Normalize config options for guest support

[linux-2.6] / drivers / lguest / core.c

/*P:400 This contains run_guest() which actually calls into the Host<->Guest
 * Switcher and analyzes the return, such as determining if the Guest wants the
 * Host to do something.  This file also contains useful helper routines, and a
 * couple of non-obvious setup and teardown pieces which were implemented after
 * days of debugging pain. :*/
#include <linux/module.h>
#include <linux/stringify.h>
#include <linux/stddef.h>
#include <linux/io.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/cpu.h>
#include <linux/freezer.h>
#include <asm/paravirt.h>
#include <asm/desc.h>
#include <asm/pgtable.h>
#include <asm/uaccess.h>
#include <asm/poll.h>
#include <asm/highmem.h>
#include <asm/asm-offsets.h>
#include <asm/i387.h>
#include "lg.h"

/* Found in switcher.S */
extern char start_switcher_text[], end_switcher_text[], switch_to_guest[];
extern unsigned long default_idt_entries[];

/* Every guest maps the core switcher code. */
#define SHARED_SWITCHER_PAGES \
        DIV_ROUND_UP(end_switcher_text - start_switcher_text, PAGE_SIZE)
/* Pages for switcher itself, then two pages per cpu */
#define TOTAL_SWITCHER_PAGES (SHARED_SWITCHER_PAGES + 2 * NR_CPUS)

/* We map at -4M for ease of mapping into the guest (one PTE page). */
#define SWITCHER_ADDR 0xFFC00000

static struct vm_struct *switcher_vma;
static struct page **switcher_page;

static int cpu_had_pge;
static struct {
        unsigned long offset;
        unsigned short segment;
} lguest_entry;

/* This One Big lock protects all inter-guest data structures. */
DEFINE_MUTEX(lguest_lock);
static DEFINE_PER_CPU(struct lguest *, last_guest);

/* FIXME: Make dynamic. */
#define MAX_LGUEST_GUESTS 16
struct lguest lguests[MAX_LGUEST_GUESTS];

/* Offset from where switcher.S was compiled to where we've copied it */
static unsigned long switcher_offset(void)
{
        return SWITCHER_ADDR - (unsigned long)start_switcher_text;
}

/* This cpu's struct lguest_pages. */
static struct lguest_pages *lguest_pages(unsigned int cpu)
{
        return &(((struct lguest_pages *)
                  (SWITCHER_ADDR + SHARED_SWITCHER_PAGES*PAGE_SIZE))[cpu]);
}

/*H:010 We need to set up the Switcher at a high virtual address.  Remember the
 * Switcher is a few hundred bytes of assembler code which actually changes the
 * CPU to run the Guest, and then changes back to the Host when a trap or
 * interrupt happens.
 *
 * The Switcher code must be at the same virtual address in the Guest as the
 * Host since it will be running as the switchover occurs.
 *
 * Trying to map memory at a particular address is an unusual thing to do, so
 * it's not a simple one-liner.  We also set up the per-cpu parts of the
 * Switcher here.
 */
static __init int map_switcher(void)
{
        int i, err;
        struct page **pagep;

        /*
         * Map the Switcher in to high memory.
         *
         * It turns out that if we choose the address 0xFFC00000 (4MB under the
         * top virtual address), it makes setting up the page tables really
         * easy.
         */

        /* We allocate an array of "struct page"s.  map_vm_area() wants the
         * pages in this form, rather than just an array of pointers. */
        switcher_page = kmalloc(sizeof(switcher_page[0])*TOTAL_SWITCHER_PAGES,
                                GFP_KERNEL);
        if (!switcher_page) {
                err = -ENOMEM;
                goto out;
        }

        /* Now we actually allocate the pages.  The Guest will see these pages,
         * so we make sure they're zeroed. */
        for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
                unsigned long addr = get_zeroed_page(GFP_KERNEL);
                if (!addr) {
                        err = -ENOMEM;
                        goto free_some_pages;
                }
                switcher_page[i] = virt_to_page(addr);
        }

        /* Now we reserve the "virtual memory area" we want: 0xFFC00000
         * (SWITCHER_ADDR).  We might not get it in theory, but in practice
         * it's worked so far. */
        switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
                                       VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
        if (!switcher_vma) {
                err = -ENOMEM;
                printk("lguest: could not map switcher pages high\n");
                goto free_pages;
        }

        /* This code actually sets up the pages we've allocated to appear at
         * SWITCHER_ADDR.  map_vm_area() takes the vma we allocated above, the
         * kind of pages we're mapping (kernel pages), and a pointer to our
         * array of struct pages.  It increments that pointer, but we don't
         * care. */
        pagep = switcher_page;
        err = map_vm_area(switcher_vma, PAGE_KERNEL, &pagep);
        if (err) {
                printk("lguest: map_vm_area failed: %i\n", err);
                goto free_vma;
        }

        /* Now the switcher is mapped at the right address, we can't fail!
         * Copy in the compiled-in Switcher code (from switcher.S). */
        memcpy(switcher_vma->addr, start_switcher_text,
               end_switcher_text - start_switcher_text);

        /* Most of the switcher.S doesn't care that it's been moved; on Intel,
         * jumps are relative, and it doesn't access any references to external
         * code or data.
         *
         * The only exception is the interrupt handlers in switcher.S: their
         * addresses are placed in a table (default_idt_entries), so we need to
         * update the table with the new addresses.  switcher_offset() is a
         * convenience function which returns the distance between the builtin
         * switcher code and the high-mapped copy we just made. */
        for (i = 0; i < IDT_ENTRIES; i++)
                default_idt_entries[i] += switcher_offset();

        /*
         * Set up the Switcher's per-cpu areas.
         *
         * Each CPU gets two pages of its own within the high-mapped region
         * (aka. "struct lguest_pages").  Much of this can be initialized now,
         * but some depends on what Guest we are running (which is set up in
         * copy_in_guest_info()).
         */
        for_each_possible_cpu(i) {
                /* lguest_pages() returns this CPU's two pages. */
                struct lguest_pages *pages = lguest_pages(i);
                /* This is a convenience pointer to make the code fit one
                 * statement to a line. */
                struct lguest_ro_state *state = &pages->state;

                /* The Global Descriptor Table: the Host has a different one
                 * for each CPU.  We keep a descriptor for the GDT which says
                 * where it is and how big it is (the size is actually the last
                 * byte, not the size, hence the "-1"). */
                state->host_gdt_desc.size = GDT_SIZE-1;
                state->host_gdt_desc.address = (long)get_cpu_gdt_table(i);

                /* All CPUs on the Host use the same Interrupt Descriptor
                 * Table, so we just use store_idt(), which gets this CPU's IDT
                 * descriptor. */
                store_idt(&state->host_idt_desc);

                /* The descriptors for the Guest's GDT and IDT can be filled
                 * out now, too.  We copy the GDT & IDT into ->guest_gdt and
                 * ->guest_idt before actually running the Guest. */
                state->guest_idt_desc.size = sizeof(state->guest_idt)-1;
                state->guest_idt_desc.address = (long)&state->guest_idt;
                state->guest_gdt_desc.size = sizeof(state->guest_gdt)-1;
                state->guest_gdt_desc.address = (long)&state->guest_gdt;

                /* We know where we want the stack to be when the Guest enters
                 * the switcher: in pages->regs.  The stack grows upwards, so
                 * we start it at the end of that structure. */
                state->guest_tss.esp0 = (long)(&pages->regs + 1);
                /* And this is the GDT entry to use for the stack: we keep a
                 * couple of special LGUEST entries. */
                state->guest_tss.ss0 = LGUEST_DS;

                /* x86 can have a finegrained bitmap which indicates what I/O
                 * ports the process can use.  We set it to the end of our
                 * structure, meaning "none". */
                state->guest_tss.io_bitmap_base = sizeof(state->guest_tss);

                /* Some GDT entries are the same across all Guests, so we can
                 * set them up now. */
                setup_default_gdt_entries(state);
                /* Most IDT entries are the same for all Guests, too.*/
                setup_default_idt_entries(state, default_idt_entries);

                /* The Host needs to be able to use the LGUEST segments on this
                 * CPU, too, so put them in the Host GDT. */
                get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
                get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
        }

        /* In the Switcher, we want the %cs segment register to use the
         * LGUEST_CS GDT entry: we've put that in the Host and Guest GDTs, so
         * it will be undisturbed when we switch.  To change %cs and jump we
         * need this structure to feed to Intel's "lcall" instruction. */
        lguest_entry.offset = (long)switch_to_guest + switcher_offset();
        lguest_entry.segment = LGUEST_CS;

        printk(KERN_INFO "lguest: mapped switcher at %p\n",
               switcher_vma->addr);
        /* And we succeeded... */
        return 0;

free_vma:
        vunmap(switcher_vma->addr);
free_pages:
        i = TOTAL_SWITCHER_PAGES;
free_some_pages:
        for (--i; i >= 0; i--)
                __free_pages(switcher_page[i], 0);
        kfree(switcher_page);
out:
        return err;
}
/*:*/

/* Cleaning up the mapping when the module is unloaded is almost...
 * too easy. */
static void unmap_switcher(void)
{
        unsigned int i;

        /* vunmap() undoes *both* map_vm_area() and __get_vm_area(). */
        vunmap(switcher_vma->addr);
        /* Now we just need to free the pages we copied the switcher into */
        for (i = 0; i < TOTAL_SWITCHER_PAGES; i++)
                __free_pages(switcher_page[i], 0);
}

/*H:130 Our Guest is usually so well behaved; it never tries to do things it
 * isn't allowed to.  Unfortunately, Linux's paravirtual infrastructure isn't
 * quite complete, because it doesn't contain replacements for the Intel I/O
 * instructions.  As a result, the Guest sometimes fumbles across one during
 * the boot process as it probes for various things which are usually attached
 * to a PC.
 *
 * When the Guest uses one of these instructions, we get trap #13 (General
 * Protection Fault) and come here.  We see if it's one of those troublesome
 * instructions and skip over it.  We return true if we did. */
static int emulate_insn(struct lguest *lg)
{
        u8 insn;
        unsigned int insnlen = 0, in = 0, shift = 0;
        /* The eip contains the *virtual* address of the Guest's instruction:
         * guest_pa just subtracts the Guest's page_offset. */
        unsigned long physaddr = guest_pa(lg, lg->regs->eip);

        /* The guest_pa() function only works for Guest kernel addresses, but
         * that's all we're trying to do anyway. */
        if (lg->regs->eip < lg->page_offset)
                return 0;

        /* Decoding x86 instructions is icky. */
        lgread(lg, &insn, physaddr, 1);

        /* 0x66 is an "operand prefix".  It means it's using the upper 16 bits
           of the eax register. */
        if (insn == 0x66) {
                shift = 16;
                /* The instruction is 1 byte so far, read the next byte. */
                insnlen = 1;
                lgread(lg, &insn, physaddr + insnlen, 1);
        }

        /* We can ignore the lower bit for the moment and decode the 4 opcodes
         * we need to emulate. */
        switch (insn & 0xFE) {
        case 0xE4: /* in     <next byte>,%al */
                insnlen += 2;
                in = 1;
                break;
        case 0xEC: /* in     (%dx),%al */
                insnlen += 1;
                in = 1;
                break;
        case 0xE6: /* out    %al,<next byte> */
                insnlen += 2;
                break;
        case 0xEE: /* out    %al,(%dx) */
                insnlen += 1;
                break;
        default:
                /* OK, we don't know what this is, can't emulate. */
                return 0;
        }

        /* If it was an "IN" instruction, they expect the result to be read
         * into %eax, so we change %eax.  We always return all-ones, which
         * traditionally means "there's nothing there". */
        if (in) {
                /* Lower bit tells is whether it's a 16 or 32 bit access */
                if (insn & 0x1)
                        lg->regs->eax = 0xFFFFFFFF;
                else
                        lg->regs->eax |= (0xFFFF << shift);
        }
        /* Finally, we've "done" the instruction, so move past it. */
        lg->regs->eip += insnlen;
        /* Success! */
        return 1;
}
/*:*/

/*L:305
 * Dealing With Guest Memory.
 *
 * When the Guest gives us (what it thinks is) a physical address, we can use
 * the normal copy_from_user() & copy_to_user() on that address: remember,
 * Guest physical == Launcher virtual.
 *
 * But we can't trust the Guest: it might be trying to access the Launcher
 * code.  We have to check that the range is below the pfn_limit the Launcher
 * gave us.  We have to make sure that addr + len doesn't give us a false
 * positive by overflowing, too. */
int lguest_address_ok(const struct lguest *lg,
                      unsigned long addr, unsigned long len)
{
        return (addr+len) / PAGE_SIZE < lg->pfn_limit && (addr+len >= addr);
}

/* This is a convenient routine to get a 32-bit value from the Guest (a very
 * common operation).  Here we can see how useful the kill_lguest() routine we
 * met in the Launcher can be: we return a random value (0) instead of needing
 * to return an error. */
u32 lgread_u32(struct lguest *lg, unsigned long addr)
{
        u32 val = 0;

        /* Don't let them access lguest binary. */
        if (!lguest_address_ok(lg, addr, sizeof(val))
            || get_user(val, (u32 __user *)addr) != 0)
                kill_guest(lg, "bad read address %#lx", addr);
        return val;
}

/* Same thing for writing a value. */
void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
{
        if (!lguest_address_ok(lg, addr, sizeof(val))
            || put_user(val, (u32 __user *)addr) != 0)
                kill_guest(lg, "bad write address %#lx", addr);
}

/* This routine is more generic, and copies a range of Guest bytes into a
 * buffer.  If the copy_from_user() fails, we fill the buffer with zeroes, so
 * the caller doesn't end up using uninitialized kernel memory. */
void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)
{
        if (!lguest_address_ok(lg, addr, bytes)
            || copy_from_user(b, (void __user *)addr, bytes) != 0) {
                /* copy_from_user should do this, but as we rely on it... */
                memset(b, 0, bytes);
                kill_guest(lg, "bad read address %#lx len %u", addr, bytes);
        }
}

/* Similarly, our generic routine to copy into a range of Guest bytes. */
void lgwrite(struct lguest *lg, unsigned long addr, const void *b,
             unsigned bytes)
{
        if (!lguest_address_ok(lg, addr, bytes)
            || copy_to_user((void __user *)addr, b, bytes) != 0)
                kill_guest(lg, "bad write address %#lx len %u", addr, bytes);
}
/* (end of memory access helper routines) :*/

static void set_ts(void)
{
        u32 cr0;

        cr0 = read_cr0();
        if (!(cr0 & 8))
                write_cr0(cr0|8);
}

/*S:010
 * We are getting close to the Switcher.
 *
 * Remember that each CPU has two pages which are visible to the Guest when it
 * runs on that CPU.  This has to contain the state for that Guest: we copy the
 * state in just before we run the Guest.
 *
 * Each Guest has "changed" flags which indicate what has changed in the Guest
 * since it last ran.  We saw this set in interrupts_and_traps.c and
 * segments.c.
 */
static void copy_in_guest_info(struct lguest *lg, struct lguest_pages *pages)
{
        /* Copying all this data can be quite expensive.  We usually run the
         * same Guest we ran last time (and that Guest hasn't run anywhere else
         * meanwhile).  If that's not the case, we pretend everything in the
         * Guest has changed. */
        if (__get_cpu_var(last_guest) != lg || lg->last_pages != pages) {
                __get_cpu_var(last_guest) = lg;
                lg->last_pages = pages;
                lg->changed = CHANGED_ALL;
        }

        /* These copies are pretty cheap, so we do them unconditionally: */
        /* Save the current Host top-level page directory. */
        pages->state.host_cr3 = __pa(current->mm->pgd);
        /* Set up the Guest's page tables to see this CPU's pages (and no
         * other CPU's pages). */
        map_switcher_in_guest(lg, pages);
        /* Set up the two "TSS" members which tell the CPU what stack to use
         * for traps which do directly into the Guest (ie. traps at privilege
         * level 1). */
        pages->state.guest_tss.esp1 = lg->esp1;
        pages->state.guest_tss.ss1 = lg->ss1;

        /* Copy direct-to-Guest trap entries. */
        if (lg->changed & CHANGED_IDT)
                copy_traps(lg, pages->state.guest_idt, default_idt_entries);

        /* Copy all GDT entries which the Guest can change. */
        if (lg->changed & CHANGED_GDT)
                copy_gdt(lg, pages->state.guest_gdt);
        /* If only the TLS entries have changed, copy them. */
        else if (lg->changed & CHANGED_GDT_TLS)
                copy_gdt_tls(lg, pages->state.guest_gdt);

        /* Mark the Guest as unchanged for next time. */
        lg->changed = 0;
}

/* Finally: the code to actually call into the Switcher to run the Guest. */
static void run_guest_once(struct lguest *lg, struct lguest_pages *pages)
{
        /* This is a dummy value we need for GCC's sake. */
        unsigned int clobber;

        /* Copy the guest-specific information into this CPU's "struct
         * lguest_pages". */
        copy_in_guest_info(lg, pages);

        /* Set the trap number to 256 (impossible value).  If we fault while
         * switching to the Guest (bad segment registers or bug), this will
         * cause us to abort the Guest. */
        lg->regs->trapnum = 256;

        /* Now: we push the "eflags" register on the stack, then do an "lcall".
         * This is how we change from using the kernel code segment to using
         * the dedicated lguest code segment, as well as jumping into the
         * Switcher.
         *
         * The lcall also pushes the old code segment (KERNEL_CS) onto the
         * stack, then the address of this call.  This stack layout happens to
         * exactly match the stack of an interrupt... */
        asm volatile("pushf; lcall *lguest_entry"
                     /* This is how we tell GCC that %eax ("a") and %ebx ("b")
                      * are changed by this routine.  The "=" means output. */
                     : "=a"(clobber), "=b"(clobber)
                     /* %eax contains the pages pointer.  ("0" refers to the
                      * 0-th argument above, ie "a").  %ebx contains the
                      * physical address of the Guest's top-level page
                      * directory. */
                     : "0"(pages), "1"(__pa(lg->pgdirs[lg->pgdidx].pgdir))
                     /* We tell gcc that all these registers could change,
                      * which means we don't have to save and restore them in
                      * the Switcher. */
                     : "memory", "%edx", "%ecx", "%edi", "%esi");
}
/*:*/

/*H:030 Let's jump straight to the the main loop which runs the Guest.
 * Remember, this is called by the Launcher reading /dev/lguest, and we keep
 * going around and around until something interesting happens. */
int run_guest(struct lguest *lg, unsigned long __user *user)
{
        /* We stop running once the Guest is dead. */
        while (!lg->dead) {
                /* We need to initialize this, otherwise gcc complains.  It's
                 * not (yet) clever enough to see that it's initialized when we
                 * need it. */
                unsigned int cr2 = 0; /* Damn gcc */

                /* First we run any hypercalls the Guest wants done: either in
                 * the hypercall ring in "struct lguest_data", or directly by
                 * using int 31 (LGUEST_TRAP_ENTRY). */
                do_hypercalls(lg);
                /* It's possible the Guest did a SEND_DMA hypercall to the
                 * Launcher, in which case we return from the read() now. */
                if (lg->dma_is_pending) {
                        if (put_user(lg->pending_dma, user) ||
                            put_user(lg->pending_key, user+1))
                                return -EFAULT;
                        return sizeof(unsigned long)*2;
                }

                /* Check for signals */
                if (signal_pending(current))
                        return -ERESTARTSYS;

                /* If Waker set break_out, return to Launcher. */
                if (lg->break_out)
                        return -EAGAIN;

                /* Check if there are any interrupts which can be delivered
                 * now: if so, this sets up the hander to be executed when we
                 * next run the Guest. */
                maybe_do_interrupt(lg);

                /* All long-lived kernel loops need to check with this horrible
                 * thing called the freezer.  If the Host is trying to suspend,
                 * it stops us. */
                try_to_freeze();

                /* Just make absolutely sure the Guest is still alive.  One of
                 * those hypercalls could have been fatal, for example. */
                if (lg->dead)
                        break;

                /* If the Guest asked to be stopped, we sleep.  The Guest's
                 * clock timer or LHCALL_BREAK from the Waker will wake us. */
                if (lg->halted) {
                        set_current_state(TASK_INTERRUPTIBLE);
                        schedule();
                        continue;
                }

                /* OK, now we're ready to jump into the Guest.  First we put up
                 * the "Do Not Disturb" sign: */
                local_irq_disable();

                /* Remember the awfully-named TS bit?  If the Guest has asked
                 * to set it we set it now, so we can trap and pass that trap
                 * to the Guest if it uses the FPU. */
                if (lg->ts)
                        set_ts();

                /* SYSENTER is an optimized way of doing system calls.  We
                 * can't allow it because it always jumps to privilege level 0.
                 * A normal Guest won't try it because we don't advertise it in
                 * CPUID, but a malicious Guest (or malicious Guest userspace
                 * program) could, so we tell the CPU to disable it before
                 * running the Guest. */
                if (boot_cpu_has(X86_FEATURE_SEP))
                        wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);

                /* Now we actually run the Guest.  It will pop back out when
                 * something interesting happens, and we can examine its
                 * registers to see what it was doing. */
                run_guest_once(lg, lguest_pages(raw_smp_processor_id()));

                /* The "regs" pointer contains two extra entries which are not
                 * really registers: a trap number which says what interrupt or
                 * trap made the switcher code come back, and an error code
                 * which some traps set.  */

                /* If the Guest page faulted, then the cr2 register will tell
                 * us the bad virtual address.  We have to grab this now,
                 * because once we re-enable interrupts an interrupt could
                 * fault and thus overwrite cr2, or we could even move off to a
                 * different CPU. */
                if (lg->regs->trapnum == 14)
                        cr2 = read_cr2();
                /* Similarly, if we took a trap because the Guest used the FPU,
                 * we have to restore the FPU it expects to see. */
                else if (lg->regs->trapnum == 7)
                        math_state_restore();

                /* Restore SYSENTER if it's supposed to be on. */
                if (boot_cpu_has(X86_FEATURE_SEP))
                        wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);

                /* Now we're ready to be interrupted or moved to other CPUs */
                local_irq_enable();

                /* OK, so what happened? */
                switch (lg->regs->trapnum) {
                case 13: /* We've intercepted a GPF. */
                        /* Check if this was one of those annoying IN or OUT
                         * instructions which we need to emulate.  If so, we
                         * just go back into the Guest after we've done it. */
                        if (lg->regs->errcode == 0) {
                                if (emulate_insn(lg))
                                        continue;
                        }
                        break;
                case 14: /* We've intercepted a page fault. */
                        /* The Guest accessed a virtual address that wasn't
                         * mapped.  This happens a lot: we don't actually set
                         * up most of the page tables for the Guest at all when
                         * we start: as it runs it asks for more and more, and
                         * we set them up as required. In this case, we don't
                         * even tell the Guest that the fault happened.
                         *
                         * The errcode tells whether this was a read or a
                         * write, and whether kernel or userspace code. */
                        if (demand_page(lg, cr2, lg->regs->errcode))
                                continue;

                        /* OK, it's really not there (or not OK): the Guest
                         * needs to know.  We write out the cr2 value so it
                         * knows where the fault occurred.
                         *
                         * Note that if the Guest were really messed up, this
                         * could happen before it's done the INITIALIZE
                         * hypercall, so lg->lguest_data will be NULL, so
                         * &lg->lguest_data->cr2 will be address 8.  Writing
                         * into that address won't hurt the Host at all,
                         * though. */
                        if (put_user(cr2, &lg->lguest_data->cr2))
                                kill_guest(lg, "Writing cr2");
                        break;
                case 7: /* We've intercepted a Device Not Available fault. */
                        /* If the Guest doesn't want to know, we already
                         * restored the Floating Point Unit, so we just
                         * continue without telling it. */
                        if (!lg->ts)
                                continue;
                        break;
                case 32 ... 255:
                        /* These values mean a real interrupt occurred, in
                         * which case the Host handler has already been run.
                         * We just do a friendly check if another process
                         * should now be run, then fall through to loop
                         * around: */
                        cond_resched();
                case LGUEST_TRAP_ENTRY: /* Handled at top of loop */
                        continue;
                }

                /* If we get here, it's a trap the Guest wants to know
                 * about. */
                if (deliver_trap(lg, lg->regs->trapnum))
                        continue;

                /* If the Guest doesn't have a handler (either it hasn't
                 * registered any yet, or it's one of the faults we don't let
                 * it handle), it dies with a cryptic error message. */
                kill_guest(lg, "unhandled trap %li at %#lx (%#lx)",
                           lg->regs->trapnum, lg->regs->eip,
                           lg->regs->trapnum == 14 ? cr2 : lg->regs->errcode);
        }
        /* The Guest is dead => "No such file or directory" */
        return -ENOENT;
}

/* Now we can look at each of the routines this calls, in increasing order of
 * complexity: do_hypercalls(), emulate_insn(), maybe_do_interrupt(),
 * deliver_trap() and demand_page().  After all those, we'll be ready to
 * examine the Switcher, and our philosophical understanding of the Host/Guest
 * duality will be complete. :*/

int find_free_guest(void)
{
        unsigned int i;
        for (i = 0; i < MAX_LGUEST_GUESTS; i++)
                if (!lguests[i].tsk)
                        return i;
        return -1;
}

static void adjust_pge(void *on)
{
        if (on)
                write_cr4(read_cr4() | X86_CR4_PGE);
        else
                write_cr4(read_cr4() & ~X86_CR4_PGE);
}

/*H:000
 * Welcome to the Host!
 *
 * By this point your brain has been tickled by the Guest code and numbed by
 * the Launcher code; prepare for it to be stretched by the Host code.  This is
 * the heart.  Let's begin at the initialization routine for the Host's lg
 * module.
 */
static int __init init(void)
{
        int err;

        /* Lguest can't run under Xen, VMI or itself.  It does Tricky Stuff. */
        if (paravirt_enabled()) {
                printk("lguest is afraid of %s\n", pv_info.name);
                return -EPERM;
        }

        /* First we put the Switcher up in very high virtual memory. */
        err = map_switcher();
        if (err)
                return err;

        /* Now we set up the pagetable implementation for the Guests. */
        err = init_pagetables(switcher_page, SHARED_SWITCHER_PAGES);
        if (err) {
                unmap_switcher();
                return err;
        }

        /* The I/O subsystem needs some things initialized. */
        lguest_io_init();

        /* /dev/lguest needs to be registered. */
        err = lguest_device_init();
        if (err) {
                free_pagetables();
                unmap_switcher();
                return err;
        }

        /* Finally, we need to turn off "Page Global Enable".  PGE is an
         * optimization where page table entries are specially marked to show
         * they never change.  The Host kernel marks all the kernel pages this
         * way because it's always present, even when userspace is running.
         *
         * Lguest breaks this: unbeknownst to the rest of the Host kernel, we
         * switch to the Guest kernel.  If you don't disable this on all CPUs,
         * you'll get really weird bugs that you'll chase for two days.
         *
         * I used to turn PGE off every time we switched to the Guest and back
         * on when we return, but that slowed the Switcher down noticibly. */

        /* We don't need the complexity of CPUs coming and going while we're
         * doing this. */
        lock_cpu_hotplug();
        if (cpu_has_pge) { /* We have a broader idea of "global". */
                /* Remember that this was originally set (for cleanup). */
                cpu_had_pge = 1;
                /* adjust_pge is a helper function which sets or unsets the PGE
                 * bit on its CPU, depending on the argument (0 == unset). */
                on_each_cpu(adjust_pge, (void *)0, 0, 1);
                /* Turn off the feature in the global feature set. */
                clear_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
        }
        unlock_cpu_hotplug();

        /* All good! */
        return 0;
}

/* Cleaning up is just the same code, backwards.  With a little French. */
static void __exit fini(void)
{
        lguest_device_remove();
        free_pagetables();
        unmap_switcher();

        /* If we had PGE before we started, turn it back on now. */
        lock_cpu_hotplug();
        if (cpu_had_pge) {
                set_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
                /* adjust_pge's argument "1" means set PGE. */
                on_each_cpu(adjust_pge, (void *)1, 0, 1);
        }
        unlock_cpu_hotplug();
}

/* The Host side of lguest can be a module.  This is a nice way for people to
 * play with it.  */
module_init(init);
module_exit(fini);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Rusty Russell <rusty@rustcorp.com.au>");