aboutsummaryrefslogtreecommitdiff
path: root/drivers/lguest/x86/core.c
blob: e989b8358864289db0d3c0e71818bacb6ce7d7d3 (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
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
/*
 * Copyright (C) 2006, Rusty Russell <rusty@rustcorp.com.au> IBM Corporation.
 * Copyright (C) 2007, Jes Sorensen <jes@sgi.com> SGI.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
 * NON INFRINGEMENT.  See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */
#include <linux/kernel.h>
#include <linux/start_kernel.h>
#include <linux/string.h>
#include <linux/console.h>
#include <linux/screen_info.h>
#include <linux/irq.h>
#include <linux/interrupt.h>
#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/cpu.h>
#include <linux/lguest.h>
#include <linux/lguest_launcher.h>
#include <asm/paravirt.h>
#include <asm/param.h>
#include <asm/page.h>
#include <asm/pgtable.h>
#include <asm/desc.h>
#include <asm/setup.h>
#include <asm/lguest.h>
#include <asm/uaccess.h>
#include <asm/i387.h>
#include "../lg.h"

static int cpu_had_pge;

static struct {
	unsigned long offset;
	unsigned short segment;
} lguest_entry;

/* Offset from where switcher.S was compiled to where we've copied it */
static unsigned long switcher_offset(void)
{
	return SWITCHER_ADDR - (unsigned long)start_switcher_text;
}

/* This cpu's struct lguest_pages. */
static struct lguest_pages *lguest_pages(unsigned int cpu)
{
	return &(((struct lguest_pages *)
		  (SWITCHER_ADDR + SHARED_SWITCHER_PAGES*PAGE_SIZE))[cpu]);
}

static DEFINE_PER_CPU(struct lguest *, last_guest);

/*S:010
 * We approach the Switcher.
 *
 * Remember that each CPU has two pages which are visible to the Guest when it
 * runs on that CPU.  This has to contain the state for that Guest: we copy the
 * state in just before we run the Guest.
 *
 * Each Guest has "changed" flags which indicate what has changed in the Guest
 * since it last ran.  We saw this set in interrupts_and_traps.c and
 * segments.c.
 */
static void copy_in_guest_info(struct lg_cpu *cpu, struct lguest_pages *pages)
{
	struct lguest *lg = cpu->lg;
	/* Copying all this data can be quite expensive.  We usually run the
	 * same Guest we ran last time (and that Guest hasn't run anywhere else
	 * meanwhile).  If that's not the case, we pretend everything in the
	 * Guest has changed. */
	if (__get_cpu_var(last_guest) != lg || lg->last_pages != pages) {
		__get_cpu_var(last_guest) = lg;
		lg->last_pages = pages;
		lg->changed = CHANGED_ALL;
	}

	/* These copies are pretty cheap, so we do them unconditionally: */
	/* Save the current Host top-level page directory. */
	pages->state.host_cr3 = __pa(current->mm->pgd);
	/* Set up the Guest's page tables to see this CPU's pages (and no
	 * other CPU's pages). */
	map_switcher_in_guest(cpu, pages);
	/* Set up the two "TSS" members which tell the CPU what stack to use
	 * for traps which do directly into the Guest (ie. traps at privilege
	 * level 1). */
	pages->state.guest_tss.esp1 = lg->esp1;
	pages->state.guest_tss.ss1 = lg->ss1;

	/* Copy direct-to-Guest trap entries. */
	if (lg->changed & CHANGED_IDT)
		copy_traps(cpu, pages->state.guest_idt, default_idt_entries);

	/* Copy all GDT entries which the Guest can change. */
	if (lg->changed & CHANGED_GDT)
		copy_gdt(cpu, pages->state.guest_gdt);
	/* If only the TLS entries have changed, copy them. */
	else if (lg->changed & CHANGED_GDT_TLS)
		copy_gdt_tls(cpu, pages->state.guest_gdt);

	/* Mark the Guest as unchanged for next time. */
	lg->changed = 0;
}

/* Finally: the code to actually call into the Switcher to run the Guest. */
static void run_guest_once(struct lg_cpu *cpu, struct lguest_pages *pages)
{
	/* This is a dummy value we need for GCC's sake. */
	unsigned int clobber;
	struct lguest *lg = cpu->lg;

	/* Copy the guest-specific information into this CPU's "struct
	 * lguest_pages". */
	copy_in_guest_info(cpu, pages);

	/* Set the trap number to 256 (impossible value).  If we fault while
	 * switching to the Guest (bad segment registers or bug), this will
	 * cause us to abort the Guest. */
	cpu->regs->trapnum = 256;

	/* Now: we push the "eflags" register on the stack, then do an "lcall".
	 * This is how we change from using the kernel code segment to using
	 * the dedicated lguest code segment, as well as jumping into the
	 * Switcher.
	 *
	 * The lcall also pushes the old code segment (KERNEL_CS) onto the
	 * stack, then the address of this call.  This stack layout happens to
	 * exactly match the stack layout created by an interrupt... */
	asm volatile("pushf; lcall *lguest_entry"
		     /* This is how we tell GCC that %eax ("a") and %ebx ("b")
		      * are changed by this routine.  The "=" means output. */
		     : "=a"(clobber), "=b"(clobber)
		     /* %eax contains the pages pointer.  ("0" refers to the
		      * 0-th argument above, ie "a").  %ebx contains the
		      * physical address of the Guest's top-level page
		      * directory. */
		     : "0"(pages), "1"(__pa(lg->pgdirs[lg->pgdidx].pgdir))
		     /* We tell gcc that all these registers could change,
		      * which means we don't have to save and restore them in
		      * the Switcher. */
		     : "memory", "%edx", "%ecx", "%edi", "%esi");
}
/*:*/

/*M:002 There are hooks in the scheduler which we can register to tell when we
 * get kicked off the CPU (preempt_notifier_register()).  This would allow us
 * to lazily disable SYSENTER which would regain some performance, and should
 * also simplify copy_in_guest_info().  Note that we'd still need to restore
 * things when we exit to Launcher userspace, but that's fairly easy.
 *
 * The hooks were designed for KVM, but we can also put them to good use. :*/

/*H:040 This is the i386-specific code to setup and run the Guest.  Interrupts
 * are disabled: we own the CPU. */
void lguest_arch_run_guest(struct lg_cpu *cpu)
{
	struct lguest *lg = cpu->lg;

	/* Remember the awfully-named TS bit?  If the Guest has asked to set it
	 * we set it now, so we can trap and pass that trap to the Guest if it
	 * uses the FPU. */
	if (lg->ts)
		lguest_set_ts();

	/* SYSENTER is an optimized way of doing system calls.  We can't allow
	 * it because it always jumps to privilege level 0.  A normal Guest
	 * won't try it because we don't advertise it in CPUID, but a malicious
	 * Guest (or malicious Guest userspace program) could, so we tell the
	 * CPU to disable it before running the Guest. */
	if (boot_cpu_has(X86_FEATURE_SEP))
		wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);

	/* Now we actually run the Guest.  It will return when something
	 * interesting happens, and we can examine its registers to see what it
	 * was doing. */
	run_guest_once(cpu, lguest_pages(raw_smp_processor_id()));

	/* Note that the "regs" pointer contains two extra entries which are
	 * not really registers: a trap number which says what interrupt or
	 * trap made the switcher code come back, and an error code which some
	 * traps set.  */

	/* If the Guest page faulted, then the cr2 register will tell us the
	 * bad virtual address.  We have to grab this now, because once we
	 * re-enable interrupts an interrupt could fault and thus overwrite
	 * cr2, or we could even move off to a different CPU. */
	if (cpu->regs->trapnum == 14)
		cpu->arch.last_pagefault = read_cr2();
	/* Similarly, if we took a trap because the Guest used the FPU,
	 * we have to restore the FPU it expects to see. */
	else if (cpu->regs->trapnum == 7)
		math_state_restore();

	/* Restore SYSENTER if it's supposed to be on. */
	if (boot_cpu_has(X86_FEATURE_SEP))
		wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
}

/*H:130 Now we've examined the hypercall code; our Guest can make requests.
 * Our Guest is usually so well behaved; it never tries to do things it isn't
 * allowed to, and uses hypercalls instead.  Unfortunately, Linux's paravirtual
 * infrastructure isn't quite complete, because it doesn't contain replacements
 * for the Intel I/O instructions.  As a result, the Guest sometimes fumbles
 * across one during the boot process as it probes for various things which are
 * usually attached to a PC.
 *
 * When the Guest uses one of these instructions, we get a trap (General
 * Protection Fault) and come here.  We see if it's one of those troublesome
 * instructions and skip over it.  We return true if we did. */
static int emulate_insn(struct lg_cpu *cpu)
{
	struct lguest *lg = cpu->lg;
	u8 insn;
	unsigned int insnlen = 0, in = 0, shift = 0;
	/* The eip contains the *virtual* address of the Guest's instruction:
	 * guest_pa just subtracts the Guest's page_offset. */
	unsigned long physaddr = guest_pa(lg, cpu->regs->eip);

	/* This must be the Guest kernel trying to do something, not userspace!
	 * The bottom two bits of the CS segment register are the privilege
	 * level. */
	if ((cpu->regs->cs & 3) != GUEST_PL)
		return 0;

	/* Decoding x86 instructions is icky. */
	insn = lgread(lg, physaddr, u8);

	/* 0x66 is an "operand prefix".  It means it's using the upper 16 bits
	   of the eax register. */
	if (insn == 0x66) {
		shift = 16;
		/* The instruction is 1 byte so far, read the next byte. */
		insnlen = 1;
		insn = lgread(lg, physaddr + insnlen, u8);
	}

	/* We can ignore the lower bit for the moment and decode the 4 opcodes
	 * we need to emulate. */
	switch (insn & 0xFE) {
	case 0xE4: /* in     <next byte>,%al */
		insnlen += 2;
		in = 1;
		break;
	case 0xEC: /* in     (%dx),%al */
		insnlen += 1;
		in = 1;
		break;
	case 0xE6: /* out    %al,<next byte> */
		insnlen += 2;
		break;
	case 0xEE: /* out    %al,(%dx) */
		insnlen += 1;
		break;
	default:
		/* OK, we don't know what this is, can't emulate. */
		return 0;
	}

	/* If it was an "IN" instruction, they expect the result to be read
	 * into %eax, so we change %eax.  We always return all-ones, which
	 * traditionally means "there's nothing there". */
	if (in) {
		/* Lower bit tells is whether it's a 16 or 32 bit access */
		if (insn & 0x1)
			cpu->regs->eax = 0xFFFFFFFF;
		else
			cpu->regs->eax |= (0xFFFF << shift);
	}
	/* Finally, we've "done" the instruction, so move past it. */
	cpu->regs->eip += insnlen;
	/* Success! */
	return 1;
}

/*H:050 Once we've re-enabled interrupts, we look at why the Guest exited. */
void lguest_arch_handle_trap(struct lg_cpu *cpu)
{
	struct lguest *lg = cpu->lg;
	switch (cpu->regs->trapnum) {
	case 13: /* We've intercepted a General Protection Fault. */
		/* Check if this was one of those annoying IN or OUT
		 * instructions which we need to emulate.  If so, we just go
		 * back into the Guest after we've done it. */
		if (cpu->regs->errcode == 0) {
			if (emulate_insn(cpu))
				return;
		}
		break;
	case 14: /* We've intercepted a Page Fault. */
		/* The Guest accessed a virtual address that wasn't mapped.
		 * This happens a lot: we don't actually set up most of the
		 * page tables for the Guest at all when we start: as it runs
		 * it asks for more and more, and we set them up as
		 * required. In this case, we don't even tell the Guest that
		 * the fault happened.
		 *
		 * The errcode tells whether this was a read or a write, and
		 * whether kernel or userspace code. */
		if (demand_page(lg,cpu->arch.last_pagefault,cpu->regs->errcode))
			return;

		/* OK, it's really not there (or not OK): the Guest needs to
		 * know.  We write out the cr2 value so it knows where the
		 * fault occurred.
		 *
		 * Note that if the Guest were really messed up, this could
		 * happen before it's done the LHCALL_LGUEST_INIT hypercall, so
		 * lg->lguest_data could be NULL */
		if (lg->lguest_data &&
		    put_user(cpu->arch.last_pagefault, &lg->lguest_data->cr2))
			kill_guest(lg, "Writing cr2");
		break;
	case 7: /* We've intercepted a Device Not Available fault. */
		/* If the Guest doesn't want to know, we already restored the
		 * Floating Point Unit, so we just continue without telling
		 * it. */
		if (!lg->ts)
			return;
		break;
	case 32 ... 255:
		/* These values mean a real interrupt occurred, in which case
		 * the Host handler has already been run.  We just do a
		 * friendly check if another process should now be run, then
		 * return to run the Guest again */
		cond_resched();
		return;
	case LGUEST_TRAP_ENTRY:
		/* Our 'struct hcall_args' maps directly over our regs: we set
		 * up the pointer now to indicate a hypercall is pending. */
		cpu->hcall = (struct hcall_args *)cpu->regs;
		return;
	}

	/* We didn't handle the trap, so it needs to go to the Guest. */
	if (!deliver_trap(cpu, cpu->regs->trapnum))
		/* If the Guest doesn't have a handler (either it hasn't
		 * registered any yet, or it's one of the faults we don't let
		 * it handle), it dies with a cryptic error message. */
		kill_guest(lg, "unhandled trap %li at %#lx (%#lx)",
			   cpu->regs->trapnum, cpu->regs->eip,
			   cpu->regs->trapnum == 14 ? cpu->arch.last_pagefault
			   : cpu->regs->errcode);
}

/* Now we can look at each of the routines this calls, in increasing order of
 * complexity: do_hypercalls(), emulate_insn(), maybe_do_interrupt(),
 * deliver_trap() and demand_page().  After all those, we'll be ready to
 * examine the Switcher, and our philosophical understanding of the Host/Guest
 * duality will be complete. :*/
static void adjust_pge(void *on)
{
	if (on)
		write_cr4(read_cr4() | X86_CR4_PGE);
	else
		write_cr4(read_cr4() & ~X86_CR4_PGE);
}

/*H:020 Now the Switcher is mapped and every thing else is ready, we need to do
 * some more i386-specific initialization. */
void __init lguest_arch_host_init(void)
{
	int i;

	/* Most of the i386/switcher.S doesn't care that it's been moved; on
	 * Intel, jumps are relative, and it doesn't access any references to
	 * external code or data.
	 *
	 * The only exception is the interrupt handlers in switcher.S: their
	 * addresses are placed in a table (default_idt_entries), so we need to
	 * update the table with the new addresses.  switcher_offset() is a
	 * convenience function which returns the distance between the builtin
	 * switcher code and the high-mapped copy we just made. */
	for (i = 0; i < IDT_ENTRIES; i++)
		default_idt_entries[i] += switcher_offset();

	/*
	 * Set up the Switcher's per-cpu areas.
	 *
	 * Each CPU gets two pages of its own within the high-mapped region
	 * (aka. "struct lguest_pages").  Much of this can be initialized now,
	 * but some depends on what Guest we are running (which is set up in
	 * copy_in_guest_info()).
	 */
	for_each_possible_cpu(i) {
		/* lguest_pages() returns this CPU's two pages. */
		struct lguest_pages *pages = lguest_pages(i);
		/* This is a convenience pointer to make the code fit one
		 * statement to a line. */
		struct lguest_ro_state *state = &pages->state;

		/* The Global Descriptor Table: the Host has a different one
		 * for each CPU.  We keep a descriptor for the GDT which says
		 * where it is and how big it is (the size is actually the last
		 * byte, not the size, hence the "-1"). */
		state->host_gdt_desc.size = GDT_SIZE-1;
		state->host_gdt_desc.address = (long)get_cpu_gdt_table(i);

		/* All CPUs on the Host use the same Interrupt Descriptor
		 * Table, so we just use store_idt(), which gets this CPU's IDT
		 * descriptor. */
		store_idt(&state->host_idt_desc);

		/* The descriptors for the Guest's GDT and IDT can be filled
		 * out now, too.  We copy the GDT & IDT into ->guest_gdt and
		 * ->guest_idt before actually running the Guest. */
		state->guest_idt_desc.size = sizeof(state->guest_idt)-1;
		state->guest_idt_desc.address = (long)&state->guest_idt;
		state->guest_gdt_desc.size = sizeof(state->guest_gdt)-1;
		state->guest_gdt_desc.address = (long)&state->guest_gdt;

		/* We know where we want the stack to be when the Guest enters
		 * the switcher: in pages->regs.  The stack grows upwards, so
		 * we start it at the end of that structure. */
		state->guest_tss.esp0 = (long)(&pages->regs + 1);
		/* And this is the GDT entry to use for the stack: we keep a
		 * couple of special LGUEST entries. */
		state->guest_tss.ss0 = LGUEST_DS;

		/* x86 can have a finegrained bitmap which indicates what I/O
		 * ports the process can use.  We set it to the end of our
		 * structure, meaning "none". */
		state->guest_tss.io_bitmap_base = sizeof(state->guest_tss);

		/* Some GDT entries are the same across all Guests, so we can
		 * set them up now. */
		setup_default_gdt_entries(state);
		/* Most IDT entries are the same for all Guests, too.*/
		setup_default_idt_entries(state, default_idt_entries);

		/* The Host needs to be able to use the LGUEST segments on this
		 * CPU, too, so put them in the Host GDT. */
		get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
		get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
	}

	/* In the Switcher, we want the %cs segment register to use the
	 * LGUEST_CS GDT entry: we've put that in the Host and Guest GDTs, so
	 * it will be undisturbed when we switch.  To change %cs and jump we
	 * need this structure to feed to Intel's "lcall" instruction. */
	lguest_entry.offset = (long)switch_to_guest + switcher_offset();
	lguest_entry.segment = LGUEST_CS;

	/* Finally, we need to turn off "Page Global Enable".  PGE is an
	 * optimization where page table entries are specially marked to show
	 * they never change.  The Host kernel marks all the kernel pages this
	 * way because it's always present, even when userspace is running.
	 *
	 * Lguest breaks this: unbeknownst to the rest of the Host kernel, we
	 * switch to the Guest kernel.  If you don't disable this on all CPUs,
	 * you'll get really weird bugs that you'll chase for two days.
	 *
	 * I used to turn PGE off every time we switched to the Guest and back
	 * on when we return, but that slowed the Switcher down noticibly. */

	/* We don't need the complexity of CPUs coming and going while we're
	 * doing this. */
	get_online_cpus();
	if (cpu_has_pge) { /* We have a broader idea of "global". */
		/* Remember that this was originally set (for cleanup). */
		cpu_had_pge = 1;
		/* adjust_pge is a helper function which sets or unsets the PGE
		 * bit on its CPU, depending on the argument (0 == unset). */
		on_each_cpu(adjust_pge, (void *)0, 0, 1);
		/* Turn off the feature in the global feature set. */
		clear_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
	}
	put_online_cpus();
};
/*:*/

void __exit lguest_arch_host_fini(void)
{
	/* If we had PGE before we started, turn it back on now. */
	get_online_cpus();
	if (cpu_had_pge) {
		set_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
		/* adjust_pge's argument "1" means set PGE. */
		on_each_cpu(adjust_pge, (void *)1, 0, 1);
	}
	put_online_cpus();
}


/*H:122 The i386-specific hypercalls simply farm out to the right functions. */
int lguest_arch_do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
{
	switch (args->arg0) {
	case LHCALL_LOAD_GDT:
		load_guest_gdt(cpu, args->arg1, args->arg2);
		break;
	case LHCALL_LOAD_IDT_ENTRY:
		load_guest_idt_entry(cpu, args->arg1, args->arg2, args->arg3);
		break;
	case LHCALL_LOAD_TLS:
		guest_load_tls(cpu, args->arg1);
		break;
	default:
		/* Bad Guest.  Bad! */
		return -EIO;
	}
	return 0;
}

/*H:126 i386-specific hypercall initialization: */
int lguest_arch_init_hypercalls(struct lg_cpu *cpu)
{
	u32 tsc_speed;
	struct lguest *lg = cpu->lg;

	/* The pointer to the Guest's "struct lguest_data" is the only
	 * argument.  We check that address now. */
	if (!lguest_address_ok(lg, cpu->hcall->arg1, sizeof(*lg->lguest_data)))
		return -EFAULT;

	/* Having checked it, we simply set lg->lguest_data to point straight
	 * into the Launcher's memory at the right place and then use
	 * copy_to_user/from_user from now on, instead of lgread/write.  I put
	 * this in to show that I'm not immune to writing stupid
	 * optimizations. */
	lg->lguest_data = lg->mem_base + cpu->hcall->arg1;

	/* We insist that the Time Stamp Counter exist and doesn't change with
	 * cpu frequency.  Some devious chip manufacturers decided that TSC
	 * changes could be handled in software.  I decided that time going
	 * backwards might be good for benchmarks, but it's bad for users.
	 *
	 * We also insist that the TSC be stable: the kernel detects unreliable
	 * TSCs for its own purposes, and we use that here. */
	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && !check_tsc_unstable())
		tsc_speed = tsc_khz;
	else
		tsc_speed = 0;
	if (put_user(tsc_speed, &lg->lguest_data->tsc_khz))
		return -EFAULT;

	/* The interrupt code might not like the system call vector. */
	if (!check_syscall_vector(lg))
		kill_guest(lg, "bad syscall vector");

	return 0;
}

/*L:030 lguest_arch_setup_regs()
 *
 * Most of the Guest's registers are left alone: we used get_zeroed_page() to
 * allocate the structure, so they will be 0. */
void lguest_arch_setup_regs(struct lg_cpu *cpu, unsigned long start)
{
	struct lguest_regs *regs = cpu->regs;

	/* There are four "segment" registers which the Guest needs to boot:
	 * The "code segment" register (cs) refers to the kernel code segment
	 * __KERNEL_CS, and the "data", "extra" and "stack" segment registers
	 * refer to the kernel data segment __KERNEL_DS.
	 *
	 * The privilege level is packed into the lower bits.  The Guest runs
	 * at privilege level 1 (GUEST_PL).*/
	regs->ds = regs->es = regs->ss = __KERNEL_DS|GUEST_PL;
	regs->cs = __KERNEL_CS|GUEST_PL;

	/* The "eflags" register contains miscellaneous flags.  Bit 1 (0x002)
	 * is supposed to always be "1".  Bit 9 (0x200) controls whether
	 * interrupts are enabled.  We always leave interrupts enabled while
	 * running the Guest. */
	regs->eflags = X86_EFLAGS_IF | 0x2;

	/* The "Extended Instruction Pointer" register says where the Guest is
	 * running. */
	regs->eip = start;

	/* %esi points to our boot information, at physical address 0, so don't
	 * touch it. */

	/* There are a couple of GDT entries the Guest expects when first
	 * booting. */
	setup_guest_gdt(cpu);
}