aboutsummaryrefslogtreecommitdiff
path: root/drivers/lguest/x86/switcher_32.S
blob: 6dec097938360ab8c063db74d79caf94b488da90 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
/*P:900
 * This is the Switcher: code which sits at 0xFFC00000 astride both the
 * Host and Guest 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".
 :*/

/*M:012
 * Lguest is meant to be simple: my rule of thumb is that 1% more LOC must
 * gain at least 1% more performance.  Since neither LOC nor performance can be
 * measured beforehand, it generally means implementing a feature then deciding
 * if it's worth it.  And once it's implemented, who can say no?
 *
 * This is why I haven't implemented this idea myself.  I want to, but I
 * haven't.  You could, though.
 *
 * The main place where lguest performance sucks is Guest page faulting.  When
 * a Guest userspace process hits an unmapped page we switch back to the Host,
 * walk the page tables, find it's not mapped, switch back to the Guest page
 * fault handler, which calls a hypercall to set the page table entry, then
 * finally returns to userspace.  That's two round-trips.
 *
 * If we had a small walker in the Switcher, we could quickly check the Guest
 * page table and if the page isn't mapped, immediately reflect the fault back
 * into the Guest.  This means the Switcher would have to know the top of the
 * Guest page table and the page fault handler address.
 *
 * For simplicity, the Guest should only handle the case where the privilege
 * level of the fault is 3 and probably only not present or write faults.  It
 * should also detect recursive faults, and hand the original fault to the
 * Host (which is actually really easy).
 *
 * Two questions remain.  Would the performance gain outweigh the complexity?
 * And who would write the verse documenting it?
:*/

/*M:011
 * Lguest64 handles NMI.  This gave me NMI envy (until I looked at their
 * code).  It's worth doing though, since it would let us use oprofile in the
 * Host when a Guest is 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 <asm/page.h>
#include <asm/segment.h>
#include <asm/lguest.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
	// Then 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
	// Copy to "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:
	// The GDT entry that TSS uses 
	// Changes type when we load it: damn Intel!
	// For after we switch over our page tables
	// That entry will be read-only: we'd crash.
	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 for our 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 "used" from type field (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 under our %esp (stack) --
	// We can simply pop off all Guest regs.
	popl	%eax
	popl	%ebx
	popl	%ecx
	popl	%edx
	popl	%esi
	popl	%edi
	popl	%ebp
	popl	%gs
	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 switch privilege
	// From Switcher's level 0 to Guest's 1,
	// And the stack where the Guest had last left it.
	// Interrupts are turned back on: we are Guest.
	iret

// We tread two paths to switch back to the Host
// Yet both must save Guest state and restore Host
// So we put the routine in a macro.
#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	%gs;							\
	pushl	%ebp;							\
	pushl	%edi;							\
	pushl	%esi;							\
	pushl	%edx;							\
	pushl	%ecx;							\
	pushl	%ebx;							\
	pushl	%eax;							\
	/* 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		\
	 * (In the Host the Guest regs are not mapped here)		\
	 * %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 its saved regs lie */		\
	movl	LGUEST_PAGES_host_sp(%eax), %esp;			\
	/* Last the TSS: our Host is returned */			\
	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

// The first path is trod when the Guest has trapped:
// (Which trap it was 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

// We are lead to the second path like so:
// 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 trickness of run_guest_once():
	// 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" drops 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)