals, but we don't implement them. */ /* The interrupt frame's code segment value tells us where the exception originated. */ switch (f->cs) { case SEL_UCSEG: /* User's code segment, so it's a user exception, as we expected. Kill the user process. */ printf ("%s: dying due to interrupt %#04x (%s).\n", thread_name (), f->vec_no, intr_name (f->vec_no)); intr_dump_frame (f); thread_exit (); case SEL_KCSEG: /* Kernel's code segment, which indicates a kernel bug. Kernel code shouldn't throw exceptions. (Page faults may cause kernel exceptions--but they shouldn't arrive here.) Panic the kernel to make the point. */ intr_dump_frame (f); PANIC ("Kernel bug - unexpected interrupt in kernel"); default: /* Some other code segment? Shouldn't happen. Panic the kernel. */ printf ("Interrupt %#04x (%s) in unknown segment %04x\n", f->vec_no, intr_name (f->vec_no), f->cs); thread_exit (); } } /* Page fault handler. This is a skeleton that must be filled in to implement virtual memory. Some solutions to project 2 may also require modifying this code. At entry, the address that faulted is in CR2 (Control Register 2) and information about the fault, formatted as described in the PF_* macros in exception.h, is in F's error_code member. The example code here shows how to parse that information. You can find more information about both of these in the description of "Interrupt 14--Page Fault Exception (#PF)" in [IA32-v3a] section 5.15 "Exception and Interrupt Reference". */ static void page_fault (struct intr_frame *f) { bool not_present; /* True: not-present page, false: writing r/o page. */ bool write; /* True: access was write, false: access was read. */ bool user; /* True: access by user, false: access by kernel. */ void *fault_addr; /* Fault address. */ /* Obtain faulting address, the virtual address that was accessed to cause the fault. It may point to code or to data. It is not necessarily the address of the instruction that caused the fault (that's f->eip). See [IA32-v2a] "MOV--Move to/from Control Registers" and [IA32-v3a] 5.15 "Interrupt 14--Page Fault Exception (#PF)". */ asm ("movl %%cr2, %0" : "=r" (fault_addr)); /* Turn interrupts back on (they were only off so that we could be assured of reading CR2 before it changed). */ intr_enable (); /* Count page faults. */ page_fault_cnt++; /* Determine cause. */ not_present = (f->error_code & PF_P) == 0; write = (f->error_code & PF_W) != 0; user = (f->error_code & PF_U) != 0; /* To implement virtual memory, delete the rest of the function body, and replace it with code that brings in the page to which fault_addr refers. */ printf ("Page fault at %p: %s error %s page in %s context.\n", fault_addr, not_present ? "not present" : "rights violation", write ? "writing" : "reading", user ? "user" : "kernel"); kill (f); }