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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. :*/ | |
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12 | #include "lg.h" |
13 | ||
bff672e6 | 14 | /*H:600 |
bff672e6 RR |
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 | ||
bff672e6 RR |
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. */ | |
d7e28ffe RR |
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 | ||
e1e72965 | 56 | /*H:630 Once the Guest gave us new GDT entries, we fix them up a little. We |
0d027c01 RR |
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". */ | |
fc708b3e | 61 | static void fixup_gdt_table(struct lg_cpu *cpu, unsigned start, unsigned end) |
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62 | { |
63 | unsigned int i; | |
64 | ||
65 | for (i = start; i < end; i++) { | |
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66 | /* We never copy these ones to real GDT, so we don't care what |
67 | * they say */ | |
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68 | if (ignored_gdt(i)) |
69 | continue; | |
70 | ||
bff672e6 RR |
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. */ | |
fc708b3e GOC |
74 | if ((cpu->arch.gdt[i].b & 0x00006000) == 0) |
75 | cpu->arch.gdt[i].b |= (GUEST_PL << 13); | |
d7e28ffe | 76 | |
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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. */ | |
fc708b3e | 81 | cpu->arch.gdt[i].b |= 0x00000100; |
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82 | } |
83 | } | |
84 | ||
e1e72965 RR |
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. */ | |
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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 | ||
e1e72965 | 97 | /* The Switcher segments are full 0-4G segments, privilege level 0 */ |
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98 | gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT; |
99 | gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT; | |
100 | ||
e1e72965 RR |
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. */ | |
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105 | gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16); |
106 | gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000) | |
107 | | ((tss >> 16) & 0x000000FF); | |
108 | } | |
109 | ||
e1e72965 RR |
110 | /* This routine sets up the initial Guest GDT for booting. All entries start |
111 | * as 0 (unusable). */ | |
fc708b3e | 112 | void setup_guest_gdt(struct lg_cpu *cpu) |
d7e28ffe | 113 | { |
bff672e6 | 114 | /* Start with full 0-4G segments... */ |
fc708b3e GOC |
115 | cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT; |
116 | cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT; | |
bff672e6 RR |
117 | /* ...except the Guest is allowed to use them, so set the privilege |
118 | * level appropriately in the flags. */ | |
fc708b3e GOC |
119 | cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13); |
120 | cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13); | |
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121 | } |
122 | ||
e1e72965 RR |
123 | /*H:650 An optimization of copy_gdt(), for just the three "thead-local storage" |
124 | * entries. */ | |
fc708b3e | 125 | void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt) |
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126 | { |
127 | unsigned int i; | |
128 | ||
129 | for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++) | |
fc708b3e | 130 | gdt[i] = cpu->arch.gdt[i]; |
d7e28ffe RR |
131 | } |
132 | ||
e1e72965 RR |
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. */ | |
fc708b3e | 136 | void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt) |
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137 | { |
138 | unsigned int i; | |
139 | ||
bff672e6 RR |
140 | /* The default entries from setup_default_gdt_entries() are not |
141 | * replaced. See ignored_gdt() above. */ | |
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142 | for (i = 0; i < GDT_ENTRIES; i++) |
143 | if (!ignored_gdt(i)) | |
fc708b3e | 144 | gdt[i] = cpu->arch.gdt[i]; |
d7e28ffe RR |
145 | } |
146 | ||
e1e72965 RR |
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. */ | |
fc708b3e | 149 | void load_guest_gdt(struct lg_cpu *cpu, unsigned long table, u32 num) |
d7e28ffe | 150 | { |
bff672e6 RR |
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. */ | |
fc708b3e | 153 | if (num > ARRAY_SIZE(cpu->arch.gdt)) |
382ac6b3 | 154 | kill_guest(cpu, "too many gdt entries %i", num); |
d7e28ffe | 155 | |
bff672e6 | 156 | /* We read the whole thing in, then fix it up. */ |
382ac6b3 | 157 | __lgread(cpu, cpu->arch.gdt, table, num * sizeof(cpu->arch.gdt[0])); |
fc708b3e | 158 | fixup_gdt_table(cpu, 0, ARRAY_SIZE(cpu->arch.gdt)); |
bff672e6 RR |
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. */ | |
ae3749dc | 161 | cpu->changed |= CHANGED_GDT; |
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162 | } |
163 | ||
e1e72965 RR |
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? */ | |
fc708b3e | 168 | void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls) |
d7e28ffe | 169 | { |
fc708b3e | 170 | struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN]; |
d7e28ffe | 171 | |
382ac6b3 | 172 | __lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES); |
fc708b3e | 173 | fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1); |
e1e72965 | 174 | /* Note that just the TLS entries have changed. */ |
ae3749dc | 175 | cpu->changed |= CHANGED_GDT_TLS; |
d7e28ffe | 176 | } |
e1e72965 | 177 | /*:*/ |
bff672e6 | 178 | |
e1e72965 | 179 | /*H:660 |
bff672e6 RR |
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 | */ |