lguest: documentation VI: Switcher

Documentation: The Switcher

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 229 insertions, 42 deletions

diff --git a/drivers/lguest/switcher.S b/drivers/lguest/switcher.S
index e7cb8c123558..d418179ea6b5 100644
--- a/drivers/lguest/switcher.S
+++ b/drivers/lguest/switcher.S
@@ -6,41 +6,131 @@
  * 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 "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)
 
-/* %eax points to lguest pages for this CPU.  %ebx contains cr3 value.
-   All normal registers can be clobbered! */
+// 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)
-        /* Save host segments on host stack. */
+        // 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
-        /* With CONFIG_FRAME_POINTER, gcc doesn't let us clobber this! */
+        // 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
-        /* Save host stack. */
+        // 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)
-        /* Switch to guest stack: if we get NMI we expect to be there. */
+
+        // 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
-        /* Switch to guest's GDT, IDT. */
+
+        // 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)
-        /* Switch to guest's TSS while GDT still writable. */
+
+        // 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
-        /* Set host's TSS GDT entry to available (clear byte 5 bit 2). */
+
+        // 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)
-        /* Switch to guest page tables: lguest_pages->state now read-only. */
+
+        // 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
-        /* Restore guest regs */
+
+        // 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
@@ -52,12 +142,27 @@ ENTRY(switch_to_guest)
         popl    %fs
         popl    %ds
         popl    %es
-        /* Skip error code and trap number */
+
+        // 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                                                  \
-        /* Save guest state */                                          \
+        /* We save the Guest state: all registers first                 \
+         * Laid out just as "struct lguest_regs" defines */             \
         pushl   %es;                                                    \
         pushl   %ds;                                                    \
         pushl   %fs;                                                    \
@@ -69,58 +174,119 @@ ENTRY(switch_to_guest)
         pushl   %edx;                                                   \
         pushl   %ecx;                                                   \
         pushl   %ebx;                                                   \
-        /* Load lguest ds segment for convenience. */                   \
+        /* 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;                                              \
-        /* Figure out where we are, based on stack (at top of regs). */ \
+        /* So where are we?  Which CPU, which struct?                   \
+         * The stack is our clue: our TSS sets                          \
+         * It at the end of "struct lguest_pages"                       \
+         * And we then pushed and pushed and pushed Guest regs:         \
+         * Now stack points atop the "struct lguest_regs".              \
+         * Subtract that offset, and we find our struct. */             \
         movl    %esp, %eax;                                             \
         subl    $LGUEST_PAGES_regs, %eax;                               \
-        /* Put trap number in %ebx before we switch cr3 and lose it. */ \
+        /* 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;                  \
-        /* Switch to host page tables (host GDT, IDT and stack are in host   \
-           mem, so need this first) */                                  \
+        /* 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;                                             \
-        /* Set guest's TSS to available (clear byte 5 bit 2). */        \
+        /* 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. */                               \
+        /* Switch to Host's GDT, IDT. */                                \
         lgdt    LGUEST_PAGES_host_gdt_desc(%eax);                       \
         lidt    LGUEST_PAGES_host_idt_desc(%eax);                       \
-        /* Switch to host's stack. */                                   \
+        /* Restore the Host's stack where it's saved regs lie */        \
         movl    LGUEST_PAGES_host_sp(%eax), %esp;                       \
-        /* Switch to host's TSS */                                      \
+        /* 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
 
-/* Return to run_guest_once. */
+// 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
-        /* Decode IDT and jump to hosts' irq handler.  When that does iret, it
-         * will return to run_guest_once.  This is a feature. */
+        // 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
 
-/* Real hardware interrupts are delivered straight to the host.  Others
-   cause us to return to run_guest_once so it can decide what to do.  Note
-   that some of these are overridden by the guest to deliver directly, and
-   never enter here (see load_guest_idt_entry). */
+// 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:
- /* Make an error number for most traps, which don't have one. */
+ // 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
@@ -129,6 +295,8 @@ deliver_to_host:
         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
@@ -137,24 +305,43 @@ deliver_to_host:
  .endr
 .endm
 
-/* We intercept every interrupt, because we may need to switch back to
- * host.  Unfortunately we can't tell them apart except by entry
- * point, so we need 256 entry points.
- */
+// 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
-        IRQ_STUBS 0 1 return_to_host            /* First two traps */
-        IRQ_STUB 2 handle_nmi                   /* NMI */
-        IRQ_STUBS 3 31 return_to_host           /* Rest of traps */
-        IRQ_STUBS 32 127 deliver_to_host        /* Real interrupts */
-        IRQ_STUB 128 return_to_host             /* System call (overridden) */
-        IRQ_STUBS 129 255 deliver_to_host       /* Other real interrupts */
-
-/* We ignore NMI and return. */
+        // 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)