+/*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"
-static int desc_ok(const struct desc_struct *gdt)
-{
- /* MBZ=0, P=1, DT=1 */
- return ((gdt->b & 0x00209000) == 0x00009000);
-}
-
-static int segment_present(const struct desc_struct *gdt)
-{
- return gdt->b & 0x8000;
-}
-
+/*H:600
+ * 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_DOUBLEFAULT_TSS);
}
-/* We don't allow removal of CS, DS or SS; it doesn't make sense. */
-static void check_segment_use(struct lguest *lg, unsigned int desc)
-{
- if (lg->regs->gs / 8 == desc)
- lg->regs->gs = 0;
- if (lg->regs->fs / 8 == desc)
- lg->regs->fs = 0;
- if (lg->regs->es / 8 == desc)
- lg->regs->es = 0;
- if (lg->regs->ds / 8 == desc
- || lg->regs->cs / 8 == desc
- || lg->regs->ss / 8 == desc)
- kill_guest(lg, "Removed live GDT entry %u", desc);
-}
-
-static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
+/*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 */
+ /* We never copy these ones to real GDT, so we don't care what
+ * they say */
if (ignored_gdt(i))
continue;
- /* We could fault in switch_to_guest if they are using
- * a removed segment. */
- if (!segment_present(&lg->gdt[i])) {
- check_segment_use(lg, i);
- continue;
- }
-
- if (!desc_ok(&lg->gdt[i]))
- kill_guest(lg, "Bad GDT descriptor %i", i);
-
- /* DPL 0 presumably means "for use by guest". */
- if ((lg->gdt[i].b & 0x00006000) == 0)
- lg->gdt[i].b |= (GUEST_PL << 13);
-
- /* Set accessed bit, since gdt isn't writable. */
- lg->gdt[i].b |= 0x00000100;
+ /* 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;
- /* Hypervisor segments. */
+ /* The Switcher segments are full 0-4G segments, privilege level 0 */
gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
- /* This is the one which we *cannot* copy from guest, since tss
- is depended on this lguest_ro_state, ie. this cpu. */
+ /* 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);
}
-void setup_guest_gdt(struct lguest *lg)
+/* This routine sets up the initial Guest GDT for booting. All entries start
+ * as 0 (unusable). */
+void setup_guest_gdt(struct lg_cpu *cpu)
{
- lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
- lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
- lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
- lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
+ /* Start with full 0-4G segments... */
+ cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
+ cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
+ /* ...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);
}
-/* This is a fast version for the common case where only the three TLS entries
- * have changed. */
-void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
+/*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] = lg->gdt[i];
+ gdt[i] = cpu->arch.gdt[i];
}
-void copy_gdt(const struct lguest *lg, struct desc_struct *gdt)
+/*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] = lg->gdt[i];
+ gdt[i] = cpu->arch.gdt[i];
}
-void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num)
+/*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)
{
- if (num > ARRAY_SIZE(lg->gdt))
- kill_guest(lg, "too many gdt entries %i", num);
-
- lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0]));
- fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt));
- lg->changed |= CHANGED_GDT;
+ /* 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;
}
-void guest_load_tls(struct lguest *lg, unsigned long gtls)
+/* 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 = &lg->gdt[GDT_ENTRY_TLS_MIN];
+ struct desc_struct *tls = &cpu->arch.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;
+ __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;
}
+/*:*/
+
+/*H:660
+ * 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.
+ */
git.openpandora.org / pandora-kernel.git / blobdiff