| /*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 | |
| * We've almost completed the Host; there's just one file to go! | |
| * | |
| * 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 int 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:610 Once the GDT has been changed, we fix the new entries 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 lguest *lg, 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 ((lg->gdt[i].b & 0x00006000) == 0) | |
| lg->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. */ | |
| lg->gdt[i].b |= 0x00000100; | |
| } | |
| } | |
| /* This routine is called at boot or modprobe time for each CPU to set up the | |
| * "constant" GDT entries for Guests running on that CPU. */ | |
| 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 hypervisor 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 CPU, so we cannot | |
| * copy it from the Guest. Forgive the magic flags */ | |
| gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16); | |
| gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000) | |
| | ((tss >> 16) & 0x000000FF); | |
| } | |
| /* This routine is called before the Guest is run for the first time. */ | |
| void setup_guest_gdt(struct lguest *lg) | |
| { | |
| /* Start with full 0-4G segments... */ | |
| lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT; | |
| lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT; | |
| /* ...except the Guest is allowed to use them, so set the privilege | |
| * level appropriately in the flags. */ | |
| lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13); | |
| lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13); | |
| } | |
| /* Like the IDT, we never simply use the GDT the Guest gives us. We set up the | |
| * GDTs for each CPU, then we copy across the entries each time we want to run | |
| * a different Guest on that CPU. */ | |
| /* A partial GDT load, for the three "thead-local storage" entries. Otherwise | |
| * it's just like load_guest_gdt(). So much, in fact, it would probably be | |
| * neater to have a single hypercall to cover both. */ | |
| void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt) | |
| { | |
| unsigned int i; | |
| for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++) | |
| gdt[i] = lg->gdt[i]; | |
| } | |
| /* This is the full version */ | |
| void copy_gdt(const struct lguest *lg, 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] = lg->gdt[i]; | |
| } | |
| /* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */ | |
| void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num) | |
| { | |
| /* 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(lg->gdt)) | |
| kill_guest(lg, "too many gdt entries %i", num); | |
| /* We read the whole thing in, then fix it up. */ | |
| lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0])); | |
| fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt)); | |
| /* 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. */ | |
| lg->changed |= CHANGED_GDT; | |
| } | |
| void guest_load_tls(struct lguest *lg, unsigned long gtls) | |
| { | |
| struct desc_struct *tls = &lg->gdt[GDT_ENTRY_TLS_MIN]; | |
| lgread(lg, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES); | |
| fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1); | |
| lg->changed |= CHANGED_GDT_TLS; | |
| } | |
| /* | |
| * 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. | |
| */ |