--- /dev/null
+/*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
+ * 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 bool 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: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))
+ 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 (cpu->arch.gdt[i].dpl == 0)
+ cpu->arch.gdt[i].dpl |= GUEST_PL;
+
+ /*
+ * 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].type |= 0x1;
+ }
+}
+
+/*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 */
+ 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 particular CPU.
+ */
+ gdt[GDT_ENTRY_TSS].a = 0;
+ gdt[GDT_ENTRY_TSS].b = 0;
+
+ gdt[GDT_ENTRY_TSS].limit0 = 0x67;
+ gdt[GDT_ENTRY_TSS].base0 = tss & 0xFFFF;
+ gdt[GDT_ENTRY_TSS].base1 = (tss >> 16) & 0xFF;
+ gdt[GDT_ENTRY_TSS].base2 = tss >> 24;
+ gdt[GDT_ENTRY_TSS].type = 0x9; /* 32-bit TSS (available) */
+ gdt[GDT_ENTRY_TSS].p = 0x1; /* Entry is present */
+ gdt[GDT_ENTRY_TSS].dpl = 0x0; /* Privilege level 0 */
+ gdt[GDT_ENTRY_TSS].s = 0x0; /* system segment */
+
+}
+
+/*
+ * 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] = FULL_EXEC_SEGMENT;
+ cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
+ cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].dpl |= GUEST_PL;
+ cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].dpl |= GUEST_PL;
+}
+
+/*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 entry
+ * (LHCALL_LOAD_GDT_ENTRY). We tweak the entry and copy it in.
+ */
+void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi)
+{
+ /*
+ * 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);
+ return;
+ }
+
+ /* Set it up, then fix it. */
+ cpu->arch.gdt[num].a = lo;
+ cpu->arch.gdt[num].b = hi;
+ fixup_gdt_table(cpu, num, num+1);
+ /*
+ * 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;
+}
+
+/*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.
+ */
Code Review / kvmfornfv.git / blobdiff