/*P:600
   * The x86 architecture has segments, which involve a table of descriptors

   * which can be used to do funky things with virtual address interpretation.
   * We originally used to use segments so the Guest couldn't alter the
   * Guest<->Host Switcher, and then we had to trim Guest segments, and restore
   * for userspace per-thread segments, but trim again for on userspace->kernel
   * transitions...  This nightmarish creation was contained within this file,
   * where we knew not to tread without heavy armament and a change of underwear.
   *
   * In these modern times, the segment handling code consists of simple sanity
   * checks, and the worst you'll experience reading this code is butterfly-rash

   * from frolicking through its parklike serenity.
  :*/

  #include "lg.h"

  /*H:600

   * Segments & The Global Descriptor Table
   *
   * (That title sounds like a bad Nerdcore group.  Not to suggest that there are
   * any good Nerdcore groups, but in high school a friend of mine had a band
   * called Joe Fish and the Chips, so there are definitely worse band names).
   *
   * To refresh: the GDT is a table of 8-byte values describing segments.  Once
   * set up, these segments can be loaded into one of the 6 "segment registers".
   *
   * GDT entries are passed around as "struct desc_struct"s, which like IDT
   * entries are split into two 32-bit members, "a" and "b".  One day, someone
   * will clean that up, and be declared a Hero.  (No pressure, I'm just saying).
   *
   * Anyway, the GDT entry contains a base (the start address of the segment), a
   * limit (the size of the segment - 1), and some flags.  Sounds simple, and it
   * would be, except those zany Intel engineers decided that it was too boring
   * to put the base at one end, the limit at the other, and the flags in
   * between.  They decided to shotgun the bits at random throughout the 8 bytes,
   * like so:
   *
   * 0               16                     40       48  52  56     63
   * [ limit part 1 ][     base part 1     ][ flags ][li][fl][base ]
   *                                                  mit ags part 2
   *                                                part 2
   *
   * As a result, this file contains a certain amount of magic numeracy.  Let's
   * begin.
   */

  /*
   * There are several entries we don't let the Guest set.  The TSS entry is the

   * "Task State Segment" which controls all kinds of delicate things.  The
   * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the

   * the Guest can't be trusted to deal with double faults.
   */

  static bool ignored_gdt(unsigned int num)

  {
  	return (num == GDT_ENTRY_TSS
  		|| num == GDT_ENTRY_LGUEST_CS
  		|| num == GDT_ENTRY_LGUEST_DS
  		|| num == GDT_ENTRY_DOUBLEFAULT_TSS);
  }

  /*H:630
   * Once the Guest gave us new GDT entries, we fix them up a little.  We

   * don't care if they're invalid: the worst that can happen is a General
   * Protection Fault in the Switcher when it restores a Guest segment register
   * which tries to use that entry.  Then we kill the Guest for causing such a

   * mess: the message will be "unhandled trap 256".
   */

  static void fixup_gdt_table(struct lg_cpu *cpu, unsigned start, unsigned end)

  {
  	unsigned int i;
  
  	for (i = start; i < end; i++) {

  		/*
  		 * We never copy these ones to real GDT, so we don't care what
  		 * they say
  		 */

  		if (ignored_gdt(i))
  			continue;

  		/*
  		 * Segment descriptors contain a privilege level: the Guest is

  		 * sometimes careless and leaves this as 0, even though it's

  		 * running at privilege level 1.  If so, we fix it here.
  		 */

  		if ((cpu->arch.gdt[i].b & 0x00006000) == 0)
  			cpu->arch.gdt[i].b |= (GUEST_PL << 13);

  		/*
  		 * Each descriptor has an "accessed" bit.  If we don't set it

  		 * now, the CPU will try to set it when the Guest first loads
  		 * that entry into a segment register.  But the GDT isn't

  		 * writable by the Guest, so bad things can happen.
  		 */

  		cpu->arch.gdt[i].b |= 0x00000100;

  	}
  }

  /*H:610
   * Like the IDT, we never simply use the GDT the Guest gives us.  We keep

   * a GDT for each CPU, and copy across the Guest's entries each time we want to
   * run the Guest on that CPU.
   *
   * This routine is called at boot or modprobe time for each CPU to set up the
   * constant GDT entries: the ones which are the same no matter what Guest we're

   * running.
   */

  void setup_default_gdt_entries(struct lguest_ro_state *state)
  {
  	struct desc_struct *gdt = state->guest_gdt;
  	unsigned long tss = (unsigned long)&state->guest_tss;

  	/* The Switcher segments are full 0-4G segments, privilege level 0 */

  	gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
  	gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;

  	/*
  	 * The TSS segment refers to the TSS entry for this particular CPU.

  	 * Forgive the magic flags: the 0x8900 means the entry is Present, it's
  	 * privilege level 0 Available 386 TSS system segment, and the 0x67

  	 * means Saturn is eclipsed by Mercury in the twelfth house.
  	 */

  	gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
  	gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
  		| ((tss >> 16) & 0x000000FF);
  }

  /*
   * This routine sets up the initial Guest GDT for booting.  All entries start
   * as 0 (unusable).
   */

  void setup_guest_gdt(struct lg_cpu *cpu)

  {

  	/*
  	 * Start with full 0-4G segments...except the Guest is allowed to use
  	 * them, so set the privilege level appropriately in the flags.
  	 */

  	cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
  	cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;

  	cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
  	cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);

  }

  /*H:650
   * An optimization of copy_gdt(), for just the three "thead-local storage"
   * entries.
   */

  void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt)

  {
  	unsigned int i;
  
  	for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)

  		gdt[i] = cpu->arch.gdt[i];

  }

  /*H:640
   * When the Guest is run on a different CPU, or the GDT entries have changed,
   * copy_gdt() is called to copy the Guest's GDT entries across to this CPU's
   * GDT.
   */

  void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt)

  {
  	unsigned int i;

  	/*
  	 * The default entries from setup_default_gdt_entries() are not
  	 * replaced.  See ignored_gdt() above.
  	 */

  	for (i = 0; i < GDT_ENTRIES; i++)
  		if (!ignored_gdt(i))

  			gdt[i] = cpu->arch.gdt[i];

  }

  /*H:620
   * This is where the Guest asks us to load a new GDT entry
   * (LHCALL_LOAD_GDT_ENTRY).  We tweak the entry and copy it in.
   */

  void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi)

  {

  	/*
  	 * We assume the Guest has the same number of GDT entries as the
  	 * Host, otherwise we'd have to dynamically allocate the Guest GDT.
  	 */

  	if (num >= ARRAY_SIZE(cpu->arch.gdt)) {

  		kill_guest(cpu, "too many gdt entries %i", num);

  		return;
  	}

  	/* Set it up, then fix it. */
  	cpu->arch.gdt[num].a = lo;
  	cpu->arch.gdt[num].b = hi;
  	fixup_gdt_table(cpu, num, num+1);

  	/*
  	 * Mark that the GDT changed so the core knows it has to copy it again,
  	 * even if the Guest is run on the same CPU.
  	 */

  	cpu->changed |= CHANGED_GDT;

  }

  /*
   * This is the fast-track version for just changing the three TLS entries.

   * Remember that this happens on every context switch, so it's worth
   * optimizing.  But wouldn't it be neater to have a single hypercall to cover

   * both cases?
   */

  void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls)

  {

  	struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN];

  	__lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);

  	fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);

  	/* Note that just the TLS entries have changed. */

  	cpu->changed |= CHANGED_GDT_TLS;

  }

  /*H:660

   * With this, we have finished the Host.
   *
   * Five of the seven parts of our task are complete.  You have made it through
   * the Bit of Despair (I think that's somewhere in the page table code,
   * myself).
   *
   * Next, we examine "make Switcher".  It's short, but intense.
   */