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