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diff --git a/drivers/lguest/x86/switcher_32.S b/drivers/lguest/x86/switcher_32.S
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+/*P:900 This is the Switcher: code which sits at 0xFFC00000 to do the low-level
+ * Guest<->Host switch.  It is as simple as it can be made, but it's naturally
+ * very specific to x86.
+ *
+ * You have now completed Preparation.  If this has whet your appetite; if you
+ * are feeling invigorated and refreshed then the next, more challenging stage
+ * can be found in "make Guest". :*/
+
+/*S:100
+ * Welcome to the Switcher itself!
+ *
+ * This file contains the low-level code which changes the CPU to run the Guest
+ * code, and returns to the Host when something happens.  Understand this, and
+ * you understand the heart of our journey.
+ *
+ * Because this is in assembler rather than C, our tale switches from prose to
+ * verse.  First I tried limericks:
+ *
+ *      There once was an eax reg,
+ *      To which our pointer was fed,
+ *      It needed an add,
+ *      Which asm-offsets.h had
+ *      But this limerick is hurting my head.
+ *
+ * Next I tried haikus, but fitting the required reference to the seasons in
+ * every stanza was quickly becoming tiresome:
+ *
+ *      The %eax reg
+ *      Holds "struct lguest_pages" now:
+ *      Cherry blossoms fall.
+ *
+ * Then I started with Heroic Verse, but the rhyming requirement leeched away
+ * the content density and led to some uniquely awful oblique rhymes:
+ *
+ *      These constants are coming from struct offsets
+ *      For use within the asm switcher text.
+ *
+ * Finally, I settled for something between heroic hexameter, and normal prose
+ * with inappropriate linebreaks.  Anyway, it aint no Shakespeare.
+ */
+
+// Not all kernel headers work from assembler
+// But these ones are needed: the ENTRY() define
+// And constants extracted from struct offsets
+// To avoid magic numbers and breakage:
+// Should they change the compiler can't save us
+// Down here in the depths of assembler code.
+#include <linux/linkage.h>
+#include <asm/asm-offsets.h>
+#include <asm/page.h>
+#include "../lg.h"
+
+// We mark the start of the code to copy
+// It's placed in .text tho it's never run here
+// You'll see the trick macro at the end
+// Which interleaves data and text to effect.
+.text
+ENTRY(start_switcher_text)
+
+// When we reach switch_to_guest we have just left
+// The safe and comforting shores of C code
+// %eax has the "struct lguest_pages" to use
+// Where we save state and still see it from the Guest
+// And %ebx holds the Guest shadow pagetable:
+// Once set we have truly left Host behind.
+ENTRY(switch_to_guest)
+        // We told gcc all its regs could fade,
+        // Clobbered by our journey into the Guest
+        // We could have saved them, if we tried
+        // But time is our master and cycles count.
+
+        // Segment registers must be saved for the Host
+        // We push them on the Host stack for later
+        pushl   %es
+        pushl   %ds
+        pushl   %gs
+        pushl   %fs
+        // But the compiler is fickle, and heeds
+        // No warning of %ebp clobbers
+        // When frame pointers are used.  That register
+        // Must be saved and restored or chaos strikes.
+        pushl   %ebp
+        // The Host's stack is done, now save it away
+        // In our "struct lguest_pages" at offset
+        // Distilled into asm-offsets.h
+        movl    %esp, LGUEST_PAGES_host_sp(%eax)
+
+        // All saved and there's now five steps before us:
+        // Stack, GDT, IDT, TSS
+        // And last of all the page tables are flipped.
+
+        // Yet beware that our stack pointer must be
+        // Always valid lest an NMI hits
+        // %edx does the duty here as we juggle
+        // %eax is lguest_pages: our stack lies within.
+        movl    %eax, %edx
+        addl    $LGUEST_PAGES_regs, %edx
+        movl    %edx, %esp
+
+        // The Guest's GDT we so carefully
+        // Placed in the "struct lguest_pages" before
+        lgdt    LGUEST_PAGES_guest_gdt_desc(%eax)
+
+        // The Guest's IDT we did partially
+        // Move to the "struct lguest_pages" as well.
+        lidt    LGUEST_PAGES_guest_idt_desc(%eax)
+
+        // The TSS entry which controls traps
+        // Must be loaded up with "ltr" now:
+        // For after we switch over our page tables
+        // It (as the rest) will be writable no more.
+        // (The GDT entry TSS needs
+        // Changes type when we load it: damn Intel!)
+        movl    $(GDT_ENTRY_TSS*8), %edx
+        ltr     %dx
+
+        // Look back now, before we take this last step!
+        // The Host's TSS entry was also marked used;
+        // Let's clear it again, ere we return.
+        // The GDT descriptor of the Host
+        // Points to the table after two "size" bytes
+        movl    (LGUEST_PAGES_host_gdt_desc+2)(%eax), %edx
+        // Clear the type field of "used" (byte 5, bit 2)
+        andb    $0xFD, (GDT_ENTRY_TSS*8 + 5)(%edx)
+
+        // Once our page table's switched, the Guest is live!
+        // The Host fades as we run this final step.
+        // Our "struct lguest_pages" is now read-only.
+        movl    %ebx, %cr3
+
+        // The page table change did one tricky thing:
+        // The Guest's register page has been mapped
+        // Writable onto our %esp (stack) --
+        // We can simply pop off all Guest regs.
+        popl    %ebx
+        popl    %ecx
+        popl    %edx
+        popl    %esi
+        popl    %edi
+        popl    %ebp
+        popl    %gs
+        popl    %eax
+        popl    %fs
+        popl    %ds
+        popl    %es
+
+        // Near the base of the stack lurk two strange fields
+        // Which we fill as we exit the Guest
+        // These are the trap number and its error
+        // We can simply step past them on our way.
+        addl    $8, %esp
+
+        // The last five stack slots hold return address
+        // And everything needed to change privilege
+        // Into the Guest privilege level of 1,
+        // And the stack where the Guest had last left it.
+        // Interrupts are turned back on: we are Guest.
+        iret
+
+// There are two paths where we switch to the Host
+// So we put the routine in a macro.
+// We are on our way home, back to the Host
+// Interrupted out of the Guest, we come here.
+#define SWITCH_TO_HOST                                                  \
+        /* We save the Guest state: all registers first                 \
+         * Laid out just as "struct lguest_regs" defines */             \
+        pushl   %es;                                                    \
+        pushl   %ds;                                                    \
+        pushl   %fs;                                                    \
+        pushl   %eax;                                                   \
+        pushl   %gs;                                                    \
+        pushl   %ebp;                                                   \
+        pushl   %edi;                                                   \
+        pushl   %esi;                                                   \
+        pushl   %edx;                                                   \
+        pushl   %ecx;                                                   \
+        pushl   %ebx;                                                   \
+        /* Our stack and our code are using segments                    \
+         * Set in the TSS and IDT                                       \
+         * Yet if we were to touch data we'd use                        \
+         * Whatever data segment the Guest had.                         \
+         * Load the lguest ds segment for now. */                       \
+        movl    $(LGUEST_DS), %eax;                                     \
+        movl    %eax, %ds;                                              \
+        /* So where are we?  Which CPU, which struct?                   \
+         * The stack is our clue: our TSS starts                        \
+         * It at the end of "struct lguest_pages".                      \
+         * Or we may have stumbled while restoring                      \
+         * Our Guest segment regs while in switch_to_guest,             \
+         * The fault pushed atop that part-unwound stack.               \
+         * If we round the stack down to the page start                 \
+         * We're at the start of "struct lguest_pages". */              \
+        movl    %esp, %eax;                                             \
+        andl    $(~(1 << PAGE_SHIFT - 1)), %eax;                        \
+        /* Save our trap number: the switch will obscure it             \
+         * (The Guest regs are not mapped here in the Host)             \
+         * %ebx holds it safe for deliver_to_host */                    \
+        movl    LGUEST_PAGES_regs_trapnum(%eax), %ebx;                  \
+        /* The Host GDT, IDT and stack!                                 \
+         * All these lie safely hidden from the Guest:                  \
+         * We must return to the Host page tables                       \
+         * (Hence that was saved in struct lguest_pages) */             \
+        movl    LGUEST_PAGES_host_cr3(%eax), %edx;                      \
+        movl    %edx, %cr3;                                             \
+        /* As before, when we looked back at the Host                   \
+         * As we left and marked TSS unused                             \
+         * So must we now for the Guest left behind. */                 \
+        andb    $0xFD, (LGUEST_PAGES_guest_gdt+GDT_ENTRY_TSS*8+5)(%eax); \
+        /* Switch to Host's GDT, IDT. */                                \
+        lgdt    LGUEST_PAGES_host_gdt_desc(%eax);                       \
+        lidt    LGUEST_PAGES_host_idt_desc(%eax);                       \
+        /* Restore the Host's stack where it's saved regs lie */        \
+        movl    LGUEST_PAGES_host_sp(%eax), %esp;                       \
+        /* Last the TSS: our Host is complete */                        \
+        movl    $(GDT_ENTRY_TSS*8), %edx;                               \
+        ltr     %dx;                                                    \
+        /* Restore now the regs saved right at the first. */            \
+        popl    %ebp;                                                   \
+        popl    %fs;                                                    \
+        popl    %gs;                                                    \
+        popl    %ds;                                                    \
+        popl    %es
+
+// Here's where we come when the Guest has just trapped:
+// (Which trap we'll see has been pushed on the stack).
+// We need only switch back, and the Host will decode
+// Why we came home, and what needs to be done.
+return_to_host:
+        SWITCH_TO_HOST
+        iret
+
+// An interrupt, with some cause external
+// Has ajerked us rudely from the Guest's code
+// Again we must return home to the Host
+deliver_to_host:
+        SWITCH_TO_HOST
+        // But now we must go home via that place
+        // Where that interrupt was supposed to go
+        // Had we not been ensconced, running the Guest.
+        // Here we see the cleverness of our stack:
+        // The Host stack is formed like an interrupt
+        // With EIP, CS and EFLAGS layered.
+        // Interrupt handlers end with "iret"
+        // And that will take us home at long long last.
+
+        // But first we must find the handler to call!
+        // The IDT descriptor for the Host
+        // Has two bytes for size, and four for address:
+        // %edx will hold it for us for now.
+        movl    (LGUEST_PAGES_host_idt_desc+2)(%eax), %edx
+        // We now know the table address we need,
+        // And saved the trap's number inside %ebx.
+        // Yet the pointer to the handler is smeared
+        // Across the bits of the table entry.
+        // What oracle can tell us how to extract
+        // From such a convoluted encoding?
+        // I consulted gcc, and it gave
+        // These instructions, which I gladly credit:
+        leal    (%edx,%ebx,8), %eax
+        movzwl  (%eax),%edx
+        movl    4(%eax), %eax
+        xorw    %ax, %ax
+        orl     %eax, %edx
+        // Now the address of the handler's in %edx
+        // We call it now: its "iret" takes us home.
+        jmp     *%edx
+
+// Every interrupt can come to us here
+// But we must truly tell each apart.
+// They number two hundred and fifty six
+// And each must land in a different spot,
+// Push its number on stack, and join the stream.
+
+// And worse, a mere six of the traps stand apart
+// And push on their stack an addition:
+// An error number, thirty two bits long
+// So we punish the other two fifty
+// And make them push a zero so they match.
+
+// Yet two fifty six entries is long
+// And all will look most the same as the last
+// So we create a macro which can make
+// As many entries as we need to fill.
+
+// Note the change to .data then .text:
+// We plant the address of each entry
+// Into a (data) table for the Host
+// To know where each Guest interrupt should go.
+.macro IRQ_STUB N TARGET
+        .data; .long 1f; .text; 1:
+ // Trap eight, ten through fourteen and seventeen
+ // Supply an error number.  Else zero.
+ .if (\N <> 8) && (\N < 10 || \N > 14) && (\N <> 17)
+        pushl   $0
+ .endif
+        pushl   $\N
+        jmp     \TARGET
+        ALIGN
+.endm
+
+// This macro creates numerous entries
+// Using GAS macros which out-power C's.
+.macro IRQ_STUBS FIRST LAST TARGET
+ irq=\FIRST
+ .rept \LAST-\FIRST+1
+        IRQ_STUB irq \TARGET
+  irq=irq+1
+ .endr
+.endm
+
+// Here's the marker for our pointer table
+// Laid in the data section just before
+// Each macro places the address of code
+// Forming an array: each one points to text
+// Which handles interrupt in its turn.
+.data
+.global default_idt_entries
+default_idt_entries:
+.text
+        // The first two traps go straight back to the Host
+        IRQ_STUBS 0 1 return_to_host
+        // We'll say nothing, yet, about NMI
+        IRQ_STUB 2 handle_nmi
+        // Other traps also return to the Host
+        IRQ_STUBS 3 31 return_to_host
+        // All interrupts go via their handlers
+        IRQ_STUBS 32 127 deliver_to_host
+        // 'Cept system calls coming from userspace
+        // Are to go to the Guest, never the Host.
+        IRQ_STUB 128 return_to_host
+        IRQ_STUBS 129 255 deliver_to_host
+
+// The NMI, what a fabulous beast
+// Which swoops in and stops us no matter that
+// We're suspended between heaven and hell,
+// (Or more likely between the Host and Guest)
+// When in it comes!  We are dazed and confused
+// So we do the simplest thing which one can.
+// Though we've pushed the trap number and zero
+// We discard them, return, and hope we live.
+handle_nmi:
+        addl    $8, %esp
+        iret
+
+// We are done; all that's left is Mastery
+// And "make Mastery" is a journey long
+// Designed to make your fingers itch to code.
+
+// Here ends the text, the file and poem.
+ENTRY(end_switcher_text)