/*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)