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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". :*/

/*M:012 Lguest is meant to be simple: my rule of thumb is that 1% more LOC must
 * gain at least 1% more performance.  Since neither LOC nor performance can be
 * measured beforehand, it generally means implementing a feature then deciding
 * if it's worth it.  And once it's implemented, who can say no?
 *
 * This is why I haven't implemented this idea myself.  I want to, but I
 * haven't.  You could, though.
 *
 * The main place where lguest performance sucks is Guest page faulting.  When
 * a Guest userspace process hits an unmapped page we switch back to the Host,
 * walk the page tables, find it's not mapped, switch back to the Guest page
 * fault handler, which calls a hypercall to set the page table entry, then
 * finally returns to userspace.  That's two round-trips.
 *
 * If we had a small walker in the Switcher, we could quickly check the Guest
 * page table and if the page isn't mapped, immediately reflect the fault back
 * into the Guest.  This means the Switcher would have to know the top of the
 * Guest page table and the page fault handler address.
 *
 * For simplicity, the Guest should only handle the case where the privilege
 * level of the fault is 3 and probably only not present or write faults.  It
 * should also detect recursive faults, and hand the original fault to the
 * Host (which is actually really easy).
 *
 * Two questions remain.  Would the performance gain outweigh the complexity?
 * And who would write the verse documenting it? :*/

/*M:011 Lguest64 handles NMI.  This gave me NMI envy (until I looked at their
 * code).  It's worth doing though, since it would let us use oprofile in the
 * Host when a Guest is running. :*/

/*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 <asm/segment.h>
#include <asm/lguest.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
        // Then 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
        // Copy to "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:
        // The GDT entry that TSS uses 
        // Changes type when we load it: damn Intel!
        // For after we switch over our page tables
        // That entry will be read-only: we'd crash.
        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 for our 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 "used" from type field (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 under our %esp (stack) --
        // We can simply pop off all Guest regs.
        popl    %eax
        popl    %ebx
        popl    %ecx
        popl    %edx
        popl    %esi
        popl    %edi
        popl    %ebp
        popl    %gs
        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 switch privilege
        // From Switcher's level 0 to Guest's 1,
        // And the stack where the Guest had last left it.
        // Interrupts are turned back on: we are Guest.
        iret

// We treat two paths to switch back to the Host
// Yet both must save Guest state and restore Host
// So we put the routine in a macro.
#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   %gs;                                                    \
        pushl   %ebp;                                                   \
        pushl   %edi;                                                   \
        pushl   %esi;                                                   \
        pushl   %edx;                                                   \
        pushl   %ecx;                                                   \
        pushl   %ebx;                                                   \
        pushl   %eax;                                                   \
        /* 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             \
         * (In the Host the Guest regs are not mapped here)             \
         * %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 its saved regs lie */         \
        movl    LGUEST_PAGES_host_sp(%eax), %esp;                       \
        /* Last the TSS: our Host is returned */                        \
        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

// The first path is trod when the Guest has trapped:
// (Which trap it was 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

// We are lead to the second path like so:
// 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 trickness of run_guest_once():
        // 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" drops 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)