1// SPDX-License-Identifier: GPL-2.0+
2/*
3 * xen console driver interface to hvc_console.c
4 *
5 * (c) 2007 Gerd Hoffmann <kraxel@suse.de>
6 */
7
8#include <linux/console.h>
9#include <linux/delay.h>
10#include <linux/err.h>
11#include <linux/irq.h>
12#include <linux/init.h>
13#include <linux/types.h>
14#include <linux/list.h>
15#include <linux/serial_core.h>
16
17#include <asm/io.h>
18#include <asm/xen/hypervisor.h>
19
20#include <xen/xen.h>
21#include <xen/interface/xen.h>
22#include <xen/hvm.h>
23#include <xen/grant_table.h>
24#include <xen/page.h>
25#include <xen/events.h>
26#include <xen/interface/io/console.h>
27#include <xen/interface/sched.h>
28#include <xen/hvc-console.h>
29#include <xen/xenbus.h>
30
31#include "hvc_console.h"
32
33#define HVC_COOKIE 0x58656e /* "Xen" in hex */
34
35struct xencons_info {
36 struct list_head list;
37 struct xenbus_device *xbdev;
38 struct xencons_interface *intf;
39 unsigned int evtchn;
40 XENCONS_RING_IDX out_cons;
41 unsigned int out_cons_same;
42 struct hvc_struct *hvc;
43 int irq;
44 int vtermno;
45 grant_ref_t gntref;
46 spinlock_t ring_lock;
47};
48
49static LIST_HEAD(xenconsoles);
50static DEFINE_SPINLOCK(xencons_lock);
51
52/* ------------------------------------------------------------------ */
53
54static struct xencons_info *vtermno_to_xencons(int vtermno)
55{
56 struct xencons_info *entry, *ret = NULL;
57 unsigned long flags;
58
59 spin_lock_irqsave(&xencons_lock, flags);
60 if (list_empty(head: &xenconsoles)) {
61 spin_unlock_irqrestore(lock: &xencons_lock, flags);
62 return NULL;
63 }
64
65 list_for_each_entry(entry, &xenconsoles, list) {
66 if (entry->vtermno == vtermno) {
67 ret = entry;
68 break;
69 }
70 }
71 spin_unlock_irqrestore(lock: &xencons_lock, flags);
72
73 return ret;
74}
75
76static inline int xenbus_devid_to_vtermno(int devid)
77{
78 return devid + HVC_COOKIE;
79}
80
81static inline void notify_daemon(struct xencons_info *cons)
82{
83 /* Use evtchn: this is called early, before irq is set up. */
84 notify_remote_via_evtchn(port: cons->evtchn);
85}
86
87static int __write_console(struct xencons_info *xencons,
88 const char *data, int len)
89{
90 XENCONS_RING_IDX cons, prod;
91 struct xencons_interface *intf = xencons->intf;
92 int sent = 0;
93 unsigned long flags;
94
95 spin_lock_irqsave(&xencons->ring_lock, flags);
96 cons = intf->out_cons;
97 prod = intf->out_prod;
98 mb(); /* update queue values before going on */
99
100 if ((prod - cons) > sizeof(intf->out)) {
101 spin_unlock_irqrestore(lock: &xencons->ring_lock, flags);
102 pr_err_once("xencons: Illegal ring page indices");
103 return -EINVAL;
104 }
105
106 while ((sent < len) && ((prod - cons) < sizeof(intf->out)))
107 intf->out[MASK_XENCONS_IDX(prod++, intf->out)] = data[sent++];
108
109 wmb(); /* write ring before updating pointer */
110 intf->out_prod = prod;
111 spin_unlock_irqrestore(lock: &xencons->ring_lock, flags);
112
113 if (sent)
114 notify_daemon(cons: xencons);
115 return sent;
116}
117
118static int domU_write_console(uint32_t vtermno, const char *data, int len)
119{
120 int ret = len;
121 struct xencons_info *cons = vtermno_to_xencons(vtermno);
122 if (cons == NULL)
123 return -EINVAL;
124
125 /*
126 * Make sure the whole buffer is emitted, polling if
127 * necessary. We don't ever want to rely on the hvc daemon
128 * because the most interesting console output is when the
129 * kernel is crippled.
130 */
131 while (len) {
132 int sent = __write_console(xencons: cons, data, len);
133
134 if (sent < 0)
135 return sent;
136
137 data += sent;
138 len -= sent;
139
140 if (unlikely(len))
141 HYPERVISOR_sched_op(SCHEDOP_yield, NULL);
142 }
143
144 return ret;
145}
146
147static int domU_read_console(uint32_t vtermno, char *buf, int len)
148{
149 struct xencons_interface *intf;
150 XENCONS_RING_IDX cons, prod;
151 int recv = 0;
152 struct xencons_info *xencons = vtermno_to_xencons(vtermno);
153 unsigned int eoiflag = 0;
154 unsigned long flags;
155
156 if (xencons == NULL)
157 return -EINVAL;
158 intf = xencons->intf;
159
160 spin_lock_irqsave(&xencons->ring_lock, flags);
161 cons = intf->in_cons;
162 prod = intf->in_prod;
163 mb(); /* get pointers before reading ring */
164
165 if ((prod - cons) > sizeof(intf->in)) {
166 spin_unlock_irqrestore(lock: &xencons->ring_lock, flags);
167 pr_err_once("xencons: Illegal ring page indices");
168 return -EINVAL;
169 }
170
171 while (cons != prod && recv < len)
172 buf[recv++] = intf->in[MASK_XENCONS_IDX(cons++, intf->in)];
173
174 mb(); /* read ring before consuming */
175 intf->in_cons = cons;
176
177 /*
178 * When to mark interrupt having been spurious:
179 * - there was no new data to be read, and
180 * - the backend did not consume some output bytes, and
181 * - the previous round with no read data didn't see consumed bytes
182 * (we might have a race with an interrupt being in flight while
183 * updating xencons->out_cons, so account for that by allowing one
184 * round without any visible reason)
185 */
186 if (intf->out_cons != xencons->out_cons) {
187 xencons->out_cons = intf->out_cons;
188 xencons->out_cons_same = 0;
189 }
190 if (!recv && xencons->out_cons_same++ > 1) {
191 eoiflag = XEN_EOI_FLAG_SPURIOUS;
192 }
193 spin_unlock_irqrestore(lock: &xencons->ring_lock, flags);
194
195 if (recv) {
196 notify_daemon(cons: xencons);
197 }
198
199 xen_irq_lateeoi(irq: xencons->irq, eoi_flags: eoiflag);
200
201 return recv;
202}
203
204static const struct hv_ops domU_hvc_ops = {
205 .get_chars = domU_read_console,
206 .put_chars = domU_write_console,
207 .notifier_add = notifier_add_irq,
208 .notifier_del = notifier_del_irq,
209 .notifier_hangup = notifier_hangup_irq,
210};
211
212static int dom0_read_console(uint32_t vtermno, char *buf, int len)
213{
214 return HYPERVISOR_console_io(CONSOLEIO_read, count: len, str: buf);
215}
216
217/*
218 * Either for a dom0 to write to the system console, or a domU with a
219 * debug version of Xen
220 */
221static int dom0_write_console(uint32_t vtermno, const char *str, int len)
222{
223 int rc = HYPERVISOR_console_io(CONSOLEIO_write, count: len, str: (char *)str);
224 if (rc < 0)
225 return rc;
226
227 return len;
228}
229
230static const struct hv_ops dom0_hvc_ops = {
231 .get_chars = dom0_read_console,
232 .put_chars = dom0_write_console,
233 .notifier_add = notifier_add_irq,
234 .notifier_del = notifier_del_irq,
235 .notifier_hangup = notifier_hangup_irq,
236};
237
238static int xen_hvm_console_init(void)
239{
240 int r;
241 uint64_t v = 0;
242 unsigned long gfn, flags;
243 struct xencons_info *info;
244
245 if (!xen_hvm_domain())
246 return -ENODEV;
247
248 info = vtermno_to_xencons(HVC_COOKIE);
249 if (!info) {
250 info = kzalloc(size: sizeof(struct xencons_info), GFP_KERNEL);
251 if (!info)
252 return -ENOMEM;
253 spin_lock_init(&info->ring_lock);
254 } else if (info->intf != NULL) {
255 /* already configured */
256 return 0;
257 }
258 /*
259 * If the toolstack (or the hypervisor) hasn't set these values, the
260 * default value is 0. Even though gfn = 0 and evtchn = 0 are
261 * theoretically correct values, in practice they never are and they
262 * mean that a legacy toolstack hasn't initialized the pv console correctly.
263 */
264 r = hvm_get_parameter(HVM_PARAM_CONSOLE_EVTCHN, value: &v);
265 if (r < 0 || v == 0)
266 goto err;
267 info->evtchn = v;
268 v = 0;
269 r = hvm_get_parameter(HVM_PARAM_CONSOLE_PFN, value: &v);
270 if (r < 0 || v == 0)
271 goto err;
272 gfn = v;
273 info->intf = memremap(offset: gfn << XEN_PAGE_SHIFT, XEN_PAGE_SIZE, flags: MEMREMAP_WB);
274 if (info->intf == NULL)
275 goto err;
276 info->vtermno = HVC_COOKIE;
277
278 spin_lock_irqsave(&xencons_lock, flags);
279 list_add_tail(new: &info->list, head: &xenconsoles);
280 spin_unlock_irqrestore(lock: &xencons_lock, flags);
281
282 return 0;
283err:
284 kfree(objp: info);
285 return -ENODEV;
286}
287
288static int xencons_info_pv_init(struct xencons_info *info, int vtermno)
289{
290 spin_lock_init(&info->ring_lock);
291 info->evtchn = xen_start_info->console.domU.evtchn;
292 /* GFN == MFN for PV guest */
293 info->intf = gfn_to_virt(xen_start_info->console.domU.mfn);
294 info->vtermno = vtermno;
295
296 list_add_tail(new: &info->list, head: &xenconsoles);
297
298 return 0;
299}
300
301static int xen_pv_console_init(void)
302{
303 struct xencons_info *info;
304 unsigned long flags;
305
306 if (!xen_pv_domain())
307 return -ENODEV;
308
309 if (!xen_start_info->console.domU.evtchn)
310 return -ENODEV;
311
312 info = vtermno_to_xencons(HVC_COOKIE);
313 if (!info) {
314 info = kzalloc(size: sizeof(struct xencons_info), GFP_KERNEL);
315 if (!info)
316 return -ENOMEM;
317 } else if (info->intf != NULL) {
318 /* already configured */
319 return 0;
320 }
321 spin_lock_irqsave(&xencons_lock, flags);
322 xencons_info_pv_init(info, HVC_COOKIE);
323 spin_unlock_irqrestore(lock: &xencons_lock, flags);
324
325 return 0;
326}
327
328static int xen_initial_domain_console_init(void)
329{
330 struct xencons_info *info;
331 unsigned long flags;
332
333 if (!xen_initial_domain())
334 return -ENODEV;
335
336 info = vtermno_to_xencons(HVC_COOKIE);
337 if (!info) {
338 info = kzalloc(size: sizeof(struct xencons_info), GFP_KERNEL);
339 if (!info)
340 return -ENOMEM;
341 spin_lock_init(&info->ring_lock);
342 }
343
344 info->irq = bind_virq_to_irq(VIRQ_CONSOLE, cpu: 0, percpu: false);
345 info->vtermno = HVC_COOKIE;
346
347 spin_lock_irqsave(&xencons_lock, flags);
348 list_add_tail(new: &info->list, head: &xenconsoles);
349 spin_unlock_irqrestore(lock: &xencons_lock, flags);
350
351 return 0;
352}
353
354static void xen_console_update_evtchn(struct xencons_info *info)
355{
356 if (xen_hvm_domain()) {
357 uint64_t v = 0;
358 int err;
359
360 err = hvm_get_parameter(HVM_PARAM_CONSOLE_EVTCHN, value: &v);
361 if (!err && v)
362 info->evtchn = v;
363 } else
364 info->evtchn = xen_start_info->console.domU.evtchn;
365}
366
367void xen_console_resume(void)
368{
369 struct xencons_info *info = vtermno_to_xencons(HVC_COOKIE);
370 if (info != NULL && info->irq) {
371 if (!xen_initial_domain())
372 xen_console_update_evtchn(info);
373 rebind_evtchn_irq(evtchn: info->evtchn, irq: info->irq);
374 }
375}
376
377#ifdef CONFIG_HVC_XEN_FRONTEND
378static void xencons_disconnect_backend(struct xencons_info *info)
379{
380 if (info->hvc != NULL)
381 hvc_remove(hp: info->hvc);
382 info->hvc = NULL;
383 if (info->irq > 0) {
384 evtchn_put(evtchn: info->evtchn);
385 info->irq = 0;
386 info->evtchn = 0;
387 }
388 /* evtchn_put() will also close it so this is only an error path */
389 if (info->evtchn > 0)
390 xenbus_free_evtchn(dev: info->xbdev, port: info->evtchn);
391 info->evtchn = 0;
392 if (info->gntref > 0)
393 gnttab_free_grant_references(head: info->gntref);
394 info->gntref = 0;
395}
396
397static void xencons_free(struct xencons_info *info)
398{
399 free_page((unsigned long)info->intf);
400 info->intf = NULL;
401 info->vtermno = 0;
402 kfree(objp: info);
403}
404
405static int xen_console_remove(struct xencons_info *info)
406{
407 unsigned long flags;
408
409 xencons_disconnect_backend(info);
410 spin_lock_irqsave(&xencons_lock, flags);
411 list_del(entry: &info->list);
412 spin_unlock_irqrestore(lock: &xencons_lock, flags);
413 if (info->xbdev != NULL)
414 xencons_free(info);
415 else {
416 if (xen_hvm_domain())
417 iounmap(addr: info->intf);
418 kfree(objp: info);
419 }
420 return 0;
421}
422
423static void xencons_remove(struct xenbus_device *dev)
424{
425 xen_console_remove(info: dev_get_drvdata(dev: &dev->dev));
426}
427
428static int xencons_connect_backend(struct xenbus_device *dev,
429 struct xencons_info *info)
430{
431 int ret, evtchn, devid, ref, irq;
432 struct xenbus_transaction xbt;
433 grant_ref_t gref_head;
434
435 ret = xenbus_alloc_evtchn(dev, port: &evtchn);
436 if (ret)
437 return ret;
438 info->evtchn = evtchn;
439 irq = bind_evtchn_to_irq_lateeoi(evtchn);
440 if (irq < 0)
441 return irq;
442 info->irq = irq;
443 devid = dev->nodename[strlen(dev->nodename) - 1] - '0';
444 info->hvc = hvc_alloc(vtermno: xenbus_devid_to_vtermno(devid),
445 data: irq, ops: &domU_hvc_ops, outbuf_size: 256);
446 if (IS_ERR(ptr: info->hvc))
447 return PTR_ERR(ptr: info->hvc);
448 ret = gnttab_alloc_grant_references(count: 1, pprivate_head: &gref_head);
449 if (ret < 0)
450 return ret;
451 info->gntref = gref_head;
452 ref = gnttab_claim_grant_reference(pprivate_head: &gref_head);
453 if (ref < 0)
454 return ref;
455 gnttab_grant_foreign_access_ref(ref, domid: info->xbdev->otherend_id,
456 virt_to_gfn(info->intf), readonly: 0);
457
458 again:
459 ret = xenbus_transaction_start(t: &xbt);
460 if (ret) {
461 xenbus_dev_fatal(dev, err: ret, fmt: "starting transaction");
462 return ret;
463 }
464 ret = xenbus_printf(t: xbt, dir: dev->nodename, node: "ring-ref", fmt: "%d", ref);
465 if (ret)
466 goto error_xenbus;
467 ret = xenbus_printf(t: xbt, dir: dev->nodename, node: "port", fmt: "%u",
468 evtchn);
469 if (ret)
470 goto error_xenbus;
471 ret = xenbus_transaction_end(t: xbt, abort: 0);
472 if (ret) {
473 if (ret == -EAGAIN)
474 goto again;
475 xenbus_dev_fatal(dev, err: ret, fmt: "completing transaction");
476 return ret;
477 }
478
479 xenbus_switch_state(dev, new_state: XenbusStateInitialised);
480 return 0;
481
482 error_xenbus:
483 xenbus_transaction_end(t: xbt, abort: 1);
484 xenbus_dev_fatal(dev, err: ret, fmt: "writing xenstore");
485 return ret;
486}
487
488static int xencons_probe(struct xenbus_device *dev,
489 const struct xenbus_device_id *id)
490{
491 int ret, devid;
492 struct xencons_info *info;
493 unsigned long flags;
494
495 devid = dev->nodename[strlen(dev->nodename) - 1] - '0';
496 if (devid == 0)
497 return -ENODEV;
498
499 info = kzalloc(size: sizeof(struct xencons_info), GFP_KERNEL);
500 if (!info)
501 return -ENOMEM;
502 spin_lock_init(&info->ring_lock);
503 dev_set_drvdata(dev: &dev->dev, data: info);
504 info->xbdev = dev;
505 info->vtermno = xenbus_devid_to_vtermno(devid);
506 info->intf = (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
507 if (!info->intf)
508 goto error_nomem;
509
510 ret = xencons_connect_backend(dev, info);
511 if (ret < 0)
512 goto error;
513 spin_lock_irqsave(&xencons_lock, flags);
514 list_add_tail(new: &info->list, head: &xenconsoles);
515 spin_unlock_irqrestore(lock: &xencons_lock, flags);
516
517 return 0;
518
519 error_nomem:
520 ret = -ENOMEM;
521 xenbus_dev_fatal(dev, err: ret, fmt: "allocating device memory");
522 error:
523 xencons_disconnect_backend(info);
524 xencons_free(info);
525 return ret;
526}
527
528static int xencons_resume(struct xenbus_device *dev)
529{
530 struct xencons_info *info = dev_get_drvdata(dev: &dev->dev);
531
532 xencons_disconnect_backend(info);
533 memset(info->intf, 0, XEN_PAGE_SIZE);
534 return xencons_connect_backend(dev, info);
535}
536
537static void xencons_backend_changed(struct xenbus_device *dev,
538 enum xenbus_state backend_state)
539{
540 switch (backend_state) {
541 case XenbusStateReconfiguring:
542 case XenbusStateReconfigured:
543 case XenbusStateInitialising:
544 case XenbusStateInitialised:
545 case XenbusStateUnknown:
546 break;
547
548 case XenbusStateInitWait:
549 break;
550
551 case XenbusStateConnected:
552 xenbus_switch_state(dev, new_state: XenbusStateConnected);
553 break;
554
555 case XenbusStateClosed:
556 if (dev->state == XenbusStateClosed)
557 break;
558 fallthrough; /* Missed the backend's CLOSING state */
559 case XenbusStateClosing: {
560 struct xencons_info *info = dev_get_drvdata(dev: &dev->dev);;
561
562 /*
563 * Don't tear down the evtchn and grant ref before the other
564 * end has disconnected, but do stop userspace from trying
565 * to use the device before we allow the backend to close.
566 */
567 if (info->hvc) {
568 hvc_remove(hp: info->hvc);
569 info->hvc = NULL;
570 }
571
572 xenbus_frontend_closed(dev);
573 break;
574 }
575 }
576}
577
578static const struct xenbus_device_id xencons_ids[] = {
579 { "console" },
580 { "" }
581};
582
583static struct xenbus_driver xencons_driver = {
584 .name = "xenconsole",
585 .ids = xencons_ids,
586 .probe = xencons_probe,
587 .remove = xencons_remove,
588 .resume = xencons_resume,
589 .otherend_changed = xencons_backend_changed,
590 .not_essential = true,
591};
592#endif /* CONFIG_HVC_XEN_FRONTEND */
593
594static int __init xen_hvc_init(void)
595{
596 int r;
597 struct xencons_info *info;
598 const struct hv_ops *ops;
599
600 if (!xen_domain())
601 return -ENODEV;
602
603 if (xen_initial_domain()) {
604 ops = &dom0_hvc_ops;
605 r = xen_initial_domain_console_init();
606 if (r < 0)
607 goto register_fe;
608 info = vtermno_to_xencons(HVC_COOKIE);
609 } else {
610 ops = &domU_hvc_ops;
611 if (xen_hvm_domain())
612 r = xen_hvm_console_init();
613 else
614 r = xen_pv_console_init();
615 if (r < 0)
616 goto register_fe;
617
618 info = vtermno_to_xencons(HVC_COOKIE);
619 info->irq = bind_evtchn_to_irq_lateeoi(evtchn: info->evtchn);
620 }
621 if (info->irq < 0)
622 info->irq = 0; /* NO_IRQ */
623 else
624 irq_set_noprobe(irq: info->irq);
625
626 info->hvc = hvc_alloc(HVC_COOKIE, data: info->irq, ops, outbuf_size: 256);
627 if (IS_ERR(ptr: info->hvc)) {
628 unsigned long flags;
629
630 r = PTR_ERR(ptr: info->hvc);
631 spin_lock_irqsave(&xencons_lock, flags);
632 list_del(entry: &info->list);
633 spin_unlock_irqrestore(lock: &xencons_lock, flags);
634 if (info->irq)
635 evtchn_put(evtchn: info->evtchn);
636 kfree(objp: info);
637 return r;
638 }
639
640 r = 0;
641 register_fe:
642#ifdef CONFIG_HVC_XEN_FRONTEND
643 r = xenbus_register_frontend(&xencons_driver);
644#endif
645 return r;
646}
647device_initcall(xen_hvc_init);
648
649static int xen_cons_init(void)
650{
651 const struct hv_ops *ops;
652
653 if (!xen_domain())
654 return 0;
655
656 if (xen_initial_domain())
657 ops = &dom0_hvc_ops;
658 else {
659 int r;
660 ops = &domU_hvc_ops;
661
662 if (xen_hvm_domain())
663 r = xen_hvm_console_init();
664 else
665 r = xen_pv_console_init();
666 if (r < 0)
667 return r;
668 }
669
670 hvc_instantiate(HVC_COOKIE, index: 0, ops);
671 return 0;
672}
673console_initcall(xen_cons_init);
674
675#ifdef CONFIG_X86
676static void xen_hvm_early_write(uint32_t vtermno, const char *str, int len)
677{
678 if (xen_cpuid_base())
679 outsb(port: 0xe9, addr: str, count: len);
680}
681#else
682static void xen_hvm_early_write(uint32_t vtermno, const char *str, int len) { }
683#endif
684
685#ifdef CONFIG_EARLY_PRINTK
686static int __init xenboot_console_setup(struct console *console, char *string)
687{
688 static struct xencons_info xenboot;
689
690 if (xen_initial_domain() || !xen_pv_domain())
691 return 0;
692
693 return xencons_info_pv_init(info: &xenboot, vtermno: 0);
694}
695
696static void xenboot_write_console(struct console *console, const char *string,
697 unsigned len)
698{
699 unsigned int linelen, off = 0;
700 const char *pos;
701
702 if (dom0_write_console(vtermno: 0, str: string, len) >= 0)
703 return;
704
705 if (!xen_pv_domain()) {
706 xen_hvm_early_write(vtermno: 0, str: string, len);
707 return;
708 }
709
710 if (domU_write_console(vtermno: 0, data: "(early) ", len: 8) < 0)
711 return;
712 while (off < len && NULL != (pos = strchr(string+off, '\n'))) {
713 linelen = pos-string+off;
714 if (off + linelen > len)
715 break;
716 domU_write_console(vtermno: 0, data: string+off, len: linelen);
717 domU_write_console(vtermno: 0, data: "\r\n", len: 2);
718 off += linelen + 1;
719 }
720 if (off < len)
721 domU_write_console(vtermno: 0, data: string+off, len: len-off);
722}
723
724struct console xenboot_console = {
725 .name = "xenboot",
726 .write = xenboot_write_console,
727 .setup = xenboot_console_setup,
728 .flags = CON_PRINTBUFFER | CON_BOOT | CON_ANYTIME,
729 .index = -1,
730};
731#endif /* CONFIG_EARLY_PRINTK */
732
733void xen_raw_console_write(const char *str)
734{
735 ssize_t len = strlen(str);
736 int rc = 0;
737
738 if (xen_domain()) {
739 rc = dom0_write_console(vtermno: 0, str, len);
740 if (rc != -ENOSYS || !xen_hvm_domain())
741 return;
742 }
743 xen_hvm_early_write(vtermno: 0, str, len);
744}
745
746void xen_raw_printk(const char *fmt, ...)
747{
748 static char buf[512];
749 va_list ap;
750
751 va_start(ap, fmt);
752 vsnprintf(buf, size: sizeof(buf), fmt, args: ap);
753 va_end(ap);
754
755 xen_raw_console_write(str: buf);
756}
757
758static void xenboot_earlycon_write(struct console *console,
759 const char *string,
760 unsigned len)
761{
762 dom0_write_console(vtermno: 0, str: string, len);
763}
764
765static int __init xenboot_earlycon_setup(struct earlycon_device *device,
766 const char *opt)
767{
768 device->con->write = xenboot_earlycon_write;
769 return 0;
770}
771EARLYCON_DECLARE(xenboot, xenboot_earlycon_setup);
772

source code of linux/drivers/tty/hvc/hvc_xen.c