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/*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"
* 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
/*H:630 Once the Guest gave us new GDT entries, we fix them 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 lg_cpu *cpu, 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))
/* 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 ((cpu->arch.gdt[i].b & 0x00006000) == 0)
cpu->arch.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. */
cpu->arch.gdt[i].b |= 0x00000100;
/*H:610 Like the IDT, we never simply use the GDT the Guest gives us. We keep
* a GDT for each CPU, and copy across the Guest's entries each time we want to
* run the Guest on that CPU.
* This routine is called at boot or modprobe time for each CPU to set up the
* constant GDT entries: the ones which are the same no matter what Guest we're
* running. */
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 Switcher segments are full 0-4G segments, privilege level 0 */
/* The TSS segment refers to the TSS entry for this particular CPU.
* Forgive the magic flags: the 0x8900 means the entry is Present, it's
* privilege level 0 Available 386 TSS system segment, and the 0x67
* means Saturn is eclipsed by Mercury in the twelfth house. */
gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
| ((tss >> 16) & 0x000000FF);
/* This routine sets up the initial Guest GDT for booting. All entries start
* as 0 (unusable). */
void setup_guest_gdt(struct lg_cpu *cpu)
/* Start with full 0-4G segments... */
/* ...except the Guest is allowed to use them, so set the privilege
* level appropriately in the flags. */
cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
/*H:650 An optimization of copy_gdt(), for just the three "thead-local storage"
* entries. */
void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt)
unsigned int i;
for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
gdt[i] = cpu->arch.gdt[i];
/*H:640 When the Guest is run on a different CPU, or the GDT entries have
* changed, copy_gdt() is called to copy the Guest's GDT entries across to this
* CPU's GDT. */
void copy_gdt(const struct lg_cpu *cpu, 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] = cpu->arch.gdt[i];
/*H:620 This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT).
* We copy it from the Guest and tweak the entries. */
void load_guest_gdt(struct lg_cpu *cpu, 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(cpu->arch.gdt))
kill_guest(cpu, "too many gdt entries %i", num);
/* We read the whole thing in, then fix it up. */
__lgread(cpu, cpu->arch.gdt, table, num * sizeof(cpu->arch.gdt[0]));
fixup_gdt_table(cpu, 0, ARRAY_SIZE(cpu->arch.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. */
cpu->changed |= CHANGED_GDT;
/* This is the fast-track version for just changing the three TLS entries.
* Remember that this happens on every context switch, so it's worth
* optimizing. But wouldn't it be neater to have a single hypercall to cover
* both cases? */
void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls)
struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN];
__lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
/* Note that just the TLS entries have changed. */
cpu->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.