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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"

/*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_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 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->arch.gdt[i].b & 0x00006000) == 0)

                        lg->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. */

                lg->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 */

        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.

         * 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 lguest *lg)

{

        /* Start with full 0-4G segments... */

        lg->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;

        lg->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. */

        lg->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);

        lg->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 lguest *lg, struct desc_struct *gdt)

{

        unsigned int i;

        for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)

                gdt[i] = lg->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 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->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 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->arch.gdt))

                kill_guest(lg, "too many gdt entries %i", num);

        /* We read the whole thing in, then fix it up. */

        __lgread(lg, lg->arch.gdt, table, num * sizeof(lg->arch.gdt[0]));

        fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->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. */

        lg->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 lguest *lg, unsigned long gtls)

{

        struct desc_struct *tls = &lg->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);

        /* Note that just the TLS entries have changed. */

        lg->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.

 */