1/*P:600
  2 * The x86 architecture has segments, which involve a table of descriptors
  3 * which can be used to do funky things with virtual address interpretation.
  4 * We originally used to use segments so the Guest couldn't alter the
  5 * Guest<->Host Switcher, and then we had to trim Guest segments, and restore
  6 * for userspace per-thread segments, but trim again for on userspace->kernel
  7 * transitions...  This nightmarish creation was contained within this file,
  8 * where we knew not to tread without heavy armament and a change of underwear.
  9 *
 10 * In these modern times, the segment handling code consists of simple sanity
 11 * checks, and the worst you'll experience reading this code is butterfly-rash
 12 * from frolicking through its parklike serenity.
 13:*/
 14#include "lg.h"
 15
 16/*H:600
 17 * Segments & The Global Descriptor Table
 18 *
 19 * (That title sounds like a bad Nerdcore group.  Not to suggest that there are
 20 * any good Nerdcore groups, but in high school a friend of mine had a band
 21 * called Joe Fish and the Chips, so there are definitely worse band names).
 22 *
 23 * To refresh: the GDT is a table of 8-byte values describing segments.  Once
 24 * set up, these segments can be loaded into one of the 6 "segment registers".
 25 *
 26 * GDT entries are passed around as "struct desc_struct"s, which like IDT
 27 * entries are split into two 32-bit members, "a" and "b".  One day, someone
 28 * will clean that up, and be declared a Hero.  (No pressure, I'm just saying).
 29 *
 30 * Anyway, the GDT entry contains a base (the start address of the segment), a
 31 * limit (the size of the segment - 1), and some flags.  Sounds simple, and it
 32 * would be, except those zany Intel engineers decided that it was too boring
 33 * to put the base at one end, the limit at the other, and the flags in
 34 * between.  They decided to shotgun the bits at random throughout the 8 bytes,
 35 * like so:
 36 *
 37 * 0               16                     40       48  52  56     63
 38 * [ limit part 1 ][     base part 1     ][ flags ][li][fl][base ]
 39 *                                                  mit ags part 2
 40 *                                                part 2
 41 *
 42 * As a result, this file contains a certain amount of magic numeracy.  Let's
 43 * begin.
 44 */
 45
 46/*
 47 * There are several entries we don't let the Guest set.  The TSS entry is the
 48 * "Task State Segment" which controls all kinds of delicate things.  The
 49 * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
 50 * the Guest can't be trusted to deal with double faults.
 51 */
 52static bool ignored_gdt(unsigned int num)
 53{
 54	return (num == GDT_ENTRY_TSS
 55		|| num == GDT_ENTRY_LGUEST_CS
 56		|| num == GDT_ENTRY_LGUEST_DS
 57		|| num == GDT_ENTRY_DOUBLEFAULT_TSS);
 58}
 59
 60/*H:630
 61 * Once the Guest gave us new GDT entries, we fix them up a little.  We
 62 * don't care if they're invalid: the worst that can happen is a General
 63 * Protection Fault in the Switcher when it restores a Guest segment register
 64 * which tries to use that entry.  Then we kill the Guest for causing such a
 65 * mess: the message will be "unhandled trap 256".
 66 */
 67static void fixup_gdt_table(struct lg_cpu *cpu, unsigned start, unsigned end)
 68{
 69	unsigned int i;
 70
 71	for (i = start; i < end; i++) {
 72		/*
 73		 * We never copy these ones to real GDT, so we don't care what
 74		 * they say
 75		 */
 76		if (ignored_gdt(i))
 77			continue;
 78
 79		/*
 80		 * Segment descriptors contain a privilege level: the Guest is
 81		 * sometimes careless and leaves this as 0, even though it's
 82		 * running at privilege level 1.  If so, we fix it here.
 83		 */
 84		if ((cpu->arch.gdt[i].b & 0x00006000) == 0)
 85			cpu->arch.gdt[i].b |= (GUEST_PL << 13);
 86
 87		/*
 88		 * Each descriptor has an "accessed" bit.  If we don't set it
 89		 * now, the CPU will try to set it when the Guest first loads
 90		 * that entry into a segment register.  But the GDT isn't
 91		 * writable by the Guest, so bad things can happen.
 92		 */
 93		cpu->arch.gdt[i].b |= 0x00000100;
 94	}
 95}
 96
 97/*H:610
 98 * Like the IDT, we never simply use the GDT the Guest gives us.  We keep
 99 * a GDT for each CPU, and copy across the Guest's entries each time we want to
100 * run the Guest on that CPU.
101 *
102 * This routine is called at boot or modprobe time for each CPU to set up the
103 * constant GDT entries: the ones which are the same no matter what Guest we're
104 * running.
105 */
106void setup_default_gdt_entries(struct lguest_ro_state *state)
107{
108	struct desc_struct *gdt = state->guest_gdt;
109	unsigned long tss = (unsigned long)&state->guest_tss;
110
111	/* The Switcher segments are full 0-4G segments, privilege level 0 */
112	gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
113	gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
114
115	/*
116	 * The TSS segment refers to the TSS entry for this particular CPU.
117	 * Forgive the magic flags: the 0x8900 means the entry is Present, it's
118	 * privilege level 0 Available 386 TSS system segment, and the 0x67
119	 * means Saturn is eclipsed by Mercury in the twelfth house.
120	 */
121	gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
122	gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
123		| ((tss >> 16) & 0x000000FF);
124}
125
126/*
127 * This routine sets up the initial Guest GDT for booting.  All entries start
128 * as 0 (unusable).
129 */
130void setup_guest_gdt(struct lg_cpu *cpu)
131{
132	/*
133	 * Start with full 0-4G segments...except the Guest is allowed to use
134	 * them, so set the privilege level appropriately in the flags.
135	 */
136	cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
137	cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
138	cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
139	cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
140}
141
142/*H:650
143 * An optimization of copy_gdt(), for just the three "thead-local storage"
144 * entries.
145 */
146void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt)
147{
148	unsigned int i;
149
150	for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
151		gdt[i] = cpu->arch.gdt[i];
152}
153
154/*H:640
155 * When the Guest is run on a different CPU, or the GDT entries have changed,
156 * copy_gdt() is called to copy the Guest's GDT entries across to this CPU's
157 * GDT.
158 */
159void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt)
160{
161	unsigned int i;
162
163	/*
164	 * The default entries from setup_default_gdt_entries() are not
165	 * replaced.  See ignored_gdt() above.
166	 */
167	for (i = 0; i < GDT_ENTRIES; i++)
168		if (!ignored_gdt(i))
169			gdt[i] = cpu->arch.gdt[i];
170}
171
172/*H:620
173 * This is where the Guest asks us to load a new GDT entry
174 * (LHCALL_LOAD_GDT_ENTRY).  We tweak the entry and copy it in.
175 */
176void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi)
177{
178	/*
179	 * We assume the Guest has the same number of GDT entries as the
180	 * Host, otherwise we'd have to dynamically allocate the Guest GDT.
181	 */
182	if (num >= ARRAY_SIZE(cpu->arch.gdt)) {
183		kill_guest(cpu, "too many gdt entries %i", num);
184		return;
185	}
186
187	/* Set it up, then fix it. */
188	cpu->arch.gdt[num].a = lo;
189	cpu->arch.gdt[num].b = hi;
190	fixup_gdt_table(cpu, num, num+1);
191	/*
192	 * Mark that the GDT changed so the core knows it has to copy it again,
193	 * even if the Guest is run on the same CPU.
194	 */
195	cpu->changed |= CHANGED_GDT;
196}
197
198/*
199 * This is the fast-track version for just changing the three TLS entries.
200 * Remember that this happens on every context switch, so it's worth
201 * optimizing.  But wouldn't it be neater to have a single hypercall to cover
202 * both cases?
203 */
204void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls)
205{
206	struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN];
207
208	__lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
209	fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
210	/* Note that just the TLS entries have changed. */
211	cpu->changed |= CHANGED_GDT_TLS;
212}
213
214/*H:660
215 * With this, we have finished the Host.
216 *
217 * Five of the seven parts of our task are complete.  You have made it through
218 * the Bit of Despair (I think that's somewhere in the page table code,
219 * myself).
220 *
221 * Next, we examine "make Switcher".  It's short, but intense.
222 */