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 ssize_t __write_console(struct xencons_info *xencons,
88 const u8 *data, size_t len)
89{
90 XENCONS_RING_IDX cons, prod;
91 struct xencons_interface *intf = xencons->intf;
92 unsigned long flags;
93 size_t sent = 0;
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 ssize_t domU_write_console(uint32_t vtermno, const u8 *data, size_t len)
119{
120 struct xencons_info *cons = vtermno_to_xencons(vtermno);
121 size_t ret = len;
122
123 if (cons == NULL)
124 return -EINVAL;
125
126 /*
127 * Make sure the whole buffer is emitted, polling if
128 * necessary. We don't ever want to rely on the hvc daemon
129 * because the most interesting console output is when the
130 * kernel is crippled.
131 */
132 while (len) {
133 ssize_t sent = __write_console(xencons: cons, data, len);
134
135 if (sent < 0)
136 return sent;
137
138 data += sent;
139 len -= sent;
140
141 if (unlikely(len))
142 HYPERVISOR_sched_op(SCHEDOP_yield, NULL);
143 }
144
145 return ret;
146}
147
148static ssize_t domU_read_console(uint32_t vtermno, u8 *buf, size_t len)
149{
150 struct xencons_interface *intf;
151 XENCONS_RING_IDX cons, prod;
152 struct xencons_info *xencons = vtermno_to_xencons(vtermno);
153 unsigned int eoiflag = 0;
154 unsigned long flags;
155 size_t recv = 0;
156
157 if (xencons == NULL)
158 return -EINVAL;
159 intf = xencons->intf;
160
161 spin_lock_irqsave(&xencons->ring_lock, flags);
162 cons = intf->in_cons;
163 prod = intf->in_prod;
164 mb(); /* get pointers before reading ring */
165
166 if ((prod - cons) > sizeof(intf->in)) {
167 spin_unlock_irqrestore(lock: &xencons->ring_lock, flags);
168 pr_err_once("xencons: Illegal ring page indices");
169 return -EINVAL;
170 }
171
172 while (cons != prod && recv < len)
173 buf[recv++] = intf->in[MASK_XENCONS_IDX(cons++, intf->in)];
174
175 mb(); /* read ring before consuming */
176 intf->in_cons = cons;
177
178 /*
179 * When to mark interrupt having been spurious:
180 * - there was no new data to be read, and
181 * - the backend did not consume some output bytes, and
182 * - the previous round with no read data didn't see consumed bytes
183 * (we might have a race with an interrupt being in flight while
184 * updating xencons->out_cons, so account for that by allowing one
185 * round without any visible reason)
186 */
187 if (intf->out_cons != xencons->out_cons) {
188 xencons->out_cons = intf->out_cons;
189 xencons->out_cons_same = 0;
190 }
191 if (!recv && xencons->out_cons_same++ > 1) {
192 eoiflag = XEN_EOI_FLAG_SPURIOUS;
193 }
194 spin_unlock_irqrestore(lock: &xencons->ring_lock, flags);
195
196 if (recv) {
197 notify_daemon(cons: xencons);
198 }
199
200 xen_irq_lateeoi(irq: xencons->irq, eoi_flags: eoiflag);
201
202 return recv;
203}
204
205static const struct hv_ops domU_hvc_ops = {
206 .get_chars = domU_read_console,
207 .put_chars = domU_write_console,
208 .notifier_add = notifier_add_irq,
209 .notifier_del = notifier_del_irq,
210 .notifier_hangup = notifier_hangup_irq,
211};
212
213static ssize_t dom0_read_console(uint32_t vtermno, u8 *buf, size_t len)
214{
215 return HYPERVISOR_console_io(CONSOLEIO_read, count: len, str: buf);
216}
217
218/*
219 * Either for a dom0 to write to the system console, or a domU with a
220 * debug version of Xen
221 */
222static ssize_t dom0_write_console(uint32_t vtermno, const u8 *str, size_t len)
223{
224 int rc = HYPERVISOR_console_io(CONSOLEIO_write, count: len, str: (u8 *)str);
225 if (rc < 0)
226 return rc;
227
228 return len;
229}
230
231static const struct hv_ops dom0_hvc_ops = {
232 .get_chars = dom0_read_console,
233 .put_chars = dom0_write_console,
234 .notifier_add = notifier_add_irq,
235 .notifier_del = notifier_del_irq,
236 .notifier_hangup = notifier_hangup_irq,
237};
238
239static int xen_hvm_console_init(void)
240{
241 int r;
242 uint64_t v = 0;
243 unsigned long gfn, flags;
244 struct xencons_info *info;
245
246 if (!xen_hvm_domain())
247 return -ENODEV;
248
249 info = vtermno_to_xencons(HVC_COOKIE);
250 if (!info) {
251 info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
252 if (!info)
253 return -ENOMEM;
254 spin_lock_init(&info->ring_lock);
255 } else if (info->intf != NULL) {
256 /* already configured */
257 return 0;
258 }
259 /*
260 * If the toolstack (or the hypervisor) hasn't set these values, the
261 * default value is 0. Even though gfn = 0 and evtchn = 0 are
262 * theoretically correct values, in practice they never are and they
263 * mean that a legacy toolstack hasn't initialized the pv console correctly.
264 */
265 r = hvm_get_parameter(HVM_PARAM_CONSOLE_EVTCHN, value: &v);
266 if (r < 0 || v == 0)
267 goto err;
268 info->evtchn = v;
269 v = 0;
270 r = hvm_get_parameter(HVM_PARAM_CONSOLE_PFN, value: &v);
271 if (r < 0 || v == 0)
272 goto err;
273 gfn = v;
274 info->intf = memremap(offset: gfn << XEN_PAGE_SHIFT, XEN_PAGE_SIZE, flags: MEMREMAP_WB);
275 if (info->intf == NULL)
276 goto err;
277 info->vtermno = HVC_COOKIE;
278
279 spin_lock_irqsave(&xencons_lock, flags);
280 list_add_tail(new: &info->list, head: &xenconsoles);
281 spin_unlock_irqrestore(lock: &xencons_lock, flags);
282
283 return 0;
284err:
285 kfree(objp: info);
286 return -ENODEV;
287}
288
289static int xencons_info_pv_init(struct xencons_info *info, int vtermno)
290{
291 spin_lock_init(&info->ring_lock);
292 info->evtchn = xen_start_info->console.domU.evtchn;
293 /* GFN == MFN for PV guest */
294 info->intf = gfn_to_virt(xen_start_info->console.domU.mfn);
295 info->vtermno = vtermno;
296
297 list_add_tail(new: &info->list, head: &xenconsoles);
298
299 return 0;
300}
301
302static int xen_pv_console_init(void)
303{
304 struct xencons_info *info;
305 unsigned long flags;
306
307 if (!xen_pv_domain())
308 return -ENODEV;
309
310 if (!xen_start_info->console.domU.evtchn)
311 return -ENODEV;
312
313 info = vtermno_to_xencons(HVC_COOKIE);
314 if (!info) {
315 info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
316 if (!info)
317 return -ENOMEM;
318 } else if (info->intf != NULL) {
319 /* already configured */
320 return 0;
321 }
322 spin_lock_irqsave(&xencons_lock, flags);
323 xencons_info_pv_init(info, HVC_COOKIE);
324 spin_unlock_irqrestore(lock: &xencons_lock, flags);
325
326 return 0;
327}
328
329static int xen_initial_domain_console_init(void)
330{
331 struct xencons_info *info;
332 unsigned long flags;
333
334 if (!xen_initial_domain())
335 return -ENODEV;
336
337 info = vtermno_to_xencons(HVC_COOKIE);
338 if (!info) {
339 info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
340 if (!info)
341 return -ENOMEM;
342 spin_lock_init(&info->ring_lock);
343 }
344
345 info->irq = bind_virq_to_irq(VIRQ_CONSOLE, cpu: 0, percpu: false);
346 info->vtermno = HVC_COOKIE;
347
348 spin_lock_irqsave(&xencons_lock, flags);
349 list_add_tail(new: &info->list, head: &xenconsoles);
350 spin_unlock_irqrestore(lock: &xencons_lock, flags);
351
352 return 0;
353}
354
355static void xen_console_update_evtchn(struct xencons_info *info)
356{
357 if (xen_hvm_domain()) {
358 uint64_t v = 0;
359 int err;
360
361 err = hvm_get_parameter(HVM_PARAM_CONSOLE_EVTCHN, value: &v);
362 if (!err && v)
363 info->evtchn = v;
364 } else
365 info->evtchn = xen_start_info->console.domU.evtchn;
366}
367
368void xen_console_resume(void)
369{
370 struct xencons_info *info = vtermno_to_xencons(HVC_COOKIE);
371 if (info != NULL && info->irq) {
372 if (!xen_initial_domain())
373 xen_console_update_evtchn(info);
374 rebind_evtchn_irq(evtchn: info->evtchn, irq: info->irq);
375 }
376}
377
378#ifdef CONFIG_HVC_XEN_FRONTEND
379static void xencons_disconnect_backend(struct xencons_info *info)
380{
381 if (info->hvc != NULL)
382 hvc_remove(hp: info->hvc);
383 info->hvc = NULL;
384 if (info->irq > 0) {
385 evtchn_put(evtchn: info->evtchn);
386 info->irq = 0;
387 info->evtchn = 0;
388 }
389 /* evtchn_put() will also close it so this is only an error path */
390 if (info->evtchn > 0)
391 xenbus_free_evtchn(dev: info->xbdev, port: info->evtchn);
392 info->evtchn = 0;
393 if (info->gntref > 0)
394 gnttab_free_grant_references(head: info->gntref);
395 info->gntref = 0;
396}
397
398static void xencons_free(struct xencons_info *info)
399{
400 free_page((unsigned long)info->intf);
401 info->intf = NULL;
402 info->vtermno = 0;
403 kfree(objp: info);
404}
405
406static int xen_console_remove(struct xencons_info *info)
407{
408 unsigned long flags;
409
410 xencons_disconnect_backend(info);
411 spin_lock_irqsave(&xencons_lock, flags);
412 list_del(entry: &info->list);
413 spin_unlock_irqrestore(lock: &xencons_lock, flags);
414 if (info->xbdev != NULL)
415 xencons_free(info);
416 else {
417 if (xen_hvm_domain())
418 iounmap(addr: info->intf);
419 kfree(objp: info);
420 }
421 return 0;
422}
423
424static void xencons_remove(struct xenbus_device *dev)
425{
426 xen_console_remove(info: dev_get_drvdata(dev: &dev->dev));
427}
428
429static int xencons_connect_backend(struct xenbus_device *dev,
430 struct xencons_info *info)
431{
432 int ret, evtchn, devid, ref, irq;
433 struct xenbus_transaction xbt;
434 grant_ref_t gref_head;
435
436 ret = xenbus_alloc_evtchn(dev, port: &evtchn);
437 if (ret)
438 return ret;
439 info->evtchn = evtchn;
440 irq = bind_evtchn_to_irq_lateeoi(evtchn);
441 if (irq < 0)
442 return irq;
443 info->irq = irq;
444 devid = dev->nodename[strlen(dev->nodename) - 1] - '0';
445 info->hvc = hvc_alloc(vtermno: xenbus_devid_to_vtermno(devid),
446 data: irq, ops: &domU_hvc_ops, outbuf_size: 256);
447 if (IS_ERR(ptr: info->hvc))
448 return PTR_ERR(ptr: info->hvc);
449 ret = gnttab_alloc_grant_references(count: 1, pprivate_head: &gref_head);
450 if (ret < 0)
451 return ret;
452 info->gntref = gref_head;
453 ref = gnttab_claim_grant_reference(pprivate_head: &gref_head);
454 if (ref < 0)
455 return ref;
456 gnttab_grant_foreign_access_ref(ref, domid: info->xbdev->otherend_id,
457 virt_to_gfn(info->intf), readonly: 0);
458
459 again:
460 ret = xenbus_transaction_start(t: &xbt);
461 if (ret) {
462 xenbus_dev_fatal(dev, err: ret, fmt: "starting transaction");
463 return ret;
464 }
465 ret = xenbus_printf(t: xbt, dir: dev->nodename, node: "ring-ref", fmt: "%d", ref);
466 if (ret)
467 goto error_xenbus;
468 ret = xenbus_printf(t: xbt, dir: dev->nodename, node: "port", fmt: "%u",
469 evtchn);
470 if (ret)
471 goto error_xenbus;
472 ret = xenbus_transaction_end(t: xbt, abort: 0);
473 if (ret) {
474 if (ret == -EAGAIN)
475 goto again;
476 xenbus_dev_fatal(dev, err: ret, fmt: "completing transaction");
477 return ret;
478 }
479
480 xenbus_switch_state(dev, new_state: XenbusStateInitialised);
481 return 0;
482
483 error_xenbus:
484 xenbus_transaction_end(t: xbt, abort: 1);
485 xenbus_dev_fatal(dev, err: ret, fmt: "writing xenstore");
486 return ret;
487}
488
489static int xencons_probe(struct xenbus_device *dev,
490 const struct xenbus_device_id *id)
491{
492 int ret, devid;
493 struct xencons_info *info;
494 unsigned long flags;
495
496 devid = dev->nodename[strlen(dev->nodename) - 1] - '0';
497 if (devid == 0)
498 return -ENODEV;
499
500 info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
501 if (!info)
502 return -ENOMEM;
503 spin_lock_init(&info->ring_lock);
504 dev_set_drvdata(dev: &dev->dev, data: info);
505 info->xbdev = dev;
506 info->vtermno = xenbus_devid_to_vtermno(devid);
507 info->intf = (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
508 if (!info->intf)
509 goto error_nomem;
510
511 ret = xencons_connect_backend(dev, info);
512 if (ret < 0)
513 goto error;
514 spin_lock_irqsave(&xencons_lock, flags);
515 list_add_tail(new: &info->list, head: &xenconsoles);
516 spin_unlock_irqrestore(lock: &xencons_lock, flags);
517
518 return 0;
519
520 error_nomem:
521 ret = -ENOMEM;
522 xenbus_dev_fatal(dev, err: ret, fmt: "allocating device memory");
523 error:
524 xencons_disconnect_backend(info);
525 xencons_free(info);
526 return ret;
527}
528
529static int xencons_resume(struct xenbus_device *dev)
530{
531 struct xencons_info *info = dev_get_drvdata(dev: &dev->dev);
532
533 xencons_disconnect_backend(info);
534 memset(info->intf, 0, XEN_PAGE_SIZE);
535 return xencons_connect_backend(dev, info);
536}
537
538static void xencons_backend_changed(struct xenbus_device *dev,
539 enum xenbus_state backend_state)
540{
541 switch (backend_state) {
542 case XenbusStateReconfiguring:
543 case XenbusStateReconfigured:
544 case XenbusStateInitialising:
545 case XenbusStateInitialised:
546 case XenbusStateUnknown:
547 break;
548
549 case XenbusStateInitWait:
550 break;
551
552 case XenbusStateConnected:
553 xenbus_switch_state(dev, new_state: XenbusStateConnected);
554 break;
555
556 case XenbusStateClosed:
557 if (dev->state == XenbusStateClosed)
558 break;
559 fallthrough; /* Missed the backend's CLOSING state */
560 case XenbusStateClosing: {
561 struct xencons_info *info = dev_get_drvdata(dev: &dev->dev);
562
563 /*
564 * Don't tear down the evtchn and grant ref before the other
565 * end has disconnected, but do stop userspace from trying
566 * to use the device before we allow the backend to close.
567 */
568 if (info->hvc) {
569 hvc_remove(hp: info->hvc);
570 info->hvc = NULL;
571 }
572
573 xenbus_frontend_closed(dev);
574 break;
575 }
576 }
577}
578
579static const struct xenbus_device_id xencons_ids[] = {
580 { "console" },
581 { "" }
582};
583
584static struct xenbus_driver xencons_driver = {
585 .name = "xenconsole",
586 .ids = xencons_ids,
587 .probe = xencons_probe,
588 .remove = xencons_remove,
589 .resume = xencons_resume,
590 .otherend_changed = xencons_backend_changed,
591 .not_essential = true,
592};
593#endif /* CONFIG_HVC_XEN_FRONTEND */
594
595static int __init xen_hvc_init(void)
596{
597 int r;
598 struct xencons_info *info;
599 const struct hv_ops *ops;
600
601 if (!xen_domain())
602 return -ENODEV;
603
604 if (xen_initial_domain()) {
605 ops = &dom0_hvc_ops;
606 r = xen_initial_domain_console_init();
607 if (r < 0)
608 goto register_fe;
609 info = vtermno_to_xencons(HVC_COOKIE);
610 } else {
611 ops = &domU_hvc_ops;
612 if (xen_hvm_domain())
613 r = xen_hvm_console_init();
614 else
615 r = xen_pv_console_init();
616 if (r < 0)
617 goto register_fe;
618
619 info = vtermno_to_xencons(HVC_COOKIE);
620 info->irq = bind_evtchn_to_irq_lateeoi(evtchn: info->evtchn);
621 }
622 if (info->irq < 0)
623 info->irq = 0; /* NO_IRQ */
624 else
625 irq_set_noprobe(irq: info->irq);
626
627 info->hvc = hvc_alloc(HVC_COOKIE, data: info->irq, ops, outbuf_size: 256);
628 if (IS_ERR(ptr: info->hvc)) {
629 unsigned long flags;
630
631 r = PTR_ERR(ptr: info->hvc);
632 spin_lock_irqsave(&xencons_lock, flags);
633 list_del(entry: &info->list);
634 spin_unlock_irqrestore(lock: &xencons_lock, flags);
635 if (info->irq)
636 evtchn_put(evtchn: info->evtchn);
637 kfree(objp: info);
638 return r;
639 }
640
641 r = 0;
642 register_fe:
643#ifdef CONFIG_HVC_XEN_FRONTEND
644 r = xenbus_register_frontend(&xencons_driver);
645#endif
646 return r;
647}
648device_initcall(xen_hvc_init);
649
650static int xen_cons_init(void)
651{
652 const struct hv_ops *ops;
653
654 if (!xen_domain())
655 return 0;
656
657 if (xen_initial_domain())
658 ops = &dom0_hvc_ops;
659 else {
660 int r;
661 ops = &domU_hvc_ops;
662
663 if (xen_hvm_domain())
664 r = xen_hvm_console_init();
665 else
666 r = xen_pv_console_init();
667 if (r < 0)
668 return r;
669 }
670
671 hvc_instantiate(HVC_COOKIE, index: 0, ops);
672 return 0;
673}
674console_initcall(xen_cons_init);
675
676#ifdef CONFIG_X86
677static void xen_hvm_early_write(uint32_t vtermno, const char *str, int len)
678{
679 if (xen_cpuid_base())
680 outsb(port: 0xe9, addr: str, count: len);
681}
682#else
683static void xen_hvm_early_write(uint32_t vtermno, const char *str, int len) { }
684#endif
685
686#ifdef CONFIG_EARLY_PRINTK
687static int __init xenboot_console_setup(struct console *console, char *string)
688{
689 static struct xencons_info xenboot;
690
691 if (xen_initial_domain() || !xen_pv_domain())
692 return 0;
693
694 return xencons_info_pv_init(info: &xenboot, vtermno: 0);
695}
696
697static void xenboot_write_console(struct console *console, const char *string,
698 unsigned len)
699{
700 unsigned int linelen, off = 0;
701 const char *pos;
702
703 if (dom0_write_console(vtermno: 0, str: string, len) >= 0)
704 return;
705
706 if (!xen_pv_domain()) {
707 xen_hvm_early_write(vtermno: 0, str: string, len);
708 return;
709 }
710
711 if (domU_write_console(vtermno: 0, data: "(early) ", len: 8) < 0)
712 return;
713 while (off < len && NULL != (pos = strchr(string+off, '\n'))) {
714 linelen = pos-string+off;
715 if (off + linelen > len)
716 break;
717 domU_write_console(vtermno: 0, data: string+off, len: linelen);
718 domU_write_console(vtermno: 0, data: "\r\n", len: 2);
719 off += linelen + 1;
720 }
721 if (off < len)
722 domU_write_console(vtermno: 0, data: string+off, len: len-off);
723}
724
725struct console xenboot_console = {
726 .name = "xenboot",
727 .write = xenboot_write_console,
728 .setup = xenboot_console_setup,
729 .flags = CON_PRINTBUFFER | CON_BOOT | CON_ANYTIME,
730 .index = -1,
731};
732#endif /* CONFIG_EARLY_PRINTK */
733
734void xen_raw_console_write(const char *str)
735{
736 ssize_t len = strlen(str);
737 int rc = 0;
738
739 if (xen_domain()) {
740 rc = dom0_write_console(vtermno: 0, str, len);
741 if (rc != -ENOSYS || !xen_hvm_domain())
742 return;
743 }
744 xen_hvm_early_write(vtermno: 0, str, len);
745}
746
747void xen_raw_printk(const char *fmt, ...)
748{
749 static char buf[512];
750 va_list ap;
751
752 va_start(ap, fmt);
753 vsnprintf(buf, size: sizeof(buf), fmt, args: ap);
754 va_end(ap);
755
756 xen_raw_console_write(str: buf);
757}
758
759static void xenboot_earlycon_write(struct console *console,
760 const char *string,
761 unsigned len)
762{
763 dom0_write_console(vtermno: 0, str: string, len);
764}
765
766static int __init xenboot_earlycon_setup(struct earlycon_device *device,
767 const char *opt)
768{
769 device->con->write = xenboot_earlycon_write;
770 return 0;
771}
772EARLYCON_DECLARE(xenboot, xenboot_earlycon_setup);
773

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