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src/userprog/tss.c

+#include "userprog/tss.h"
+#include <debug.h>
+#include <stddef.h>
+#include "userprog/gdt.h"
+#include "threads/thread.h"
+#include "threads/palloc.h"
+#include "threads/vaddr.h"
+
+/* The Task-State Segment (TSS).
+
+   Instances of the TSS, an x86-specific structure, are used to
+   define "tasks", a form of support for multitasking built right
+   into the processor.  However, for various reasons including
+   portability, speed, and flexibility, most x86 OSes almost
+   completely ignore the TSS.  We are no exception.
+
+   Unfortunately, there is one thing that can only be done using
+   a TSS: stack switching for interrupts that occur in user mode.
+   When an interrupt occurs in user mode (ring 3), the processor
+   consults the ss0 and esp0 members of the current TSS to
+   determine the stack to use for handling the interrupt.  Thus,
+   we must create a TSS and initialize at least these fields, and
+   this is precisely what this file does.
+
+   When an interrupt is handled by an interrupt or trap gate
+   (which applies to all interrupts we handle), an x86 processor
+   works like this:
+
+     - If the code interrupted by the interrupt is in the same
+       ring as the interrupt handler, then no stack switch takes
+       place.  This is the case for interrupts that happen when
+       we're running in the kernel.  The contents of the TSS are
+       irrelevant for this case.
+
+     - If the interrupted code is in a different ring from the
+       handler, then the processor switches to the stack
+       specified in the TSS for the new ring.  This is the case
+       for interrupts that happen when we're in user space.  It's
+       important that we switch to a stack that's not already in
+       use, to avoid corruption.  Because we're running in user
+       space, we know that the current process's kernel stack is
+       not in use, so we can always use that.  Thus, when the
+       scheduler switches threads, it also changes the TSS's
+       stack pointer to point to the new thread's kernel stack.
+       (The call is in thread_schedule_tail() in thread.c.)
+
+   See [IA32-v3a] 6.2.1 "Task-State Segment (TSS)" for a
+   description of the TSS.  See [IA32-v3a] 5.12.1 "Exception- or
+   Interrupt-Handler Procedures" for a description of when and
+   how stack switching occurs during an interrupt. */
+struct tss
+  {
+    uint16_t back_link, :16;
+    void *esp0;                         /* Ring 0 stack virtual address. */
+    uint16_t ss0, :16;                  /* Ring 0 stack segment selector. */
+    void *esp1;
+    uint16_t ss1, :16;
+    void *esp2;
+    uint16_t ss2, :16;
+    uint32_t cr3;
+    void (*eip) (void);
+    uint32_t eflags;
+    uint32_t eax, ecx, edx, ebx;
+    uint32_t esp, ebp, esi, edi;
+    uint16_t es, :16;
+    uint16_t cs, :16;
+    uint16_t ss, :16;
+    uint16_t ds, :16;
+    uint16_t fs, :16;
+    uint16_t gs, :16;
+    uint16_t ldt, :16;
+    uint16_t trace, bitmap;
+  };
+
+/* Kernel TSS. */
+static struct tss *tss;
+
+/* Initializes the kernel TSS. */
+void
+tss_init (void)
+{
+  /* Our TSS is never used in a call gate or task gate, so only a
+     few fields of it are ever referenced, and those are the only
+     ones we initialize. */
+  tss = palloc_get_page (PAL_ASSERT | PAL_ZERO);
+  tss->ss0 = SEL_KDSEG;
+  tss->bitmap = 0xdfff;
+  tss_update ();
+}
+
+/* Returns the kernel TSS. */
+struct tss *
+tss_get (void)
+{
+  ASSERT (tss != NULL);
+  return tss;
+}
+
+/* Sets the ring 0 stack pointer in the TSS to point to the end
+   of the thread stack. */
+void
+tss_update (void)
+{
+  ASSERT (tss != NULL);
+  tss->esp0 = (uint8_t *) thread_current () + PGSIZE;
+}