389 lines
13 KiB
ArmAsm
389 lines
13 KiB
ArmAsm
/*P:900
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* This is the Switcher: code which sits at 0xFFC00000 (or 0xFFE00000) astride
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* both the Host and Guest to do the low-level Guest<->Host switch. It is as
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* simple as it can be made, but it's naturally very specific to x86.
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*
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* You have now completed Preparation. If this has whet your appetite; if you
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* are feeling invigorated and refreshed then the next, more challenging stage
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* can be found in "make Guest".
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:*/
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/*M:012
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* Lguest is meant to be simple: my rule of thumb is that 1% more LOC must
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* gain at least 1% more performance. Since neither LOC nor performance can be
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* measured beforehand, it generally means implementing a feature then deciding
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* if it's worth it. And once it's implemented, who can say no?
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*
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* This is why I haven't implemented this idea myself. I want to, but I
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* haven't. You could, though.
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*
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* The main place where lguest performance sucks is Guest page faulting. When
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* a Guest userspace process hits an unmapped page we switch back to the Host,
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* walk the page tables, find it's not mapped, switch back to the Guest page
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* fault handler, which calls a hypercall to set the page table entry, then
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* finally returns to userspace. That's two round-trips.
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*
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* If we had a small walker in the Switcher, we could quickly check the Guest
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* page table and if the page isn't mapped, immediately reflect the fault back
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* into the Guest. This means the Switcher would have to know the top of the
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* Guest page table and the page fault handler address.
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*
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* For simplicity, the Guest should only handle the case where the privilege
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* level of the fault is 3 and probably only not present or write faults. It
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* should also detect recursive faults, and hand the original fault to the
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* Host (which is actually really easy).
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*
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* Two questions remain. Would the performance gain outweigh the complexity?
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* And who would write the verse documenting it?
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:*/
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/*M:011
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* Lguest64 handles NMI. This gave me NMI envy (until I looked at their
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* code). It's worth doing though, since it would let us use oprofile in the
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* Host when a Guest is running.
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:*/
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/*S:100
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* Welcome to the Switcher itself!
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*
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* This file contains the low-level code which changes the CPU to run the Guest
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* code, and returns to the Host when something happens. Understand this, and
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* you understand the heart of our journey.
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*
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* Because this is in assembler rather than C, our tale switches from prose to
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* verse. First I tried limericks:
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*
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* There once was an eax reg,
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* To which our pointer was fed,
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* It needed an add,
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* Which asm-offsets.h had
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* But this limerick is hurting my head.
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*
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* Next I tried haikus, but fitting the required reference to the seasons in
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* every stanza was quickly becoming tiresome:
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*
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* The %eax reg
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* Holds "struct lguest_pages" now:
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* Cherry blossoms fall.
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*
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* Then I started with Heroic Verse, but the rhyming requirement leeched away
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* the content density and led to some uniquely awful oblique rhymes:
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*
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* These constants are coming from struct offsets
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* For use within the asm switcher text.
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*
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* Finally, I settled for something between heroic hexameter, and normal prose
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* with inappropriate linebreaks. Anyway, it aint no Shakespeare.
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*/
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// Not all kernel headers work from assembler
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// But these ones are needed: the ENTRY() define
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// And constants extracted from struct offsets
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// To avoid magic numbers and breakage:
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// Should they change the compiler can't save us
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// Down here in the depths of assembler code.
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#include <linux/linkage.h>
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#include <asm/asm-offsets.h>
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#include <asm/page.h>
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#include <asm/segment.h>
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#include <asm/lguest.h>
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// We mark the start of the code to copy
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// It's placed in .text tho it's never run here
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// You'll see the trick macro at the end
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// Which interleaves data and text to effect.
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.text
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ENTRY(start_switcher_text)
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// When we reach switch_to_guest we have just left
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// The safe and comforting shores of C code
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// %eax has the "struct lguest_pages" to use
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// Where we save state and still see it from the Guest
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// And %ebx holds the Guest shadow pagetable:
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// Once set we have truly left Host behind.
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ENTRY(switch_to_guest)
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// We told gcc all its regs could fade,
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// Clobbered by our journey into the Guest
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// We could have saved them, if we tried
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// But time is our master and cycles count.
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// Segment registers must be saved for the Host
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// We push them on the Host stack for later
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pushl %es
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pushl %ds
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pushl %gs
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pushl %fs
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// But the compiler is fickle, and heeds
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// No warning of %ebp clobbers
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// When frame pointers are used. That register
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// Must be saved and restored or chaos strikes.
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pushl %ebp
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// The Host's stack is done, now save it away
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// In our "struct lguest_pages" at offset
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// Distilled into asm-offsets.h
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movl %esp, LGUEST_PAGES_host_sp(%eax)
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// All saved and there's now five steps before us:
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// Stack, GDT, IDT, TSS
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// Then last of all the page tables are flipped.
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// Yet beware that our stack pointer must be
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// Always valid lest an NMI hits
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// %edx does the duty here as we juggle
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// %eax is lguest_pages: our stack lies within.
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movl %eax, %edx
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addl $LGUEST_PAGES_regs, %edx
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movl %edx, %esp
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// The Guest's GDT we so carefully
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// Placed in the "struct lguest_pages" before
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lgdt LGUEST_PAGES_guest_gdt_desc(%eax)
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// The Guest's IDT we did partially
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// Copy to "struct lguest_pages" as well.
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lidt LGUEST_PAGES_guest_idt_desc(%eax)
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// The TSS entry which controls traps
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// Must be loaded up with "ltr" now:
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// The GDT entry that TSS uses
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// Changes type when we load it: damn Intel!
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// For after we switch over our page tables
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// That entry will be read-only: we'd crash.
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movl $(GDT_ENTRY_TSS*8), %edx
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ltr %dx
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// Look back now, before we take this last step!
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// The Host's TSS entry was also marked used;
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// Let's clear it again for our return.
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// The GDT descriptor of the Host
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// Points to the table after two "size" bytes
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movl (LGUEST_PAGES_host_gdt_desc+2)(%eax), %edx
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// Clear "used" from type field (byte 5, bit 2)
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andb $0xFD, (GDT_ENTRY_TSS*8 + 5)(%edx)
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// Once our page table's switched, the Guest is live!
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// The Host fades as we run this final step.
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// Our "struct lguest_pages" is now read-only.
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movl %ebx, %cr3
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// The page table change did one tricky thing:
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// The Guest's register page has been mapped
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// Writable under our %esp (stack) --
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// We can simply pop off all Guest regs.
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popl %eax
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popl %ebx
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popl %ecx
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popl %edx
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popl %esi
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popl %edi
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popl %ebp
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popl %gs
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popl %fs
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popl %ds
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popl %es
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// Near the base of the stack lurk two strange fields
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// Which we fill as we exit the Guest
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// These are the trap number and its error
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// We can simply step past them on our way.
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addl $8, %esp
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// The last five stack slots hold return address
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// And everything needed to switch privilege
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// From Switcher's level 0 to Guest's 1,
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// And the stack where the Guest had last left it.
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// Interrupts are turned back on: we are Guest.
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iret
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// We tread two paths to switch back to the Host
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// Yet both must save Guest state and restore Host
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// So we put the routine in a macro.
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#define SWITCH_TO_HOST \
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/* We save the Guest state: all registers first \
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* Laid out just as "struct lguest_regs" defines */ \
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pushl %es; \
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pushl %ds; \
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pushl %fs; \
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pushl %gs; \
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pushl %ebp; \
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pushl %edi; \
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pushl %esi; \
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pushl %edx; \
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pushl %ecx; \
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pushl %ebx; \
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pushl %eax; \
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/* Our stack and our code are using segments \
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* Set in the TSS and IDT \
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* Yet if we were to touch data we'd use \
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* Whatever data segment the Guest had. \
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* Load the lguest ds segment for now. */ \
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movl $(LGUEST_DS), %eax; \
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movl %eax, %ds; \
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/* So where are we? Which CPU, which struct? \
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* The stack is our clue: our TSS starts \
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* It at the end of "struct lguest_pages". \
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* Or we may have stumbled while restoring \
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* Our Guest segment regs while in switch_to_guest, \
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* The fault pushed atop that part-unwound stack. \
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* If we round the stack down to the page start \
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* We're at the start of "struct lguest_pages". */ \
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movl %esp, %eax; \
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andl $(~(1 << PAGE_SHIFT - 1)), %eax; \
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/* Save our trap number: the switch will obscure it \
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* (In the Host the Guest regs are not mapped here) \
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* %ebx holds it safe for deliver_to_host */ \
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movl LGUEST_PAGES_regs_trapnum(%eax), %ebx; \
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/* The Host GDT, IDT and stack! \
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* All these lie safely hidden from the Guest: \
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* We must return to the Host page tables \
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* (Hence that was saved in struct lguest_pages) */ \
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movl LGUEST_PAGES_host_cr3(%eax), %edx; \
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movl %edx, %cr3; \
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/* As before, when we looked back at the Host \
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* As we left and marked TSS unused \
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* So must we now for the Guest left behind. */ \
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andb $0xFD, (LGUEST_PAGES_guest_gdt+GDT_ENTRY_TSS*8+5)(%eax); \
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/* Switch to Host's GDT, IDT. */ \
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lgdt LGUEST_PAGES_host_gdt_desc(%eax); \
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lidt LGUEST_PAGES_host_idt_desc(%eax); \
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/* Restore the Host's stack where its saved regs lie */ \
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movl LGUEST_PAGES_host_sp(%eax), %esp; \
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/* Last the TSS: our Host is returned */ \
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movl $(GDT_ENTRY_TSS*8), %edx; \
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ltr %dx; \
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/* Restore now the regs saved right at the first. */ \
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popl %ebp; \
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popl %fs; \
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popl %gs; \
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popl %ds; \
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popl %es
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// The first path is trod when the Guest has trapped:
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// (Which trap it was has been pushed on the stack).
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// We need only switch back, and the Host will decode
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// Why we came home, and what needs to be done.
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return_to_host:
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SWITCH_TO_HOST
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iret
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// We are lead to the second path like so:
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// An interrupt, with some cause external
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// Has ajerked us rudely from the Guest's code
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// Again we must return home to the Host
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deliver_to_host:
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SWITCH_TO_HOST
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// But now we must go home via that place
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// Where that interrupt was supposed to go
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// Had we not been ensconced, running the Guest.
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// Here we see the trickness of run_guest_once():
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// The Host stack is formed like an interrupt
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// With EIP, CS and EFLAGS layered.
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// Interrupt handlers end with "iret"
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// And that will take us home at long long last.
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// But first we must find the handler to call!
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// The IDT descriptor for the Host
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// Has two bytes for size, and four for address:
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// %edx will hold it for us for now.
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movl (LGUEST_PAGES_host_idt_desc+2)(%eax), %edx
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// We now know the table address we need,
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// And saved the trap's number inside %ebx.
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// Yet the pointer to the handler is smeared
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// Across the bits of the table entry.
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// What oracle can tell us how to extract
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// From such a convoluted encoding?
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// I consulted gcc, and it gave
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// These instructions, which I gladly credit:
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leal (%edx,%ebx,8), %eax
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movzwl (%eax),%edx
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movl 4(%eax), %eax
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xorw %ax, %ax
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orl %eax, %edx
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// Now the address of the handler's in %edx
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// We call it now: its "iret" drops us home.
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jmp *%edx
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// Every interrupt can come to us here
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// But we must truly tell each apart.
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// They number two hundred and fifty six
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// And each must land in a different spot,
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// Push its number on stack, and join the stream.
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// And worse, a mere six of the traps stand apart
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// And push on their stack an addition:
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// An error number, thirty two bits long
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// So we punish the other two fifty
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// And make them push a zero so they match.
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// Yet two fifty six entries is long
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// And all will look most the same as the last
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// So we create a macro which can make
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// As many entries as we need to fill.
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// Note the change to .data then .text:
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// We plant the address of each entry
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// Into a (data) table for the Host
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// To know where each Guest interrupt should go.
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.macro IRQ_STUB N TARGET
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.data; .long 1f; .text; 1:
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// Trap eight, ten through fourteen and seventeen
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// Supply an error number. Else zero.
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.if (\N <> 8) && (\N < 10 || \N > 14) && (\N <> 17)
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pushl $0
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.endif
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pushl $\N
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jmp \TARGET
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ALIGN
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.endm
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// This macro creates numerous entries
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// Using GAS macros which out-power C's.
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.macro IRQ_STUBS FIRST LAST TARGET
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irq=\FIRST
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.rept \LAST-\FIRST+1
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IRQ_STUB irq \TARGET
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irq=irq+1
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.endr
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.endm
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// Here's the marker for our pointer table
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// Laid in the data section just before
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// Each macro places the address of code
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// Forming an array: each one points to text
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// Which handles interrupt in its turn.
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.data
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.global default_idt_entries
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default_idt_entries:
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.text
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// The first two traps go straight back to the Host
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IRQ_STUBS 0 1 return_to_host
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// We'll say nothing, yet, about NMI
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IRQ_STUB 2 handle_nmi
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// Other traps also return to the Host
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IRQ_STUBS 3 31 return_to_host
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// All interrupts go via their handlers
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IRQ_STUBS 32 127 deliver_to_host
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// 'Cept system calls coming from userspace
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// Are to go to the Guest, never the Host.
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IRQ_STUB 128 return_to_host
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IRQ_STUBS 129 255 deliver_to_host
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// The NMI, what a fabulous beast
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// Which swoops in and stops us no matter that
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// We're suspended between heaven and hell,
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// (Or more likely between the Host and Guest)
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// When in it comes! We are dazed and confused
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// So we do the simplest thing which one can.
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// Though we've pushed the trap number and zero
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// We discard them, return, and hope we live.
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handle_nmi:
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addl $8, %esp
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iret
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// We are done; all that's left is Mastery
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// And "make Mastery" is a journey long
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// Designed to make your fingers itch to code.
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// Here ends the text, the file and poem.
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ENTRY(end_switcher_text)
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