arch/x86/lguest/boot.c

/*P:010
 * A hypervisor allows multiple Operating Systems to run on a single machine.
 * To quote David Wheeler: "Any problem in computer science can be solved with
 * another layer of indirection."
 *
 * We keep things simple in two ways.  First, we start with a normal Linux
 * kernel and insert a module (lg.ko) which allows us to run other Linux
 * kernels the same way we'd run processes.  We call the first kernel the Host,
 * and the others the Guests.  The program which sets up and configures Guests
 * (such as the example in Documentation/lguest/lguest.c) is called the
 * Launcher.
 *
 * Secondly, we only run specially modified Guests, not normal kernels: setting
 * CONFIG_LGUEST_GUEST to "y" compiles this file into the kernel so it knows
 * how to be a Guest at boot time.  This means that you can use the same kernel
 * you boot normally (ie. as a Host) as a Guest.
 *
 * These Guests know that they cannot do privileged operations, such as disable
 * interrupts, and that they have to ask the Host to do such things explicitly.
 * This file consists of all the replacements for such low-level native
 * hardware operations: these special Guest versions call the Host.
 *
 * So how does the kernel know it's a Guest?  We'll see that later, but let's
 * just say that we end up here where we replace the native functions various
 * "paravirt" structures with our Guest versions, then boot like normal. :*/

/*
 * Copyright (C) 2006, Rusty Russell <rusty@rustcorp.com.au> IBM Corporation.
 *
 * 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.
 */
#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/lguest.h>
#include <linux/lguest_launcher.h>
#include <linux/virtio_console.h>
#include <linux/pm.h>
#include <asm/apic.h>
#include <asm/lguest.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/e820.h>
#include <asm/mce.h>
#include <asm/io.h>
#include <asm/i387.h>
#include <asm/stackprotector.h>
#include <asm/reboot.h>		/* for struct machine_ops */

/*G:010 Welcome to the Guest!
 *
 * The Guest in our tale is a simple creature: identical to the Host but
 * behaving in simplified but equivalent ways.  In particular, the Guest is the
 * same kernel as the Host (or at least, built from the same source code). :*/

struct lguest_data lguest_data = {
	.hcall_status = { [0 ... LHCALL_RING_SIZE-1] = 0xFF },
	.noirq_start = (u32)lguest_noirq_start,
	.noirq_end = (u32)lguest_noirq_end,
	.kernel_address = PAGE_OFFSET,
	.blocked_interrupts = { 1 }, /* Block timer interrupts */
	.syscall_vec = SYSCALL_VECTOR,
};

/*G:037 async_hcall() is pretty simple: I'm quite proud of it really.  We have a
 * ring buffer of stored hypercalls which the Host will run though next time we
 * do a normal hypercall.  Each entry in the ring has 5 slots for the hypercall
 * arguments, and a "hcall_status" word which is 0 if the call is ready to go,
 * and 255 once the Host has finished with it.
 *
 * If we come around to a slot which hasn't been finished, then the table is
 * full and we just make the hypercall directly.  This has the nice side
 * effect of causing the Host to run all the stored calls in the ring buffer
 * which empties it for next time! */
static void async_hcall(unsigned long call, unsigned long arg1,
			unsigned long arg2, unsigned long arg3,
			unsigned long arg4)
{
	/* Note: This code assumes we're uniprocessor. */
	static unsigned int next_call;
	unsigned long flags;

	/* Disable interrupts if not already disabled: we don't want an
	 * interrupt handler making a hypercall while we're already doing
	 * one! */
	local_irq_save(flags);
	if (lguest_data.hcall_status[next_call] != 0xFF) {
		/* Table full, so do normal hcall which will flush table. */
		kvm_hypercall4(call, arg1, arg2, arg3, arg4);
	} else {
		lguest_data.hcalls[next_call].arg0 = call;
		lguest_data.hcalls[next_call].arg1 = arg1;
		lguest_data.hcalls[next_call].arg2 = arg2;
		lguest_data.hcalls[next_call].arg3 = arg3;
		lguest_data.hcalls[next_call].arg4 = arg4;
		/* Arguments must all be written before we mark it to go */
		wmb();
		lguest_data.hcall_status[next_call] = 0;
		if (++next_call == LHCALL_RING_SIZE)
			next_call = 0;
	}
	local_irq_restore(flags);
}

/*G:035 Notice the lazy_hcall() above, rather than hcall().  This is our first
 * real optimization trick!
 *
 * When lazy_mode is set, it means we're allowed to defer all hypercalls and do
 * them as a batch when lazy_mode is eventually turned off.  Because hypercalls
 * are reasonably expensive, batching them up makes sense.  For example, a
 * large munmap might update dozens of page table entries: that code calls
 * paravirt_enter_lazy_mmu(), does the dozen updates, then calls
 * lguest_leave_lazy_mode().
 *
 * So, when we're in lazy mode, we call async_hcall() to store the call for
 * future processing: */
static void lazy_hcall1(unsigned long call,
		       unsigned long arg1)
{
	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
		kvm_hypercall1(call, arg1);
	else
		async_hcall(call, arg1, 0, 0, 0);
}

static void lazy_hcall2(unsigned long call,
		       unsigned long arg1,
		       unsigned long arg2)
{
	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
		kvm_hypercall2(call, arg1, arg2);
	else
		async_hcall(call, arg1, arg2, 0, 0);
}

static void lazy_hcall3(unsigned long call,
		       unsigned long arg1,
		       unsigned long arg2,
		       unsigned long arg3)
{
	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
		kvm_hypercall3(call, arg1, arg2, arg3);
	else
		async_hcall(call, arg1, arg2, arg3, 0);
}

#ifdef CONFIG_X86_PAE
static void lazy_hcall4(unsigned long call,
		       unsigned long arg1,
		       unsigned long arg2,
		       unsigned long arg3,
		       unsigned long arg4)
{
	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
		kvm_hypercall4(call, arg1, arg2, arg3, arg4);
	else
		async_hcall(call, arg1, arg2, arg3, arg4);
}
#endif

/* When lazy mode is turned off reset the per-cpu lazy mode variable and then
 * issue the do-nothing hypercall to flush any stored calls. */
static void lguest_leave_lazy_mmu_mode(void)
{
	kvm_hypercall0(LHCALL_FLUSH_ASYNC);
	paravirt_leave_lazy_mmu();
}

static void lguest_end_context_switch(struct task_struct *next)
{
	kvm_hypercall0(LHCALL_FLUSH_ASYNC);
	paravirt_end_context_switch(next);
}

/*G:032
 * After that diversion we return to our first native-instruction
 * replacements: four functions for interrupt control.
 *
 * The simplest way of implementing these would be to have "turn interrupts
 * off" and "turn interrupts on" hypercalls.  Unfortunately, this is too slow:
 * these are by far the most commonly called functions of those we override.
 *
 * So instead we keep an "irq_enabled" field inside our "struct lguest_data",
 * which the Guest can update with a single instruction.  The Host knows to
 * check there before it tries to deliver an interrupt.
 */

/* save_flags() is expected to return the processor state (ie. "flags").  The
 * flags word contains all kind of stuff, but in practice Linux only cares
 * about the interrupt flag.  Our "save_flags()" just returns that. */
static unsigned long save_fl(void)
{
	return lguest_data.irq_enabled;
}

/* Interrupts go off... */
static void irq_disable(void)
{
	lguest_data.irq_enabled = 0;
}

/* Let's pause a moment.  Remember how I said these are called so often?
 * Jeremy Fitzhardinge optimized them so hard early in 2009 that he had to
 * break some rules.  In particular, these functions are assumed to save their
 * own registers if they need to: normal C functions assume they can trash the
 * eax register.  To use normal C functions, we use
 * PV_CALLEE_SAVE_REGS_THUNK(), which pushes %eax onto the stack, calls the
 * C function, then restores it. */
PV_CALLEE_SAVE_REGS_THUNK(save_fl);
PV_CALLEE_SAVE_REGS_THUNK(irq_disable);
/*:*/

/* These are in i386_head.S */
extern void lg_irq_enable(void);
extern void lg_restore_fl(unsigned long flags);

/*M:003 Note that we don't check for outstanding interrupts when we re-enable
 * them (or when we unmask an interrupt).  This seems to work for the moment,
 * since interrupts are rare and we'll just get the interrupt on the next timer
 * tick, but now we can run with CONFIG_NO_HZ, we should revisit this.  One way
 * would be to put the "irq_enabled" field in a page by itself, and have the
 * Host write-protect it when an interrupt comes in when irqs are disabled.
 * There will then be a page fault as soon as interrupts are re-enabled.
 *
 * A better method is to implement soft interrupt disable generally for x86:
 * instead of disabling interrupts, we set a flag.  If an interrupt does come
 * in, we then disable them for real.  This is uncommon, so we could simply use
 * a hypercall for interrupt control and not worry about efficiency. :*/

/*G:034
 * The Interrupt Descriptor Table (IDT).
 *
 * The IDT tells the processor what to do when an interrupt comes in.  Each
 * entry in the table is a 64-bit descriptor: this holds the privilege level,
 * address of the handler, and... well, who cares?  The Guest just asks the
 * Host to make the change anyway, because the Host controls the real IDT.
 */
static void lguest_write_idt_entry(gate_desc *dt,
				   int entrynum, const gate_desc *g)
{
	/* The gate_desc structure is 8 bytes long: we hand it to the Host in
	 * two 32-bit chunks.  The whole 32-bit kernel used to hand descriptors
	 * around like this; typesafety wasn't a big concern in Linux's early
	 * years. */
	u32 *desc = (u32 *)g;
	/* Keep the local copy up to date. */
	native_write_idt_entry(dt, entrynum, g);
	/* Tell Host about this new entry. */
	kvm_hypercall3(LHCALL_LOAD_IDT_ENTRY, entrynum, desc[0], desc[1]);
}

/* Changing to a different IDT is very rare: we keep the IDT up-to-date every
 * time it is written, so we can simply loop through all entries and tell the
 * Host about them. */
static void lguest_load_idt(const struct desc_ptr *desc)
{
	unsigned int i;
	struct desc_struct *idt = (void *)desc->address;

	for (i = 0; i < (desc->size+1)/8; i++)
		kvm_hypercall3(LHCALL_LOAD_IDT_ENTRY, i, idt[i].a, idt[i].b);
}

/*
 * The Global Descriptor Table.
 *
 * The Intel architecture defines another table, called the Global Descriptor
 * Table (GDT).  You tell the CPU where it is (and its size) using the "lgdt"
 * instruction, and then several other instructions refer to entries in the
 * table.  There are three entries which the Switcher needs, so the Host simply
 * controls the entire thing and the Guest asks it to make changes using the
 * LOAD_GDT hypercall.
 *
 * This is the exactly like the IDT code.
 */
static void lguest_load_gdt(const struct desc_ptr *desc)
{
	unsigned int i;
	struct desc_struct *gdt = (void *)desc->address;

	for (i = 0; i < (desc->size+1)/8; i++)
		kvm_hypercall3(LHCALL_LOAD_GDT_ENTRY, i, gdt[i].a, gdt[i].b);
}

/* For a single GDT entry which changes, we do the lazy thing: alter our GDT,
 * then tell the Host to reload the entire thing.  This operation is so rare
 * that this naive implementation is reasonable. */
static void lguest_write_gdt_entry(struct desc_struct *dt, int entrynum,
				   const void *desc, int type)
{
	native_write_gdt_entry(dt, entrynum, desc, type);
	/* Tell Host about this new entry. */
	kvm_hypercall3(LHCALL_LOAD_GDT_ENTRY, entrynum,
		       dt[entrynum].a, dt[entrynum].b);
}

/* OK, I lied.  There are three "thread local storage" GDT entries which change
 * on every context switch (these three entries are how glibc implements
 * __thread variables).  So we have a hypercall specifically for this case. */
static void lguest_load_tls(struct thread_struct *t, unsigned int cpu)
{
	/* There's one problem which normal hardware doesn't have: the Host
	 * can't handle us removing entries we're currently using.  So we clear
	 * the GS register here: if it's needed it'll be reloaded anyway. */
	lazy_load_gs(0);
	lazy_hcall2(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu);
}

/*G:038 That's enough excitement for now, back to ploughing through each of
 * the different pv_ops structures (we're about 1/3 of the way through).
 *
 * This is the Local Descriptor Table, another weird Intel thingy.  Linux only
 * uses this for some strange applications like Wine.  We don't do anything
 * here, so they'll get an informative and friendly Segmentation Fault. */
static void lguest_set_ldt(const void *addr, unsigned entries)
{
}

/* This loads a GDT entry into the "Task Register": that entry points to a
 * structure called the Task State Segment.  Some comments scattered though the
 * kernel code indicate that this used for task switching in ages past, along
 * with blood sacrifice and astrology.
 *
 * Now there's nothing interesting in here that we don't get told elsewhere.
 * But the native version uses the "ltr" instruction, which makes the Host
 * complain to the Guest about a Segmentation Fault and it'll oops.  So we
 * override the native version with a do-nothing version. */
static void lguest_load_tr_desc(void)
{
}

/* The "cpuid" instruction is a way of querying both the CPU identity
 * (manufacturer, model, etc) and its features.  It was introduced before the
 * Pentium in 1993 and keeps getting extended by both Intel, AMD and others.
 * As you might imagine, after a decade and a half this treatment, it is now a
 * giant ball of hair.  Its entry in the current Intel manual runs to 28 pages.
 *
 * This instruction even it has its own Wikipedia entry.  The Wikipedia entry
 * has been translated into 4 languages.  I am not making this up!
 *
 * We could get funky here and identify ourselves as "GenuineLguest", but
 * instead we just use the real "cpuid" instruction.  Then I pretty much turned
 * off feature bits until the Guest booted.  (Don't say that: you'll damage
 * lguest sales!)  Shut up, inner voice!  (Hey, just pointing out that this is
 * hardly future proof.)  Noone's listening!  They don't like you anyway,
 * parenthetic weirdo!
 *
 * Replacing the cpuid so we can turn features off is great for the kernel, but
 * anyone (including userspace) can just use the raw "cpuid" instruction and
 * the Host won't even notice since it isn't privileged.  So we try not to get
 * too worked up about it. */
static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
			 unsigned int *cx, unsigned int *dx)
{
	int function = *ax;

	native_cpuid(ax, bx, cx, dx);
	switch (function) {
	case 1:	/* Basic feature request. */
		/* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
		*cx &= 0x00002201;
		/* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, TSC, FPU, PAE. */
		*dx &= 0x07808151;
		/* The Host can do a nice optimization if it knows that the
		 * kernel mappings (addresses above 0xC0000000 or whatever
		 * PAGE_OFFSET is set to) haven't changed.  But Linux calls
		 * flush_tlb_user() for both user and kernel mappings unless
		 * the Page Global Enable (PGE) feature bit is set. */
		*dx |= 0x00002000;
		/* We also lie, and say we're family id 5.  6 or greater
		 * leads to a rdmsr in early_init_intel which we can't handle.
		 * Family ID is returned as bits 8-12 in ax. */
		*ax &= 0xFFFFF0FF;
		*ax |= 0x00000500;
		break;
	case 0x80000000:
		/* Futureproof this a little: if they ask how much extended
		 * processor information there is, limit it to known fields. */
		if (*ax > 0x80000008)
			*ax = 0x80000008;
		break;
	case 0x80000001:
		/* Here we should fix nx cap depending on host. */
		/* For this version of PAE, we just clear NX bit. */
		*dx &= ~(1 << 20);
		break;
	}
}

/* Intel has four control registers, imaginatively named cr0, cr2, cr3 and cr4.
 * I assume there's a cr1, but it hasn't bothered us yet, so we'll not bother
 * it.  The Host needs to know when the Guest wants to change them, so we have
 * a whole series of functions like read_cr0() and write_cr0().
 *
 * We start with cr0.  cr0 allows you to turn on and off all kinds of basic
 * features, but Linux only really cares about one: the horrifically-named Task
 * Switched (TS) bit at bit 3 (ie. 8)
 *
 * What does the TS bit do?  Well, it causes the CPU to trap (interrupt 7) if
 * the floating point unit is used.  Which allows us to restore FPU state
 * lazily after a task switch, and Linux uses that gratefully, but wouldn't a
 * name like "FPUTRAP bit" be a little less cryptic?
 *
 * We store cr0 locally because the Host never changes it.  The Guest sometimes
 * wants to read it and we'd prefer not to bother the Host unnecessarily. */
static unsigned long current_cr0;
static void lguest_write_cr0(unsigned long val)
{
	lazy_hcall1(LHCALL_TS, val & X86_CR0_TS);
	current_cr0 = val;
}

static unsigned long lguest_read_cr0(void)
{
	return current_cr0;
}

/* Intel provided a special instruction to clear the TS bit for people too cool
 * to use write_cr0() to do it.  This "clts" instruction is faster, because all
 * the vowels have been optimized out. */
static void lguest_clts(void)
{
	lazy_hcall1(LHCALL_TS, 0);
	current_cr0 &= ~X86_CR0_TS;
}

/* cr2 is the virtual address of the last page fault, which the Guest only ever
 * reads.  The Host kindly writes this into our "struct lguest_data", so we
 * just read it out of there. */
static unsigned long lguest_read_cr2(void)
{
	return lguest_data.cr2;
}

/* See lguest_set_pte() below. */
static bool cr3_changed = false;

/* cr3 is the current toplevel pagetable page: the principle is the same as
 * cr0.  Keep a local copy, and tell the Host when it changes.  The only
 * difference is that our local copy is in lguest_data because the Host needs
 * to set it upon our initial hypercall. */
static void lguest_write_cr3(unsigned long cr3)
{
	lguest_data.pgdir = cr3;
	lazy_hcall1(LHCALL_NEW_PGTABLE, cr3);
	cr3_changed = true;
}

static unsigned long lguest_read_cr3(void)
{
	return lguest_data.pgdir;
}

/* cr4 is used to enable and disable PGE, but we don't care. */
static unsigned long lguest_read_cr4(void)
{
	return 0;
}

static void lguest_write_cr4(unsigned long val)
{
}

/*
 * Page Table Handling.
 *
 * Now would be a good time to take a rest and grab a coffee or similarly
 * relaxing stimulant.  The easy parts are behind us, and the trek gradually
 * winds uphill from here.
 *
 * Quick refresher: memory is divided into "pages" of 4096 bytes each.  The CPU
 * maps virtual addresses to physical addresses using "page tables".  We could
 * use one huge index of 1 million entries: each address is 4 bytes, so that's
 * 1024 pages just to hold the page tables.   But since most virtual addresses
 * are unused, we use a two level index which saves space.  The cr3 register
 * contains the physical address of the top level "page directory" page, which
 * contains physical addresses of up to 1024 second-level pages.  Each of these
 * second level pages contains up to 1024 physical addresses of actual pages,
 * or Page Table Entries (PTEs).
 *
 * Here's a diagram, where arrows indicate physical addresses:
 *
 * cr3 ---> +---------+
 *	    |  	   --------->+---------+
 *	    |	      |	     | PADDR1  |
 *	  Top-level   |	     | PADDR2  |
 *	  (PMD) page  |	     | 	       |
 *	    |	      |	   Lower-level |
 *	    |	      |	   (PTE) page  |
 *	    |	      |	     |	       |
 *	      ....    	     	 ....
 *
 * So to convert a virtual address to a physical address, we look up the top
 * level, which points us to the second level, which gives us the physical
 * address of that page.  If the top level entry was not present, or the second
 * level entry was not present, then the virtual address is invalid (we
 * say "the page was not mapped").
 *
 * Put another way, a 32-bit virtual address is divided up like so:
 *
 *  1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
 * |<---- 10 bits ---->|<---- 10 bits ---->|<------ 12 bits ------>|
 *    Index into top     Index into second      Offset within page
 *  page directory page    pagetable page
 *
 * The kernel spends a lot of time changing both the top-level page directory
 * and lower-level pagetable pages.  The Guest doesn't know physical addresses,
 * so while it maintains these page tables exactly like normal, it also needs
 * to keep the Host informed whenever it makes a change: the Host will create
 * the real page tables based on the Guests'.
 */

/* The Guest calls this to set a second-level entry (pte), ie. to map a page
 * into a process' address space.  We set the entry then tell the Host the
 * toplevel and address this corresponds to.  The Guest uses one pagetable per
 * process, so we need to tell the Host which one we're changing (mm->pgd). */
static void lguest_pte_update(struct mm_struct *mm, unsigned long addr,
			       pte_t *ptep)
{
#ifdef CONFIG_X86_PAE
	lazy_hcall4(LHCALL_SET_PTE, __pa(mm->pgd), addr,
		    ptep->pte_low, ptep->pte_high);
#else
	lazy_hcall3(LHCALL_SET_PTE, __pa(mm->pgd), addr, ptep->pte_low);
#endif
}

static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr,
			      pte_t *ptep, pte_t pteval)
{
	native_set_pte(ptep, pteval);
	lguest_pte_update(mm, addr, ptep);
}

/* The Guest calls lguest_set_pud to set a top-level entry and lguest_set_pmd
 * to set a middle-level entry when PAE is activated.
 * Again, we set the entry then tell the Host which page we changed,
 * and the index of the entry we changed. */
#ifdef CONFIG_X86_PAE
static void lguest_set_pud(pud_t *pudp, pud_t pudval)
{
	native_set_pud(pudp, pudval);

	/* 32 bytes aligned pdpt address and the index. */
	lazy_hcall2(LHCALL_SET_PGD, __pa(pudp) & 0xFFFFFFE0,
		   (__pa(pudp) & 0x1F) / sizeof(pud_t));
}

static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
{
	native_set_pmd(pmdp, pmdval);
	lazy_hcall2(LHCALL_SET_PMD, __pa(pmdp) & PAGE_MASK,
		   (__pa(pmdp) & (PAGE_SIZE - 1)) / sizeof(pmd_t));
}
#else

/* The Guest calls lguest_set_pmd to set a top-level entry when PAE is not
 * activated. */
static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
{
	native_set_pmd(pmdp, pmdval);
	lazy_hcall2(LHCALL_SET_PGD, __pa(pmdp) & PAGE_MASK,
		   (__pa(pmdp) & (PAGE_SIZE - 1)) / sizeof(pmd_t));
}
#endif

/* There are a couple of legacy places where the kernel sets a PTE, but we
 * don't know the top level any more.  This is useless for us, since we don't
 * know which pagetable is changing or what address, so we just tell the Host
 * to forget all of them.  Fortunately, this is very rare.
 *
 * ... except in early boot when the kernel sets up the initial pagetables,
 * which makes booting astonishingly slow: 1.83 seconds!  So we don't even tell
 * the Host anything changed until we've done the first page table switch,
 * which brings boot back to 0.25 seconds. */
static void lguest_set_pte(pte_t *ptep, pte_t pteval)
{
	native_set_pte(ptep, pteval);
	if (cr3_changed)
		lazy_hcall1(LHCALL_FLUSH_TLB, 1);
}

#ifdef CONFIG_X86_PAE
static void lguest_set_pte_atomic(pte_t *ptep, pte_t pte)
{
	native_set_pte_atomic(ptep, pte);
	if (cr3_changed)
		lazy_hcall1(LHCALL_FLUSH_TLB, 1);
}

void lguest_pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
{
	native_pte_clear(mm, addr, ptep);
	lguest_pte_update(mm, addr, ptep);
}

void lguest_pmd_clear(pmd_t *pmdp)
{
	lguest_set_pmd(pmdp, __pmd(0));
}
#endif

/* Unfortunately for Lguest, the pv_mmu_ops for page tables were based on
 * native page table operations.  On native hardware you can set a new page
 * table entry whenever you want, but if you want to remove one you have to do
 * a TLB flush (a TLB is a little cache of page table entries kept by the CPU).
 *
 * So the lguest_set_pte_at() and lguest_set_pmd() functions above are only
 * called when a valid entry is written, not when it's removed (ie. marked not
 * present).  Instead, this is where we come when the Guest wants to remove a
 * page table entry: we tell the Host to set that entry to 0 (ie. the present
 * bit is zero). */
static void lguest_flush_tlb_single(unsigned long addr)
{
	/* Simply set it to zero: if it was not, it will fault back in. */
	lazy_hcall3(LHCALL_SET_PTE, lguest_data.pgdir, addr, 0);
}

/* This is what happens after the Guest has removed a large number of entries.
 * This tells the Host that any of the page table entries for userspace might
 * have changed, ie. virtual addresses below PAGE_OFFSET. */
static void lguest_flush_tlb_user(void)
{
	lazy_hcall1(LHCALL_FLUSH_TLB, 0);
}

/* This is called when the kernel page tables have changed.  That's not very
 * common (unless the Guest is using highmem, which makes the Guest extremely
 * slow), so it's worth separating this from the user flushing above. */
static void lguest_flush_tlb_kernel(void)
{
	lazy_hcall1(LHCALL_FLUSH_TLB, 1);
}

/*
 * The Unadvanced Programmable Interrupt Controller.
 *
 * This is an attempt to implement the simplest possible interrupt controller.
 * I spent some time looking though routines like set_irq_chip_and_handler,
 * set_irq_chip_and_handler_name, set_irq_chip_data and set_phasers_to_stun and
 * I *think* this is as simple as it gets.
 *
 * We can tell the Host what interrupts we want blocked ready for using the
 * lguest_data.interrupts bitmap, so disabling (aka "masking") them is as
 * simple as setting a bit.  We don't actually "ack" interrupts as such, we
 * just mask and unmask them.  I wonder if we should be cleverer?
 */
static void disable_lguest_irq(unsigned int irq)
{
	set_bit(irq, lguest_data.blocked_interrupts);
}

static void enable_lguest_irq(unsigned int irq)
{
	clear_bit(irq, lguest_data.blocked_interrupts);
}

/* This structure describes the lguest IRQ controller. */
static struct irq_chip lguest_irq_controller = {
	.name		= "lguest",
	.mask		= disable_lguest_irq,
	.mask_ack	= disable_lguest_irq,
	.unmask		= enable_lguest_irq,
};

/* This sets up the Interrupt Descriptor Table (IDT) entry for each hardware
 * interrupt (except 128, which is used for system calls), and then tells the
 * Linux infrastructure that each interrupt is controlled by our level-based
 * lguest interrupt controller. */
static void __init lguest_init_IRQ(void)
{
	unsigned int i;

	for (i = FIRST_EXTERNAL_VECTOR; i < NR_VECTORS; i++) {
		/* Some systems map "vectors" to interrupts weirdly.  Lguest has
		 * a straightforward 1 to 1 mapping, so force that here. */
		__get_cpu_var(vector_irq)[i] = i - FIRST_EXTERNAL_VECTOR;
		if (i != SYSCALL_VECTOR)
			set_intr_gate(i, interrupt[i - FIRST_EXTERNAL_VECTOR]);
	}
	/* This call is required to set up for 4k stacks, where we have
	 * separate stacks for hard and soft interrupts. */
	irq_ctx_init(smp_processor_id());
}

void lguest_setup_irq(unsigned int irq)
{
	irq_to_desc_alloc_node(irq, 0);
	set_irq_chip_and_handler_name(irq, &lguest_irq_controller,
				      handle_level_irq, "level");
}

/*
 * Time.
 *
 * It would be far better for everyone if the Guest had its own clock, but
 * until then the Host gives us the time on every interrupt.
 */
static unsigned long lguest_get_wallclock(void)
{
	return lguest_data.time.tv_sec;
}

/* The TSC is an Intel thing called the Time Stamp Counter.  The Host tells us
 * what speed it runs at, or 0 if it's unusable as a reliable clock source.
 * This matches what we want here: if we return 0 from this function, the x86
 * TSC clock will give up and not register itself. */
static unsigned long lguest_tsc_khz(void)
{
	return lguest_data.tsc_khz;
}

/* If we can't use the TSC, the kernel falls back to our lower-priority
 * "lguest_clock", where we read the time value given to us by the Host. */
static cycle_t lguest_clock_read(struct clocksource *cs)
{
	unsigned long sec, nsec;

	/* Since the time is in two parts (seconds and nanoseconds), we risk
	 * reading it just as it's changing from 99 & 0.999999999 to 100 and 0,
	 * and getting 99 and 0.  As Linux tends to come apart under the stress
	 * of time travel, we must be careful: */
	do {
		/* First we read the seconds part. */
		sec = lguest_data.time.tv_sec;
		/* This read memory barrier tells the compiler and the CPU that
		 * this can't be reordered: we have to complete the above
		 * before going on. */
		rmb();
		/* Now we read the nanoseconds part. */
		nsec = lguest_data.time.tv_nsec;
		/* Make sure we've done that. */
		rmb();
		/* Now if the seconds part has changed, try again. */
	} while (unlikely(lguest_data.time.tv_sec != sec));

	/* Our lguest clock is in real nanoseconds. */
	return sec*1000000000ULL + nsec;
}

/* This is the fallback clocksource: lower priority than the TSC clocksource. */
static struct clocksource lguest_clock = {
	.name		= "lguest",
	.rating		= 200,
	.read		= lguest_clock_read,
	.mask		= CLOCKSOURCE_MASK(64),
	.mult		= 1 << 22,
	.shift		= 22,
	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
};

/* We also need a "struct clock_event_device": Linux asks us to set it to go
 * off some time in the future.  Actually, James Morris figured all this out, I
 * just applied the patch. */
static int lguest_clockevent_set_next_event(unsigned long delta,
                                           struct clock_event_device *evt)
{
	/* FIXME: I don't think this can ever happen, but James tells me he had
	 * to put this code in.  Maybe we should remove it now.  Anyone? */
	if (delta < LG_CLOCK_MIN_DELTA) {
		if (printk_ratelimit())
			printk(KERN_DEBUG "%s: small delta %lu ns\n",
			       __func__, delta);
		return -ETIME;
	}

	/* Please wake us this far in the future. */
	kvm_hypercall1(LHCALL_SET_CLOCKEVENT, delta);
	return 0;
}

static void lguest_clockevent_set_mode(enum clock_event_mode mode,
                                      struct clock_event_device *evt)
{
	switch (mode) {
	case CLOCK_EVT_MODE_UNUSED:
	case CLOCK_EVT_MODE_SHUTDOWN:
		/* A 0 argument shuts the clock down. */
		kvm_hypercall0(LHCALL_SET_CLOCKEVENT);
		break;
	case CLOCK_EVT_MODE_ONESHOT:
		/* This is what we expect. */
		break;
	case CLOCK_EVT_MODE_PERIODIC:
		BUG();
	case CLOCK_EVT_MODE_RESUME:
		break;
	}
}

/* This describes our primitive timer chip. */
static struct clock_event_device lguest_clockevent = {
	.name                   = "lguest",
	.features               = CLOCK_EVT_FEAT_ONESHOT,
	.set_next_event         = lguest_clockevent_set_next_event,
	.set_mode               = lguest_clockevent_set_mode,
	.rating                 = INT_MAX,
	.mult                   = 1,
	.shift                  = 0,
	.min_delta_ns           = LG_CLOCK_MIN_DELTA,
	.max_delta_ns           = LG_CLOCK_MAX_DELTA,
};

/* This is the Guest timer interrupt handler (hardware interrupt 0).  We just
 * call the clockevent infrastructure and it does whatever needs doing. */
static void lguest_time_irq(unsigned int irq, struct irq_desc *desc)
{
	unsigned long flags;

	/* Don't interrupt us while this is running. */
	local_irq_save(flags);
	lguest_clockevent.event_handler(&lguest_clockevent);
	local_irq_restore(flags);
}

/* At some point in the boot process, we get asked to set up our timing
 * infrastructure.  The kernel doesn't expect timer interrupts before this, but
 * we cleverly initialized the "blocked_interrupts" field of "struct
 * lguest_data" so that timer interrupts were blocked until now. */
static void lguest_time_init(void)
{
	/* Set up the timer interrupt (0) to go to our simple timer routine */
	set_irq_handler(0, lguest_time_irq);

	clocksource_register(&lguest_clock);

	/* We can't set cpumask in the initializer: damn C limitations!  Set it
	 * here and register our timer device. */
	lguest_clockevent.cpumask = cpumask_of(0);
	clockevents_register_device(&lguest_clockevent);

	/* Finally, we unblock the timer interrupt. */
	enable_lguest_irq(0);
}

/*
 * Miscellaneous bits and pieces.
 *
 * Here is an oddball collection of functions which the Guest needs for things
 * to work.  They're pretty simple.
 */

/* The Guest needs to tell the Host what stack it expects traps to use.  For
 * native hardware, this is part of the Task State Segment mentioned above in
 * lguest_load_tr_desc(), but to help hypervisors there's this special call.
 *
 * We tell the Host the segment we want to use (__KERNEL_DS is the kernel data
 * segment), the privilege level (we're privilege level 1, the Host is 0 and
 * will not tolerate us trying to use that), the stack pointer, and the number
 * of pages in the stack. */
static void lguest_load_sp0(struct tss_struct *tss,
			    struct thread_struct *thread)
{
	lazy_hcall3(LHCALL_SET_STACK, __KERNEL_DS | 0x1, thread->sp0,
		   THREAD_SIZE / PAGE_SIZE);
}

/* Let's just say, I wouldn't do debugging under a Guest. */
static void lguest_set_debugreg(int regno, unsigned long value)
{
	/* FIXME: Implement */
}

/* There are times when the kernel wants to make sure that no memory writes are
 * caught in the cache (that they've all reached real hardware devices).  This
 * doesn't matter for the Guest which has virtual hardware.
 *
 * On the Pentium 4 and above, cpuid() indicates that the Cache Line Flush
 * (clflush) instruction is available and the kernel uses that.  Otherwise, it
 * uses the older "Write Back and Invalidate Cache" (wbinvd) instruction.
 * Unlike clflush, wbinvd can only be run at privilege level 0.  So we can
 * ignore clflush, but replace wbinvd.
 */
static void lguest_wbinvd(void)
{
}

/* If the Guest expects to have an Advanced Programmable Interrupt Controller,
 * we play dumb by ignoring writes and returning 0 for reads.  So it's no
 * longer Programmable nor Controlling anything, and I don't think 8 lines of
 * code qualifies for Advanced.  It will also never interrupt anything.  It
 * does, however, allow us to get through the Linux boot code. */
#ifdef CONFIG_X86_LOCAL_APIC
static void lguest_apic_write(u32 reg, u32 v)
{
}

static u32 lguest_apic_read(u32 reg)
{
	return 0;
}

static u64 lguest_apic_icr_read(void)
{
	return 0;
}

static void lguest_apic_icr_write(u32 low, u32 id)
{
	/* Warn to see if there's any stray references */
	WARN_ON(1);
}

static void lguest_apic_wait_icr_idle(void)
{
	return;
}

static u32 lguest_apic_safe_wait_icr_idle(void)
{
	return 0;
}

static void set_lguest_basic_apic_ops(void)
{
	apic->read = lguest_apic_read;
	apic->write = lguest_apic_write;
	apic->icr_read = lguest_apic_icr_read;
	apic->icr_write = lguest_apic_icr_write;
	apic->wait_icr_idle = lguest_apic_wait_icr_idle;
	apic->safe_wait_icr_idle = lguest_apic_safe_wait_icr_idle;
};
#endif

/* STOP!  Until an interrupt comes in. */
static void lguest_safe_halt(void)
{
	kvm_hypercall0(LHCALL_HALT);
}

/* The SHUTDOWN hypercall takes a string to describe what's happening, and
 * an argument which says whether this to restart (reboot) the Guest or not.
 *
 * Note that the Host always prefers that the Guest speak in physical addresses
 * rather than virtual addresses, so we use __pa() here. */
static void lguest_power_off(void)
{
	kvm_hypercall2(LHCALL_SHUTDOWN, __pa("Power down"),
					LGUEST_SHUTDOWN_POWEROFF);
}

/*
 * Panicing.
 *
 * Don't.  But if you did, this is what happens.
 */
static int lguest_panic(struct notifier_block *nb, unsigned long l, void *p)
{
	kvm_hypercall2(LHCALL_SHUTDOWN, __pa(p), LGUEST_SHUTDOWN_POWEROFF);
	/* The hcall won't return, but to keep gcc happy, we're "done". */
	return NOTIFY_DONE;
}

static struct notifier_block paniced = {
	.notifier_call = lguest_panic
};

/* Setting up memory is fairly easy. */
static __init char *lguest_memory_setup(void)
{
	/* We do this here and not earlier because lockcheck used to barf if we
	 * did it before start_kernel().  I think we fixed that, so it'd be
	 * nice to move it back to lguest_init.  Patch welcome... */
	atomic_notifier_chain_register(&panic_notifier_list, &paniced);

	/* The Linux bootloader header contains an "e820" memory map: the
	 * Launcher populated the first entry with our memory limit. */
	e820_add_region(boot_params.e820_map[0].addr,
			  boot_params.e820_map[0].size,
			  boot_params.e820_map[0].type);

	/* This string is for the boot messages. */
	return "LGUEST";
}

/* We will eventually use the virtio console device to produce console output,
 * but before that is set up we use LHCALL_NOTIFY on normal memory to produce
 * console output. */
static __init int early_put_chars(u32 vtermno, const char *buf, int count)
{
	char scratch[17];
	unsigned int len = count;

	/* We use a nul-terminated string, so we have to make a copy.  Icky,
	 * huh? */
	if (len > sizeof(scratch) - 1)
		len = sizeof(scratch) - 1;
	scratch[len] = '\0';
	memcpy(scratch, buf, len);
	kvm_hypercall1(LHCALL_NOTIFY, __pa(scratch));

	/* This routine returns the number of bytes actually written. */
	return len;
}

/* Rebooting also tells the Host we're finished, but the RESTART flag tells the
 * Launcher to reboot us. */
static void lguest_restart(char *reason)
{
	kvm_hypercall2(LHCALL_SHUTDOWN, __pa(reason), LGUEST_SHUTDOWN_RESTART);
}

/*G:050
 * Patching (Powerfully Placating Performance Pedants)
 *
 * We have already seen that pv_ops structures let us replace simple native
 * instructions with calls to the appropriate back end all throughout the
 * kernel.  This allows the same kernel to run as a Guest and as a native
 * kernel, but it's slow because of all the indirect branches.
 *
 * Remember that David Wheeler quote about "Any problem in computer science can
 * be solved with another layer of indirection"?  The rest of that quote is
 * "... But that usually will create another problem."  This is the first of
 * those problems.
 *
 * Our current solution is to allow the paravirt back end to optionally patch
 * over the indirect calls to replace them with something more efficient.  We
 * patch two of the simplest of the most commonly called functions: disable
 * interrupts and save interrupts.  We usually have 6 or 10 bytes to patch
 * into: the Guest versions of these operations are small enough that we can
 * fit comfortably.
 *
 * First we need assembly templates of each of the patchable Guest operations,
 * and these are in i386_head.S. */

/*G:060 We construct a table from the assembler templates: */
static const struct lguest_insns
{
	const char *start, *end;
} lguest_insns[] = {
	[PARAVIRT_PATCH(pv_irq_ops.irq_disable)] = { lgstart_cli, lgend_cli },
	[PARAVIRT_PATCH(pv_irq_ops.save_fl)] = { lgstart_pushf, lgend_pushf },
};

/* Now our patch routine is fairly simple (based on the native one in
 * paravirt.c).  If we have a replacement, we copy it in and return how much of
 * the available space we used. */
static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
			     unsigned long addr, unsigned len)
{
	unsigned int insn_len;

	/* Don't do anything special if we don't have a replacement */
	if (type >= ARRAY_SIZE(lguest_insns) || !lguest_insns[type].start)
		return paravirt_patch_default(type, clobber, ibuf, addr, len);

	insn_len = lguest_insns[type].end - lguest_insns[type].start;

	/* Similarly if we can't fit replacement (shouldn't happen, but let's
	 * be thorough). */
	if (len < insn_len)
		return paravirt_patch_default(type, clobber, ibuf, addr, len);

	/* Copy in our instructions. */
	memcpy(ibuf, lguest_insns[type].start, insn_len);
	return insn_len;
}

/*G:030 Once we get to lguest_init(), we know we're a Guest.  The various
 * pv_ops structures in the kernel provide points for (almost) every routine we
 * have to override to avoid privileged instructions. */
__init void lguest_init(void)
{
	/* We're under lguest, paravirt is enabled, and we're running at
	 * privilege level 1, not 0 as normal. */
	pv_info.name = "lguest";
	pv_info.paravirt_enabled = 1;
	pv_info.kernel_rpl = 1;
	pv_info.shared_kernel_pmd = 1;

	/* We set up all the lguest overrides for sensitive operations.  These
	 * are detailed with the operations themselves. */

	/* interrupt-related operations */
	pv_irq_ops.init_IRQ = lguest_init_IRQ;
	pv_irq_ops.save_fl = PV_CALLEE_SAVE(save_fl);
	pv_irq_ops.restore_fl = __PV_IS_CALLEE_SAVE(lg_restore_fl);
	pv_irq_ops.irq_disable = PV_CALLEE_SAVE(irq_disable);
	pv_irq_ops.irq_enable = __PV_IS_CALLEE_SAVE(lg_irq_enable);
	pv_irq_ops.safe_halt = lguest_safe_halt;

	/* init-time operations */
	pv_init_ops.memory_setup = lguest_memory_setup;
	pv_init_ops.patch = lguest_patch;

	/* Intercepts of various cpu instructions */
	pv_cpu_ops.load_gdt = lguest_load_gdt;
	pv_cpu_ops.cpuid = lguest_cpuid;
	pv_cpu_ops.load_idt = lguest_load_idt;
	pv_cpu_ops.iret = lguest_iret;
	pv_cpu_ops.load_sp0 = lguest_load_sp0;
	pv_cpu_ops.load_tr_desc = lguest_load_tr_desc;
	pv_cpu_ops.set_ldt = lguest_set_ldt;
	pv_cpu_ops.load_tls = lguest_load_tls;
	pv_cpu_ops.set_debugreg = lguest_set_debugreg;
	pv_cpu_ops.clts = lguest_clts;
	pv_cpu_ops.read_cr0 = lguest_read_cr0;
	pv_cpu_ops.write_cr0 = lguest_write_cr0;
	pv_cpu_ops.read_cr4 = lguest_read_cr4;
	pv_cpu_ops.write_cr4 = lguest_write_cr4;
	pv_cpu_ops.write_gdt_entry = lguest_write_gdt_entry;
	pv_cpu_ops.write_idt_entry = lguest_write_idt_entry;
	pv_cpu_ops.wbinvd = lguest_wbinvd;
	pv_cpu_ops.start_context_switch = paravirt_start_context_switch;
	pv_cpu_ops.end_context_switch = lguest_end_context_switch;

	/* pagetable management */
	pv_mmu_ops.write_cr3 = lguest_write_cr3;
	pv_mmu_ops.flush_tlb_user = lguest_flush_tlb_user;
	pv_mmu_ops.flush_tlb_single = lguest_flush_tlb_single;
	pv_mmu_ops.flush_tlb_kernel = lguest_flush_tlb_kernel;
	pv_mmu_ops.set_pte = lguest_set_pte;
	pv_mmu_ops.set_pte_at = lguest_set_pte_at;
	pv_mmu_ops.set_pmd = lguest_set_pmd;
#ifdef CONFIG_X86_PAE
	pv_mmu_ops.set_pte_atomic = lguest_set_pte_atomic;
	pv_mmu_ops.pte_clear = lguest_pte_clear;
	pv_mmu_ops.pmd_clear = lguest_pmd_clear;
	pv_mmu_ops.set_pud = lguest_set_pud;
#endif
	pv_mmu_ops.read_cr2 = lguest_read_cr2;
	pv_mmu_ops.read_cr3 = lguest_read_cr3;
	pv_mmu_ops.lazy_mode.enter = paravirt_enter_lazy_mmu;
	pv_mmu_ops.lazy_mode.leave = lguest_leave_lazy_mmu_mode;
	pv_mmu_ops.pte_update = lguest_pte_update;
	pv_mmu_ops.pte_update_defer = lguest_pte_update;

#ifdef CONFIG_X86_LOCAL_APIC
	/* apic read/write intercepts */
	set_lguest_basic_apic_ops();
#endif

	/* time operations */
	pv_time_ops.get_wallclock = lguest_get_wallclock;
	pv_time_ops.time_init = lguest_time_init;
	pv_time_ops.get_tsc_khz = lguest_tsc_khz;

	/* Now is a good time to look at the implementations of these functions
	 * before returning to the rest of lguest_init(). */

	/*G:070 Now we've seen all the paravirt_ops, we return to
	 * lguest_init() where the rest of the fairly chaotic boot setup
	 * occurs. */

	/* The stack protector is a weird thing where gcc places a canary
	 * value on the stack and then checks it on return.  This file is
	 * compiled with -fno-stack-protector it, so we got this far without
	 * problems.  The value of the canary is kept at offset 20 from the
	 * %gs register, so we need to set that up before calling C functions
	 * in other files. */
	setup_stack_canary_segment(0);
	/* We could just call load_stack_canary_segment(), but we might as
	 * call switch_to_new_gdt() which loads the whole table and sets up
	 * the per-cpu segment descriptor register %fs as well. */
	switch_to_new_gdt(0);

	/* As described in head_32.S, we map the first 128M of memory. */
	max_pfn_mapped = (128*1024*1024) >> PAGE_SHIFT;

	/* The Host<->Guest Switcher lives at the top of our address space, and
	 * the Host told us how big it is when we made LGUEST_INIT hypercall:
	 * it put the answer in lguest_data.reserve_mem  */
	reserve_top_address(lguest_data.reserve_mem);

	/* If we don't initialize the lock dependency checker now, it crashes
	 * paravirt_disable_iospace. */
	lockdep_init();

	/* The IDE code spends about 3 seconds probing for disks: if we reserve
	 * all the I/O ports up front it can't get them and so doesn't probe.
	 * Other device drivers are similar (but less severe).  This cuts the
	 * kernel boot time on my machine from 4.1 seconds to 0.45 seconds. */
	paravirt_disable_iospace();

	/* This is messy CPU setup stuff which the native boot code does before
	 * start_kernel, so we have to do, too: */
	cpu_detect(&new_cpu_data);
	/* head.S usually sets up the first capability word, so do it here. */
	new_cpu_data.x86_capability[0] = cpuid_edx(1);

	/* Math is always hard! */
	new_cpu_data.hard_math = 1;

	/* We don't have features.  We have puppies!  Puppies! */
#ifdef CONFIG_X86_MCE
	mce_disabled = 1;
#endif
#ifdef CONFIG_ACPI
	acpi_disabled = 1;
	acpi_ht = 0;
#endif

	/* We set the preferred console to "hvc".  This is the "hypervisor
	 * virtual console" driver written by the PowerPC people, which we also
	 * adapted for lguest's use. */
	add_preferred_console("hvc", 0, NULL);

	/* Register our very early console. */
	virtio_cons_early_init(early_put_chars);

	/* Last of all, we set the power management poweroff hook to point to
	 * the Guest routine to power off, and the reboot hook to our restart
	 * routine. */
	pm_power_off = lguest_power_off;
	machine_ops.restart = lguest_restart;

	/* Now we're set up, call i386_start_kernel() in head32.c and we proceed
	 * to boot as normal.  It never returns. */
	i386_start_kernel();
}
/*
 * This marks the end of stage II of our journey, The Guest.
 *
 * It is now time for us to explore the layer of virtual drivers and complete
 * our understanding of the Guest in "make Drivers".
 */