lguest: update comments

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / drivers / lguest / x86 / core.c

/*
 * Copyright (C) 2006, Rusty Russell <rusty@rustcorp.com.au> IBM Corporation.
 * Copyright (C) 2007, Jes Sorensen <jes@sgi.com> SGI.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
 * NON INFRINGEMENT.  See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */
/*P:450
 * This file contains the x86-specific lguest code.  It used to be all
 * mixed in with drivers/lguest/core.c but several foolhardy code slashers
 * wrestled most of the dependencies out to here in preparation for porting
 * lguest to other architectures (see what I mean by foolhardy?).
 *
 * This also contains a couple of non-obvious setup and teardown pieces which
 * were implemented after days of debugging pain.
:*/
#include <linux/kernel.h>
#include <linux/start_kernel.h>
#include <linux/string.h>
#include <linux/console.h>
#include <linux/screen_info.h>
#include <linux/irq.h>
#include <linux/interrupt.h>
#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/cpu.h>
#include <linux/lguest.h>
#include <linux/lguest_launcher.h>
#include <asm/paravirt.h>
#include <asm/param.h>
#include <asm/page.h>
#include <asm/pgtable.h>
#include <asm/desc.h>
#include <asm/setup.h>
#include <asm/lguest.h>
#include <asm/uaccess.h>
#include <asm/i387.h>
#include "../lg.h"

static int cpu_had_pge;

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

/* 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]);
}

static DEFINE_PER_CPU(struct lg_cpu *, lg_last_cpu);

/*S:010
 * We approach 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 lg_cpu *cpu, 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 (__this_cpu_read(lg_last_cpu) != cpu || cpu->last_pages != pages) {
                __this_cpu_write(lg_last_cpu, cpu);
                cpu->last_pages = pages;
                cpu->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(cpu, 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.sp1 = cpu->esp1;
        pages->state.guest_tss.ss1 = cpu->ss1;

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

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

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

/* Finally: the code to actually call into the Switcher to run the Guest. */
static void run_guest_once(struct lg_cpu *cpu, 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(cpu, 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.
         */
        cpu->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 layout created by 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(cpu->lg->pgdirs[cpu->cpu_pgd].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");
}
/*:*/

/*M:002
 * There are hooks in the scheduler which we can register to tell when we
 * get kicked off the CPU (preempt_notifier_register()).  This would allow us
 * to lazily disable SYSENTER which would regain some performance, and should
 * also simplify copy_in_guest_info().  Note that we'd still need to restore
 * things when we exit to Launcher userspace, but that's fairly easy.
 *
 * We could also try using these hooks for PGE, but that might be too expensive.
 *
 * The hooks were designed for KVM, but we can also put them to good use.
:*/

/*H:040
 * This is the i386-specific code to setup and run the Guest.  Interrupts
 * are disabled: we own the CPU.
 */
void lguest_arch_run_guest(struct lg_cpu *cpu)
{
        /*
         * 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 (cpu->ts)
                unlazy_fpu(current);

        /*
         * 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 return when something
         * interesting happens, and we can examine its registers to see what it
         * was doing.
         */
        run_guest_once(cpu, lguest_pages(raw_smp_processor_id()));

        /*
         * Note that the "regs" structure 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.
         */

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

        /*
         * 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 (cpu->regs->trapnum == 14)
                cpu->arch.last_pagefault = read_cr2();
        /*
         * Similarly, if we took a trap because the Guest used the FPU,
         * we have to restore the FPU it expects to see.
         * math_state_restore() may sleep and we may even move off to
         * a different CPU. So all the critical stuff should be done
         * before this.
         */
        else if (cpu->regs->trapnum == 7)
                math_state_restore();
}

/*H:130
 * Now we've examined the hypercall code; our Guest can make requests.
 * Our Guest is usually so well behaved; it never tries to do things it isn't
 * allowed to, and uses hypercalls instead.  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 a trap (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 lg_cpu *cpu)
{
        u8 insn;
        unsigned int insnlen = 0, in = 0, shift = 0;
        /*
         * The eip contains the *virtual* address of the Guest's instruction:
         * walk the Guest's page tables to find the "physical" address.
         */
        unsigned long physaddr = guest_pa(cpu, cpu->regs->eip);

        /*
         * This must be the Guest kernel trying to do something, not userspace!
         * The bottom two bits of the CS segment register are the privilege
         * level.
         */
        if ((cpu->regs->cs & 3) != GUEST_PL)
                return 0;

        /* Decoding x86 instructions is icky. */
        insn = lgread(cpu, physaddr, u8);

        /*
         * Around 2.6.33, the kernel started using an emulation for the
         * cmpxchg8b instruction in early boot on many configurations.  This
         * code isn't paravirtualized, and it tries to disable interrupts.
         * Ignore it, which will Mostly Work.
         */
        if (insn == 0xfa) {
                /* "cli", or Clear Interrupt Enable instruction.  Skip it. */
                cpu->regs->eip++;
                return 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;
                insn = lgread(cpu, physaddr + insnlen, u8);
        }

        /*
         * 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)
                        cpu->regs->eax = 0xFFFFFFFF;
                else
                        cpu->regs->eax |= (0xFFFF << shift);
        }
        /* Finally, we've "done" the instruction, so move past it. */
        cpu->regs->eip += insnlen;
        /* Success! */
        return 1;
}

/*H:050 Once we've re-enabled interrupts, we look at why the Guest exited. */
void lguest_arch_handle_trap(struct lg_cpu *cpu)
{
        switch (cpu->regs->trapnum) {
        case 13: /* We've intercepted a General Protection Fault. */
                /*
                 * 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 (cpu->regs->errcode == 0) {
                        if (emulate_insn(cpu))
                                return;
                }
                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(cpu, cpu->arch.last_pagefault,
                                cpu->regs->errcode))
                        return;

                /*
                 * 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 LHCALL_LGUEST_INIT hypercall, so
                 * lg->lguest_data could be NULL
                 */
                if (cpu->lg->lguest_data &&
                    put_user(cpu->arch.last_pagefault,
                             &cpu->lg->lguest_data->cr2))
                        kill_guest(cpu, "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 (!cpu->ts)
                        return;
                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
                 * return to run the Guest again.
                 */
                cond_resched();
                return;
        case LGUEST_TRAP_ENTRY:
                /*
                 * Our 'struct hcall_args' maps directly over our regs: we set
                 * up the pointer now to indicate a hypercall is pending.
                 */
                cpu->hcall = (struct hcall_args *)cpu->regs;
                return;
        }

        /* We didn't handle the trap, so it needs to go to the Guest. */
        if (!deliver_trap(cpu, cpu->regs->trapnum))
                /*
                 * 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 this cryptic error message.
                 */
                kill_guest(cpu, "unhandled trap %li at %#lx (%#lx)",
                           cpu->regs->trapnum, cpu->regs->eip,
                           cpu->regs->trapnum == 14 ? cpu->arch.last_pagefault
                           : cpu->regs->errcode);
}

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

/*H:020
 * Now the Switcher is mapped and every thing else is ready, we need to do
 * some more i386-specific initialization.
 */
void __init lguest_arch_host_init(void)
{
        int i;

        /*
         * Most of the x86/switcher_32.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
         * compiled-in 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 neater. */
                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.sp0 = (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;

        /*
         * 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.
         */
        get_online_cpus();
        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, 1);
                /* Turn off the feature in the global feature set. */
                clear_cpu_cap(&boot_cpu_data, X86_FEATURE_PGE);
        }
        put_online_cpus();
}
/*:*/

void __exit lguest_arch_host_fini(void)
{
        /* If we had PGE before we started, turn it back on now. */
        get_online_cpus();
        if (cpu_had_pge) {
                set_cpu_cap(&boot_cpu_data, X86_FEATURE_PGE);
                /* adjust_pge's argument "1" means set PGE. */
                on_each_cpu(adjust_pge, (void *)1, 1);
        }
        put_online_cpus();
}


/*H:122 The i386-specific hypercalls simply farm out to the right functions. */
int lguest_arch_do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
{
        switch (args->arg0) {
        case LHCALL_LOAD_GDT_ENTRY:
                load_guest_gdt_entry(cpu, args->arg1, args->arg2, args->arg3);
                break;
        case LHCALL_LOAD_IDT_ENTRY:
                load_guest_idt_entry(cpu, args->arg1, args->arg2, args->arg3);
                break;
        case LHCALL_LOAD_TLS:
                guest_load_tls(cpu, args->arg1);
                break;
        default:
                /* Bad Guest.  Bad! */
                return -EIO;
        }
        return 0;
}

/*H:126 i386-specific hypercall initialization: */
int lguest_arch_init_hypercalls(struct lg_cpu *cpu)
{
        u32 tsc_speed;

        /*
         * The pointer to the Guest's "struct lguest_data" is the only argument.
         * We check that address now.
         */
        if (!lguest_address_ok(cpu->lg, cpu->hcall->arg1,
                               sizeof(*cpu->lg->lguest_data)))
                return -EFAULT;

        /*
         * Having checked it, we simply set lg->lguest_data to point straight
         * into the Launcher's memory at the right place and then use
         * copy_to_user/from_user from now on, instead of lgread/write.  I put
         * this in to show that I'm not immune to writing stupid
         * optimizations.
         */
        cpu->lg->lguest_data = cpu->lg->mem_base + cpu->hcall->arg1;

        /*
         * We insist that the Time Stamp Counter exist and doesn't change with
         * cpu frequency.  Some devious chip manufacturers decided that TSC
         * changes could be handled in software.  I decided that time going
         * backwards might be good for benchmarks, but it's bad for users.
         *
         * We also insist that the TSC be stable: the kernel detects unreliable
         * TSCs for its own purposes, and we use that here.
         */
        if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && !check_tsc_unstable())
                tsc_speed = tsc_khz;
        else
                tsc_speed = 0;
        if (put_user(tsc_speed, &cpu->lg->lguest_data->tsc_khz))
                return -EFAULT;

        /* The interrupt code might not like the system call vector. */
        if (!check_syscall_vector(cpu->lg))
                kill_guest(cpu, "bad syscall vector");

        return 0;
}
/*:*/

/*L:030
 * Most of the Guest's registers are left alone: we used get_zeroed_page() to
 * allocate the structure, so they will be 0.
 */
void lguest_arch_setup_regs(struct lg_cpu *cpu, unsigned long start)
{
        struct lguest_regs *regs = cpu->regs;

        /*
         * There are four "segment" registers which the Guest needs to boot:
         * The "code segment" register (cs) refers to the kernel code segment
         * __KERNEL_CS, and the "data", "extra" and "stack" segment registers
         * refer to the kernel data segment __KERNEL_DS.
         *
         * The privilege level is packed into the lower bits.  The Guest runs
         * at privilege level 1 (GUEST_PL).
         */
        regs->ds = regs->es = regs->ss = __KERNEL_DS|GUEST_PL;
        regs->cs = __KERNEL_CS|GUEST_PL;

        /*
         * The "eflags" register contains miscellaneous flags.  Bit 1 (0x002)
         * is supposed to always be "1".  Bit 9 (0x200) controls whether
         * interrupts are enabled.  We always leave interrupts enabled while
         * running the Guest.
         */
        regs->eflags = X86_EFLAGS_IF | 0x2;

        /*
         * The "Extended Instruction Pointer" register says where the Guest is
         * running.
         */
        regs->eip = start;

        /*
         * %esi points to our boot information, at physical address 0, so don't
         * touch it.
         */

        /* There are a couple of GDT entries the Guest expects at boot. */
        setup_guest_gdt(cpu);
}