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Rusty Russellf938d2c2007-07-26 10:41:02 -07001/*P:900 This is the Switcher: code which sits at 0xFFC00000 to do the low-level
2 * Guest<->Host switch. It is as simple as it can be made, but it's naturally
3 * very specific to x86.
4 *
5 * You have now completed Preparation. If this has whet your appetite; if you
6 * are feeling invigorated and refreshed then the next, more challenging stage
7 * can be found in "make Guest". :*/
Rusty Russelld7e28ff2007-07-19 01:49:23 -07008
Rusty Russellf8f0fdc2007-07-26 10:41:04 -07009/*S:100
10 * Welcome to the Switcher itself!
11 *
12 * This file contains the low-level code which changes the CPU to run the Guest
13 * code, and returns to the Host when something happens. Understand this, and
14 * you understand the heart of our journey.
15 *
16 * Because this is in assembler rather than C, our tale switches from prose to
17 * verse. First I tried limericks:
18 *
19 * There once was an eax reg,
20 * To which our pointer was fed,
21 * It needed an add,
22 * Which asm-offsets.h had
23 * But this limerick is hurting my head.
24 *
25 * Next I tried haikus, but fitting the required reference to the seasons in
26 * every stanza was quickly becoming tiresome:
27 *
28 * The %eax reg
29 * Holds "struct lguest_pages" now:
30 * Cherry blossoms fall.
31 *
32 * Then I started with Heroic Verse, but the rhyming requirement leeched away
33 * the content density and led to some uniquely awful oblique rhymes:
34 *
35 * These constants are coming from struct offsets
36 * For use within the asm switcher text.
37 *
38 * Finally, I settled for something between heroic hexameter, and normal prose
39 * with inappropriate linebreaks. Anyway, it aint no Shakespeare.
40 */
41
42// Not all kernel headers work from assembler
43// But these ones are needed: the ENTRY() define
44// And constants extracted from struct offsets
45// To avoid magic numbers and breakage:
46// Should they change the compiler can't save us
47// Down here in the depths of assembler code.
Rusty Russelld7e28ff2007-07-19 01:49:23 -070048#include <linux/linkage.h>
49#include <asm/asm-offsets.h>
Rusty Russell0d027c02007-08-09 20:57:13 +100050#include <asm/page.h>
Jes Sorensen625efab2007-10-22 11:03:28 +100051#include <asm/segment.h>
52#include <asm/lguest.h>
Rusty Russelld7e28ff2007-07-19 01:49:23 -070053
Rusty Russellf8f0fdc2007-07-26 10:41:04 -070054// We mark the start of the code to copy
55// It's placed in .text tho it's never run here
56// You'll see the trick macro at the end
57// Which interleaves data and text to effect.
Rusty Russelld7e28ff2007-07-19 01:49:23 -070058.text
59ENTRY(start_switcher_text)
60
Rusty Russellf8f0fdc2007-07-26 10:41:04 -070061// When we reach switch_to_guest we have just left
62// The safe and comforting shores of C code
63// %eax has the "struct lguest_pages" to use
64// Where we save state and still see it from the Guest
65// And %ebx holds the Guest shadow pagetable:
66// Once set we have truly left Host behind.
Rusty Russelld7e28ff2007-07-19 01:49:23 -070067ENTRY(switch_to_guest)
Rusty Russellf8f0fdc2007-07-26 10:41:04 -070068 // We told gcc all its regs could fade,
69 // Clobbered by our journey into the Guest
70 // We could have saved them, if we tried
71 // But time is our master and cycles count.
72
73 // Segment registers must be saved for the Host
74 // We push them on the Host stack for later
Rusty Russelld7e28ff2007-07-19 01:49:23 -070075 pushl %es
76 pushl %ds
77 pushl %gs
78 pushl %fs
Rusty Russellf8f0fdc2007-07-26 10:41:04 -070079 // But the compiler is fickle, and heeds
80 // No warning of %ebp clobbers
81 // When frame pointers are used. That register
82 // Must be saved and restored or chaos strikes.
Rusty Russelld7e28ff2007-07-19 01:49:23 -070083 pushl %ebp
Rusty Russellf8f0fdc2007-07-26 10:41:04 -070084 // The Host's stack is done, now save it away
85 // In our "struct lguest_pages" at offset
86 // Distilled into asm-offsets.h
Rusty Russelld7e28ff2007-07-19 01:49:23 -070087 movl %esp, LGUEST_PAGES_host_sp(%eax)
Rusty Russellf8f0fdc2007-07-26 10:41:04 -070088
89 // All saved and there's now five steps before us:
90 // Stack, GDT, IDT, TSS
91 // And last of all the page tables are flipped.
92
93 // Yet beware that our stack pointer must be
94 // Always valid lest an NMI hits
95 // %edx does the duty here as we juggle
96 // %eax is lguest_pages: our stack lies within.
Rusty Russelld7e28ff2007-07-19 01:49:23 -070097 movl %eax, %edx
98 addl $LGUEST_PAGES_regs, %edx
99 movl %edx, %esp
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700100
101 // The Guest's GDT we so carefully
102 // Placed in the "struct lguest_pages" before
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700103 lgdt LGUEST_PAGES_guest_gdt_desc(%eax)
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700104
105 // The Guest's IDT we did partially
106 // Move to the "struct lguest_pages" as well.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700107 lidt LGUEST_PAGES_guest_idt_desc(%eax)
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700108
109 // The TSS entry which controls traps
110 // Must be loaded up with "ltr" now:
111 // For after we switch over our page tables
112 // It (as the rest) will be writable no more.
113 // (The GDT entry TSS needs
114 // Changes type when we load it: damn Intel!)
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700115 movl $(GDT_ENTRY_TSS*8), %edx
116 ltr %dx
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700117
118 // Look back now, before we take this last step!
119 // The Host's TSS entry was also marked used;
120 // Let's clear it again, ere we return.
121 // The GDT descriptor of the Host
122 // Points to the table after two "size" bytes
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700123 movl (LGUEST_PAGES_host_gdt_desc+2)(%eax), %edx
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700124 // Clear the type field of "used" (byte 5, bit 2)
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700125 andb $0xFD, (GDT_ENTRY_TSS*8 + 5)(%edx)
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700126
127 // Once our page table's switched, the Guest is live!
128 // The Host fades as we run this final step.
129 // Our "struct lguest_pages" is now read-only.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700130 movl %ebx, %cr3
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700131
132 // The page table change did one tricky thing:
133 // The Guest's register page has been mapped
134 // Writable onto our %esp (stack) --
135 // We can simply pop off all Guest regs.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700136 popl %ebx
137 popl %ecx
138 popl %edx
139 popl %esi
140 popl %edi
141 popl %ebp
142 popl %gs
143 popl %eax
144 popl %fs
145 popl %ds
146 popl %es
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700147
148 // Near the base of the stack lurk two strange fields
149 // Which we fill as we exit the Guest
150 // These are the trap number and its error
151 // We can simply step past them on our way.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700152 addl $8, %esp
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700153
154 // The last five stack slots hold return address
155 // And everything needed to change privilege
156 // Into the Guest privilege level of 1,
157 // And the stack where the Guest had last left it.
158 // Interrupts are turned back on: we are Guest.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700159 iret
160
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700161// There are two paths where we switch to the Host
162// So we put the routine in a macro.
163// We are on our way home, back to the Host
164// Interrupted out of the Guest, we come here.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700165#define SWITCH_TO_HOST \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700166 /* We save the Guest state: all registers first \
167 * Laid out just as "struct lguest_regs" defines */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700168 pushl %es; \
169 pushl %ds; \
170 pushl %fs; \
171 pushl %eax; \
172 pushl %gs; \
173 pushl %ebp; \
174 pushl %edi; \
175 pushl %esi; \
176 pushl %edx; \
177 pushl %ecx; \
178 pushl %ebx; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700179 /* Our stack and our code are using segments \
180 * Set in the TSS and IDT \
181 * Yet if we were to touch data we'd use \
182 * Whatever data segment the Guest had. \
183 * Load the lguest ds segment for now. */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700184 movl $(LGUEST_DS), %eax; \
185 movl %eax, %ds; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700186 /* So where are we? Which CPU, which struct? \
Rusty Russell0d027c02007-08-09 20:57:13 +1000187 * The stack is our clue: our TSS starts \
188 * It at the end of "struct lguest_pages". \
189 * Or we may have stumbled while restoring \
190 * Our Guest segment regs while in switch_to_guest, \
191 * The fault pushed atop that part-unwound stack. \
192 * If we round the stack down to the page start \
193 * We're at the start of "struct lguest_pages". */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700194 movl %esp, %eax; \
Rusty Russell0d027c02007-08-09 20:57:13 +1000195 andl $(~(1 << PAGE_SHIFT - 1)), %eax; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700196 /* Save our trap number: the switch will obscure it \
197 * (The Guest regs are not mapped here in the Host) \
198 * %ebx holds it safe for deliver_to_host */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700199 movl LGUEST_PAGES_regs_trapnum(%eax), %ebx; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700200 /* The Host GDT, IDT and stack! \
201 * All these lie safely hidden from the Guest: \
202 * We must return to the Host page tables \
203 * (Hence that was saved in struct lguest_pages) */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700204 movl LGUEST_PAGES_host_cr3(%eax), %edx; \
205 movl %edx, %cr3; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700206 /* As before, when we looked back at the Host \
207 * As we left and marked TSS unused \
208 * So must we now for the Guest left behind. */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700209 andb $0xFD, (LGUEST_PAGES_guest_gdt+GDT_ENTRY_TSS*8+5)(%eax); \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700210 /* Switch to Host's GDT, IDT. */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700211 lgdt LGUEST_PAGES_host_gdt_desc(%eax); \
212 lidt LGUEST_PAGES_host_idt_desc(%eax); \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700213 /* Restore the Host's stack where it's saved regs lie */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700214 movl LGUEST_PAGES_host_sp(%eax), %esp; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700215 /* Last the TSS: our Host is complete */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700216 movl $(GDT_ENTRY_TSS*8), %edx; \
217 ltr %dx; \
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700218 /* Restore now the regs saved right at the first. */ \
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700219 popl %ebp; \
220 popl %fs; \
221 popl %gs; \
222 popl %ds; \
223 popl %es
224
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700225// Here's where we come when the Guest has just trapped:
226// (Which trap we'll see has been pushed on the stack).
227// We need only switch back, and the Host will decode
228// Why we came home, and what needs to be done.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700229return_to_host:
230 SWITCH_TO_HOST
231 iret
232
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700233// An interrupt, with some cause external
234// Has ajerked us rudely from the Guest's code
235// Again we must return home to the Host
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700236deliver_to_host:
237 SWITCH_TO_HOST
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700238 // But now we must go home via that place
239 // Where that interrupt was supposed to go
240 // Had we not been ensconced, running the Guest.
241 // Here we see the cleverness of our stack:
242 // The Host stack is formed like an interrupt
243 // With EIP, CS and EFLAGS layered.
244 // Interrupt handlers end with "iret"
245 // And that will take us home at long long last.
246
247 // But first we must find the handler to call!
248 // The IDT descriptor for the Host
249 // Has two bytes for size, and four for address:
250 // %edx will hold it for us for now.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700251 movl (LGUEST_PAGES_host_idt_desc+2)(%eax), %edx
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700252 // We now know the table address we need,
253 // And saved the trap's number inside %ebx.
254 // Yet the pointer to the handler is smeared
255 // Across the bits of the table entry.
256 // What oracle can tell us how to extract
257 // From such a convoluted encoding?
258 // I consulted gcc, and it gave
259 // These instructions, which I gladly credit:
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700260 leal (%edx,%ebx,8), %eax
261 movzwl (%eax),%edx
262 movl 4(%eax), %eax
263 xorw %ax, %ax
264 orl %eax, %edx
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700265 // Now the address of the handler's in %edx
266 // We call it now: its "iret" takes us home.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700267 jmp *%edx
268
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700269// Every interrupt can come to us here
270// But we must truly tell each apart.
271// They number two hundred and fifty six
272// And each must land in a different spot,
273// Push its number on stack, and join the stream.
274
275// And worse, a mere six of the traps stand apart
276// And push on their stack an addition:
277// An error number, thirty two bits long
278// So we punish the other two fifty
279// And make them push a zero so they match.
280
281// Yet two fifty six entries is long
282// And all will look most the same as the last
283// So we create a macro which can make
284// As many entries as we need to fill.
285
286// Note the change to .data then .text:
287// We plant the address of each entry
288// Into a (data) table for the Host
289// To know where each Guest interrupt should go.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700290.macro IRQ_STUB N TARGET
291 .data; .long 1f; .text; 1:
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700292 // Trap eight, ten through fourteen and seventeen
293 // Supply an error number. Else zero.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700294 .if (\N <> 8) && (\N < 10 || \N > 14) && (\N <> 17)
295 pushl $0
296 .endif
297 pushl $\N
298 jmp \TARGET
299 ALIGN
300.endm
301
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700302// This macro creates numerous entries
303// Using GAS macros which out-power C's.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700304.macro IRQ_STUBS FIRST LAST TARGET
305 irq=\FIRST
306 .rept \LAST-\FIRST+1
307 IRQ_STUB irq \TARGET
308 irq=irq+1
309 .endr
310.endm
311
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700312// Here's the marker for our pointer table
313// Laid in the data section just before
314// Each macro places the address of code
315// Forming an array: each one points to text
316// Which handles interrupt in its turn.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700317.data
318.global default_idt_entries
319default_idt_entries:
320.text
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700321 // The first two traps go straight back to the Host
322 IRQ_STUBS 0 1 return_to_host
323 // We'll say nothing, yet, about NMI
324 IRQ_STUB 2 handle_nmi
325 // Other traps also return to the Host
326 IRQ_STUBS 3 31 return_to_host
327 // All interrupts go via their handlers
328 IRQ_STUBS 32 127 deliver_to_host
329 // 'Cept system calls coming from userspace
330 // Are to go to the Guest, never the Host.
331 IRQ_STUB 128 return_to_host
332 IRQ_STUBS 129 255 deliver_to_host
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700333
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700334// The NMI, what a fabulous beast
335// Which swoops in and stops us no matter that
336// We're suspended between heaven and hell,
337// (Or more likely between the Host and Guest)
338// When in it comes! We are dazed and confused
339// So we do the simplest thing which one can.
340// Though we've pushed the trap number and zero
341// We discard them, return, and hope we live.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700342handle_nmi:
343 addl $8, %esp
344 iret
345
Rusty Russellf8f0fdc2007-07-26 10:41:04 -0700346// We are done; all that's left is Mastery
347// And "make Mastery" is a journey long
348// Designed to make your fingers itch to code.
349
350// Here ends the text, the file and poem.
Rusty Russelld7e28ff2007-07-19 01:49:23 -0700351ENTRY(end_switcher_text)