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1 | /*P:600 |
2 | * The x86 architecture has segments, which involve a table of descriptors | |
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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 | |
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12 | * from frolicking through its parklike serenity. |
13 | :*/ | |
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14 | #include "lg.h" |
15 | ||
bff672e6 | 16 | /*H:600 |
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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 | ||
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46 | /* |
47 | * There are several entries we don't let the Guest set. The TSS entry is the | |
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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 | |
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50 | * the Guest can't be trusted to deal with double faults. |
51 | */ | |
df1693ab | 52 | static bool ignored_gdt(unsigned int num) |
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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 | ||
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60 | /*H:630 |
61 | * Once the Guest gave us new GDT entries, we fix them up a little. We | |
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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 | |
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65 | * mess: the message will be "unhandled trap 256". |
66 | */ | |
fc708b3e | 67 | static void fixup_gdt_table(struct lg_cpu *cpu, unsigned start, unsigned end) |
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68 | { |
69 | unsigned int i; | |
70 | ||
71 | for (i = start; i < end; i++) { | |
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72 | /* |
73 | * We never copy these ones to real GDT, so we don't care what | |
74 | * they say | |
75 | */ | |
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76 | if (ignored_gdt(i)) |
77 | continue; | |
78 | ||
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79 | /* |
80 | * Segment descriptors contain a privilege level: the Guest is | |
bff672e6 | 81 | * sometimes careless and leaves this as 0, even though it's |
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82 | * running at privilege level 1. If so, we fix it here. |
83 | */ | |
fc708b3e GOC |
84 | if ((cpu->arch.gdt[i].b & 0x00006000) == 0) |
85 | cpu->arch.gdt[i].b |= (GUEST_PL << 13); | |
d7e28ffe | 86 | |
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87 | /* |
88 | * Each descriptor has an "accessed" bit. If we don't set it | |
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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 | |
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91 | * writable by the Guest, so bad things can happen. |
92 | */ | |
fc708b3e | 93 | cpu->arch.gdt[i].b |= 0x00000100; |
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94 | } |
95 | } | |
96 | ||
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97 | /*H:610 |
98 | * Like the IDT, we never simply use the GDT the Guest gives us. We keep | |
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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 | |
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104 | * running. |
105 | */ | |
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106 | void 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 | ||
e1e72965 | 111 | /* The Switcher segments are full 0-4G segments, privilege level 0 */ |
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112 | gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT; |
113 | gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT; | |
114 | ||
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115 | /* |
116 | * The TSS segment refers to the TSS entry for this particular CPU. | |
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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 | |
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119 | * means Saturn is eclipsed by Mercury in the twelfth house. |
120 | */ | |
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121 | gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16); |
122 | gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000) | |
123 | | ((tss >> 16) & 0x000000FF); | |
124 | } | |
125 | ||
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126 | /* |
127 | * This routine sets up the initial Guest GDT for booting. All entries start | |
128 | * as 0 (unusable). | |
129 | */ | |
fc708b3e | 130 | void setup_guest_gdt(struct lg_cpu *cpu) |
d7e28ffe | 131 | { |
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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 | */ | |
fc708b3e GOC |
136 | cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT; |
137 | cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT; | |
fc708b3e GOC |
138 | cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13); |
139 | cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13); | |
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140 | } |
141 | ||
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142 | /*H:650 |
143 | * An optimization of copy_gdt(), for just the three "thead-local storage" | |
144 | * entries. | |
145 | */ | |
fc708b3e | 146 | void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt) |
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147 | { |
148 | unsigned int i; | |
149 | ||
150 | for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++) | |
fc708b3e | 151 | gdt[i] = cpu->arch.gdt[i]; |
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152 | } |
153 | ||
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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 | */ | |
fc708b3e | 159 | void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt) |
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160 | { |
161 | unsigned int i; | |
162 | ||
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163 | /* |
164 | * The default entries from setup_default_gdt_entries() are not | |
165 | * replaced. See ignored_gdt() above. | |
166 | */ | |
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167 | for (i = 0; i < GDT_ENTRIES; i++) |
168 | if (!ignored_gdt(i)) | |
fc708b3e | 169 | gdt[i] = cpu->arch.gdt[i]; |
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170 | } |
171 | ||
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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 | */ | |
a489f0b5 | 176 | void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi) |
d7e28ffe | 177 | { |
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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 | */ | |
3e27249c | 182 | if (num >= ARRAY_SIZE(cpu->arch.gdt)) { |
382ac6b3 | 183 | kill_guest(cpu, "too many gdt entries %i", num); |
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184 | return; |
185 | } | |
d7e28ffe | 186 | |
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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); | |
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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 | */ | |
ae3749dc | 195 | cpu->changed |= CHANGED_GDT; |
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196 | } |
197 | ||
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198 | /* |
199 | * This is the fast-track version for just changing the three TLS entries. | |
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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 | |
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202 | * both cases? |
203 | */ | |
fc708b3e | 204 | void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls) |
d7e28ffe | 205 | { |
fc708b3e | 206 | struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN]; |
d7e28ffe | 207 | |
382ac6b3 | 208 | __lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES); |
fc708b3e | 209 | fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1); |
e1e72965 | 210 | /* Note that just the TLS entries have changed. */ |
ae3749dc | 211 | cpu->changed |= CHANGED_GDT_TLS; |
d7e28ffe | 212 | } |
bff672e6 | 213 | |
e1e72965 | 214 | /*H:660 |
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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 | */ |