diff options
Diffstat (limited to 'drivers/lguest/switcher.S')
| -rw-r--r-- | drivers/lguest/switcher.S | 284 |
1 files changed, 236 insertions, 48 deletions
diff --git a/drivers/lguest/switcher.S b/drivers/lguest/switcher.S index eadd4cc299d2..d418179ea6b5 100644 --- a/drivers/lguest/switcher.S +++ b/drivers/lguest/switcher.S @@ -1,45 +1,136 @@ -/* This code sits at 0xFFC00000 to do the low-level guest<->host switch. +/*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". :*/ - There is are two pages above us for this CPU (struct lguest_pages). - The second page (struct lguest_ro_state) becomes read-only after the - context switch. The first page (the stack for traps) remains writable, - but while we're in here, the guest cannot be 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 "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 @@ -51,12 +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; \ @@ -68,58 +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 @@ -128,6 +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 @@ -136,24 +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) |