Merge git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux-2.6-lguest

* git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux-2.6-lguest: (45 commits)
  Use "struct boot_params" in example launcher
  Loading bzImage directly.
  Revert lguest magic and use hook in head.S
  Update lguest documentation to reflect the new virtual block device name.
  generalize lgread_u32/lgwrite_u32.
  Example launcher handle guests not being ready for input
  Update example launcher for virtio
  Lguest support for Virtio
  Remove old lguest I/O infrrasructure.
  Remove old lguest bus and drivers.
  Virtio helper routines for a descriptor ringbuffer implementation
  Module autoprobing support for virtio drivers.
  Virtio console driver
  Block driver using virtio.
  Net driver using virtio
  Virtio interface
  Boot with virtual == physical to get closer to native Linux.
  Allow guest to specify syscall vector to use.
  Rename "cr3" to "gpgdir" to avoid x86-specific naming.
  Pagetables to use normal kernel types
  ...

8 files changed, 1229 insertions, 12 deletions

diff --git a/arch/i386/Kconfig b/arch/i386/Kconfig
index f6e44fc5283..5bed8be34ba 100644
--- a/arch/i386/Kconfig
+++ b/arch/i386/Kconfig
@@ -227,28 +227,40 @@ config SCHED_NO_NO_OMIT_FRAME_POINTER
 	  If in doubt, say "Y".
 
 config PARAVIRT
-	bool "Paravirtualization support (EXPERIMENTAL)"
-	depends on EXPERIMENTAL
+	bool
 	depends on !(X86_VISWS || X86_VOYAGER)
 	help
-	  Paravirtualization is a way of running multiple instances of
-	  Linux on the same machine, under a hypervisor.  This option
-	  changes the kernel so it can modify itself when it is run
-	  under a hypervisor, improving performance significantly.
-	  However, when run without a hypervisor the kernel is
-	  theoretically slower.  If in doubt, say N.
+	  This changes the kernel so it can modify itself when it is run
+	  under a hypervisor, potentially improving performance significantly
+	  over full virtualization.  However, when run without a hypervisor
+	  the kernel is theoretically slower and slightly larger.
+
+menuconfig PARAVIRT_GUEST
+	bool "Paravirtualized guest support"
+	help
+	  Say Y here to get to see options related to running Linux under
+	  various hypervisors.  This option alone does not add any kernel code.
+
+	  If you say N, all options in this submenu will be skipped and disabled.
+
+if PARAVIRT_GUEST
 
 source "arch/x86/xen/Kconfig"
 
 config VMI
-	bool "VMI Paravirt-ops support"
-	depends on PARAVIRT
+	bool "VMI Guest support"
+	select PARAVIRT
+	depends on !(X86_VISWS || X86_VOYAGER)
 	help
 	  VMI provides a paravirtualized interface to the VMware ESX server
 	  (it could be used by other hypervisors in theory too, but is not
 	  at the moment), by linking the kernel to a GPL-ed ROM module
 	  provided by the hypervisor.
 
+source "arch/x86/lguest/Kconfig"
+
+endif
+
 config ACPI_SRAT
 	bool
 	default y
diff --git a/arch/i386/Makefile b/arch/i386/Makefile
index b88e47ca303..b81cb64d48e 100644
--- a/arch/i386/Makefile
+++ b/arch/i386/Makefile
@@ -99,6 +99,9 @@ core-$(CONFIG_X86_ES7000)	:= arch/x86/mach-es7000/
 # Xen paravirtualization support
 core-$(CONFIG_XEN)		+= arch/x86/xen/
 
+# lguest paravirtualization support
+core-$(CONFIG_LGUEST_GUEST)	+= arch/x86/lguest/
+
 # default subarch .h files
 mflags-y += -Iinclude/asm-x86/mach-default
 
diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
index f8764716b0c..0e45981b2dd 100644
--- a/arch/x86/kernel/asm-offsets_32.c
+++ b/arch/x86/kernel/asm-offsets_32.c
@@ -136,6 +136,7 @@ void foo(void)
 #ifdef CONFIG_LGUEST_GUEST
 	BLANK();
 	OFFSET(LGUEST_DATA_irq_enabled, lguest_data, irq_enabled);
+	OFFSET(LGUEST_DATA_pgdir, lguest_data, pgdir);
 	OFFSET(LGUEST_PAGES_host_gdt_desc, lguest_pages, state.host_gdt_desc);
 	OFFSET(LGUEST_PAGES_host_idt_desc, lguest_pages, state.host_idt_desc);
 	OFFSET(LGUEST_PAGES_host_cr3, lguest_pages, state.host_cr3);
diff --git a/arch/x86/lguest/Kconfig b/arch/x86/lguest/Kconfig
new file mode 100644
index 00000000000..c4dffbeea5e
--- /dev/null
+++ b/arch/x86/lguest/Kconfig
@@ -0,0 +1,14 @@
+config LGUEST_GUEST
+	bool "Lguest guest support"
+	select PARAVIRT
+	depends on !X86_PAE
+	select VIRTIO
+	select VIRTIO_RING
+	select VIRTIO_CONSOLE
+	help
+	  Lguest is a tiny in-kernel hypervisor.  Selecting this will
+	  allow your kernel to boot under lguest.  This option will increase
+	  your kernel size by about 6k.  If in doubt, say N.
+
+	  If you say Y here, make sure you say Y (or M) to the virtio block
+	  and net drivers which lguest needs.
diff --git a/arch/x86/lguest/Makefile b/arch/x86/lguest/Makefile
new file mode 100644
index 00000000000..27f0c9ed7f6
--- /dev/null
+++ b/arch/x86/lguest/Makefile
@@ -0,0 +1 @@
+obj-y		:= i386_head.o boot.o
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
new file mode 100644
index 00000000000..d2235db4085
--- /dev/null
+++ b/arch/x86/lguest/boot.c
@@ -0,0 +1,1070 @@
+/*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.  When
+ * you set CONFIG_LGUEST to 'y' or 'm', this automatically sets
+ * CONFIG_LGUEST_GUEST=y, which compiles this file into the kernel so it knows
+ * how to be a Guest.  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?  The Guest starts at a special
+ * entry point marked with a magic string, which sets up a few things then
+ * calls here.  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 <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>
+
+/*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). :*/
+
+/* Declarations for definitions in lguest_guest.S */
+extern char lguest_noirq_start[], lguest_noirq_end[];
+extern const char lgstart_cli[], lgend_cli[];
+extern const char lgstart_sti[], lgend_sti[];
+extern const char lgstart_popf[], lgend_popf[];
+extern const char lgstart_pushf[], lgend_pushf[];
+extern const char lgstart_iret[], lgend_iret[];
+extern void lguest_iret(void);
+
+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,
+};
+static cycle_t clock_base;
+
+/*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 mmap 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_hypercall() to store the call for
+ * future processing.  When lazy mode is turned off we issue a hypercall to
+ * flush the stored calls.
+ */
+static void lguest_leave_lazy_mode(void)
+{
+	paravirt_leave_lazy(paravirt_get_lazy_mode());
+	hcall(LHCALL_FLUSH_ASYNC, 0, 0, 0);
+}
+
+static void lazy_hcall(unsigned long call,
+		       unsigned long arg1,
+		       unsigned long arg2,
+		       unsigned long arg3)
+{
+	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
+		hcall(call, arg1, arg2, arg3);
+	else
+		async_hcall(call, arg1, arg2, arg3);
+}
+
+/* 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 4 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! */
+void async_hcall(unsigned long call,
+		 unsigned long arg1, unsigned long arg2, unsigned long arg3)
+{
+	/* 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. */
+		hcall(call, arg1, arg2, arg3);
+	} 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;
+		/* 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:033
+ * Here are 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 when it wants to deliver an interrupt.
+ */
+
+/* save_flags() is expected to return the processor state (ie. "eflags").  The
+ * eflags 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;
+}
+
+/* "restore_flags" just sets the flags back to the value given. */
+static void restore_fl(unsigned long flags)
+{
+	lguest_data.irq_enabled = flags;
+}
+
+/* Interrupts go off... */
+static void irq_disable(void)
+{
+	lguest_data.irq_enabled = 0;
+}
+
+/* Interrupts go on... */
+static void irq_enable(void)
+{
+	lguest_data.irq_enabled = X86_EFLAGS_IF;
+}
+/*:*/
+/*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 when we turn on 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. :*/
+
+/*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(struct desc_struct *dt,
+				   int entrynum, u32 low, u32 high)
+{
+	/* Keep the local copy up to date. */
+	write_dt_entry(dt, entrynum, low, high);
+	/* Tell Host about this new entry. */
+	hcall(LHCALL_LOAD_IDT_ENTRY, entrynum, low, high);
+}
+
+/* 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 Xgt_desc_struct *desc)
+{
+	unsigned int i;
+	struct desc_struct *idt = (void *)desc->address;
+
+	for (i = 0; i < (desc->size+1)/8; i++)
+		hcall(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 opposite of the IDT code where we have a LOAD_IDT_ENTRY
+ * hypercall and use that repeatedly to load a new IDT.  I don't think it
+ * really matters, but wouldn't it be nice if they were the same?
+ */
+static void lguest_load_gdt(const struct Xgt_desc_struct *desc)
+{
+	BUG_ON((desc->size+1)/8 != GDT_ENTRIES);
+	hcall(LHCALL_LOAD_GDT, __pa(desc->address), GDT_ENTRIES, 0);
+}
+
+/* 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, u32 low, u32 high)
+{
+	write_dt_entry(dt, entrynum, low, high);
+	hcall(LHCALL_LOAD_GDT, __pa(dt), GDT_ENTRIES, 0);
+}
+
+/* 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. */
+	loadsegment(gs, 0);
+	lazy_hcall(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu, 0);
+}
+
+/*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 and AMD.  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 *eax, unsigned int *ebx,
+			 unsigned int *ecx, unsigned int *edx)
+{
+	int function = *eax;
+
+	native_cpuid(eax, ebx, ecx, edx);
+	switch (function) {
+	case 1:	/* Basic feature request. */
+		/* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
+		*ecx &= 0x00002201;
+		/* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, FPU. */
+		*edx &= 0x07808101;
+		/* 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. */
+		*edx |= 0x00002000;
+		break;
+	case 0x80000000:
+		/* Futureproof this a little: if they ask how much extended
+		 * processor information there is, limit it to known fields. */
+		if (*eax > 0x80000008)
+			*eax = 0x80000008;
+		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 (and cr3) 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, current_cr3;
+static void lguest_write_cr0(unsigned long val)
+{
+	/* 8 == TS bit. */
+	lazy_hcall(LHCALL_TS, val & 8, 0, 0);
+	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_hcall(LHCALL_TS, 0, 0, 0);
+	current_cr0 &= ~8U;
+}
+
+/* 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;
+}
+
+/* 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. */
+static void lguest_write_cr3(unsigned long cr3)
+{
+	lazy_hcall(LHCALL_NEW_PGTABLE, cr3, 0, 0);
+	current_cr3 = cr3;
+}
+
+static unsigned long lguest_read_cr3(void)
+{
+	return current_cr3;
+}
+
+/* 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_set_pte_at(struct mm_struct *mm, unsigned long addr,
+			      pte_t *ptep, pte_t pteval)
+{
+	*ptep = pteval;
+	lazy_hcall(LHCALL_SET_PTE, __pa(mm->pgd), addr, pteval.pte_low);
+}
+
+/* The Guest calls this to set a top-level entry.  Again, we set the entry then
+ * tell the Host which top-level page we changed, and the index of the entry we
+ * changed. */
+static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
+{
+	*pmdp = pmdval;
+	lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK,
+		   (__pa(pmdp)&(PAGE_SIZE-1))/4, 0);
+}
+
+/* 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.  So we don't even tell the Host
+ * anything changed until we've done the first page table switch.
+ */
+static void lguest_set_pte(pte_t *ptep, pte_t pteval)
+{
+	*ptep = pteval;
+	/* Don't bother with hypercall before initial setup. */
+	if (current_cr3)
+		lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0);
+}
+
+/* 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_hcall(LHCALL_SET_PTE, current_cr3, 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_hcall(LHCALL_FLUSH_TLB, 0, 0, 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_hcall(LHCALL_FLUSH_TLB, 1, 0, 0);
+}
+
+/*
+ * 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 = 0; i < LGUEST_IRQS; i++) {
+		int vector = FIRST_EXTERNAL_VECTOR + i;
+		if (vector != SYSCALL_VECTOR) {
+			set_intr_gate(vector, interrupt[i]);
+			set_irq_chip_and_handler(i, &lguest_irq_controller,
+						 handle_level_irq);
+		}
+	}
+	/* 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());
+}
+
+/*
+ * 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;
+}
+
+static cycle_t lguest_clock_read(void)
+{
+	unsigned long sec, nsec;
+
+	/* If the Host tells the TSC speed, we can trust that. */
+	if (lguest_data.tsc_khz)
+		return native_read_tsc();
+
+	/* If we can't use the TSC, we read the time value written by the Host.
+	 * Since it's 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 non-TSC clock is in real nanoseconds. */
+	return sec*1000000000ULL + nsec;
+}
+
+/* This is what we tell the kernel is our clocksource.  */
+static struct clocksource lguest_clock = {
+	.name		= "lguest",
+	.rating		= 400,
+	.read		= lguest_clock_read,
+	.mask		= CLOCKSOURCE_MASK(64),
+	.mult		= 1 << 22,
+	.shift		= 22,
+	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
+};
+
+/* The "scheduler clock" is just our real clock, adjusted to start at zero */
+static unsigned long long lguest_sched_clock(void)
+{
+	return cyc2ns(&lguest_clock, lguest_clock_read() - clock_base);
+}
+
+/* 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)
+{
+	if (delta < LG_CLOCK_MIN_DELTA) {
+		if (printk_ratelimit())
+			printk(KERN_DEBUG "%s: small delta %lu ns\n",
+			       __FUNCTION__, delta);
+		return -ETIME;
+	}
+	hcall(LHCALL_SET_CLOCKEVENT, delta, 0, 0);
+	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. */
+		hcall(LHCALL_SET_CLOCKEVENT, 0, 0, 0);
+		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);
+
+	/* Our clock structure look like arch/i386/kernel/tsc.c if we can use
+	 * the TSC, otherwise it's a dumb nanosecond-resolution clock.  Either
+	 * way, the "rating" is initialized so high that it's always chosen
+	 * over any other clocksource. */
+	if (lguest_data.tsc_khz)
+		lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz,
+							 lguest_clock.shift);
+	clock_base = lguest_clock_read();
+	clocksource_register(&lguest_clock);
+
+	/* Now we've set up our clock, we can use it as the scheduler clock */
+	pv_time_ops.sched_clock = lguest_sched_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_cpu(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_esp0(struct tss_struct *tss,
+				     struct thread_struct *thread)
+{
+	lazy_hcall(LHCALL_SET_STACK, __KERNEL_DS|0x1, thread->esp0,
+		   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)