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158
src/userprog/exception.c
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158
src/userprog/exception.c
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#include "userprog/exception.h"
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#include <inttypes.h>
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#include <stdio.h>
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#include "userprog/gdt.h"
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#include "threads/interrupt.h"
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#include "threads/thread.h"
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/* Number of page faults processed. */
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static long long page_fault_cnt;
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static void kill (struct intr_frame *);
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static void page_fault (struct intr_frame *);
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/* Registers handlers for interrupts that can be caused by user
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programs.
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In a real Unix-like OS, most of these interrupts would be
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passed along to the user process in the form of signals, as
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described in [SV-386] 3-24 and 3-25, but we don't implement
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signals. Instead, we'll make them simply kill the user
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process.
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Page faults are an exception. Here they are treated the same
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way as other exceptions, but this will need to change to
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implement virtual memory.
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Refer to [IA32-v3a] section 5.15 "Exception and Interrupt
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Reference" for a description of each of these exceptions. */
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void
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exception_init (void)
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{
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/* These exceptions can be raised explicitly by a user program,
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e.g. via the INT, INT3, INTO, and BOUND instructions. Thus,
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we set DPL==3, meaning that user programs are allowed to
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invoke them via these instructions. */
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intr_register_int (3, 3, INTR_ON, kill, "#BP Breakpoint Exception");
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intr_register_int (4, 3, INTR_ON, kill, "#OF Overflow Exception");
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intr_register_int (5, 3, INTR_ON, kill, "#BR BOUND Range Exceeded Exception");
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/* These exceptions have DPL==0, preventing user processes from
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invoking them via the INT instruction. They can still be
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caused indirectly, e.g. #DE can be caused by dividing by
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0. */
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intr_register_int (0, 0, INTR_ON, kill, "#DE Divide Error");
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intr_register_int (1, 0, INTR_ON, kill, "#DB Debug Exception");
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intr_register_int (6, 0, INTR_ON, kill, "#UD Invalid Opcode Exception");
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intr_register_int (7, 0, INTR_ON, kill, "#NM Device Not Available Exception");
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intr_register_int (11, 0, INTR_ON, kill, "#NP Segment Not Present");
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intr_register_int (12, 0, INTR_ON, kill, "#SS Stack Fault Exception");
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intr_register_int (13, 0, INTR_ON, kill, "#GP General Protection Exception");
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intr_register_int (16, 0, INTR_ON, kill, "#MF x87 FPU Floating-Point Error");
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intr_register_int (19, 0, INTR_ON, kill, "#XF SIMD Floating-Point Exception");
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/* Most exceptions can be handled with interrupts turned on.
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We need to disable interrupts for page faults because the
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fault address is stored in CR2 and needs to be preserved. */
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intr_register_int (14, 0, INTR_OFF, page_fault, "#PF Page-Fault Exception");
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}
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/* Prints exception statistics. */
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void
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exception_print_stats (void)
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{
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printf ("Exception: %lld page faults\n", page_fault_cnt);
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}
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/* Handler for an exception (probably) caused by a user process. */
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static void
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kill (struct intr_frame *f)
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{
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/* This interrupt is one (probably) caused by a user process.
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For example, the process might have tried to access unmapped
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virtual memory (a page fault). For now, we simply kill the
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user process. Later, we'll want to handle page faults in
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the kernel. Real Unix-like operating systems pass most
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exceptions back to the process via signals, but we don't
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implement them. */
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/* The interrupt frame's code segment value tells us where the
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exception originated. */
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switch (f->cs)
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{
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case SEL_UCSEG:
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/* User's code segment, so it's a user exception, as we
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expected. Kill the user process. */
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printf ("%s: dying due to interrupt %#04x (%s).\n",
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thread_name (), f->vec_no, intr_name (f->vec_no));
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intr_dump_frame (f);
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thread_exit ();
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case SEL_KCSEG:
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/* Kernel's code segment, which indicates a kernel bug.
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Kernel code shouldn't throw exceptions. (Page faults
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may cause kernel exceptions--but they shouldn't arrive
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here.) Panic the kernel to make the point. */
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intr_dump_frame (f);
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PANIC ("Kernel bug - unexpected interrupt in kernel");
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default:
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/* Some other code segment?
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Shouldn't happen. Panic the kernel. */
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printf ("Interrupt %#04x (%s) in unknown segment %04x\n",
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f->vec_no, intr_name (f->vec_no), f->cs);
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PANIC ("Kernel bug - this shouldn't be possible!");
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}
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}
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/* Page fault handler. This is a skeleton that must be filled in
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to implement virtual memory. Some solutions to task 2 may
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also require modifying this code.
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At entry, the address that faulted is in CR2 (Control Register
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2) and information about the fault, formatted as described in
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the PF_* macros in exception.h, is in F's error_code member. The
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example code here shows how to parse that information. You
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can find more information about both of these in the
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description of "Interrupt 14--Page Fault Exception (#PF)" in
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[IA32-v3a] section 5.15 "Exception and Interrupt Reference". */
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static void
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page_fault (struct intr_frame *f)
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{
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bool not_present; /* True: not-present page, false: writing r/o page. */
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bool write; /* True: access was write, false: access was read. */
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bool user; /* True: access by user, false: access by kernel. */
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void *fault_addr; /* Fault address. */
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/* Obtain faulting address, the virtual address that was
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accessed to cause the fault. It may point to code or to
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data. It is not necessarily the address of the instruction
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that caused the fault (that's f->eip).
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See [IA32-v2a] "MOV--Move to/from Control Registers" and
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[IA32-v3a] 5.15 "Interrupt 14--Page Fault Exception
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(#PF)". */
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asm ("movl %%cr2, %0" : "=r" (fault_addr));
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/* Turn interrupts back on (they were only off so that we could
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be assured of reading CR2 before it changed). */
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intr_enable ();
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/* Count page faults. */
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page_fault_cnt++;
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/* Determine cause. */
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not_present = (f->error_code & PF_P) == 0;
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write = (f->error_code & PF_W) != 0;
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user = (f->error_code & PF_U) != 0;
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/* To implement virtual memory, delete the rest of the function
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body, and replace it with code that brings in the page to
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which fault_addr refers. */
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printf ("Page fault at %p: %s error %s page in %s context.\n",
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fault_addr,
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not_present ? "not present" : "rights violation",
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write ? "writing" : "reading",
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user ? "user" : "kernel");
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kill (f);
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}
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