1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Copyright IBM Corp. 1999, 2023
  4 */
  5
  6#include <linux/cpuhotplug.h>
  7#include <linux/sched/task.h>
  8#include <linux/errno.h>
  9#include <linux/init.h>
 10#include <linux/irq.h>
 11#include <asm/asm-extable.h>
 12#include <asm/pfault.h>
 13#include <asm/diag.h>
 14
 15#define __SUBCODE_MASK 0x0600
 16#define __PF_RES_FIELD 0x8000000000000000UL
 17
 18/*
 19 * 'pfault' pseudo page faults routines.
 20 */
 21static int pfault_disable;
 22
 23static int __init nopfault(char *str)
 24{
 25	pfault_disable = 1;
 26	return 1;
 27}
 28early_param("nopfault", nopfault);
 29
 30struct pfault_refbk {
 31	u16 refdiagc;
 32	u16 reffcode;
 33	u16 refdwlen;
 34	u16 refversn;
 35	u64 refgaddr;
 36	u64 refselmk;
 37	u64 refcmpmk;
 38	u64 reserved;
 39};
 40
 41static struct pfault_refbk pfault_init_refbk = {
 42	.refdiagc = 0x258,
 43	.reffcode = 0,
 44	.refdwlen = 5,
 45	.refversn = 2,
 46	.refgaddr = __LC_LPP,
 47	.refselmk = 1UL << 48,
 48	.refcmpmk = 1UL << 48,
 49	.reserved = __PF_RES_FIELD
 50};
 51
 52int __pfault_init(void)
 53{
 54	int rc = -EOPNOTSUPP;
 55
 56	if (pfault_disable)
 57		return rc;
 58	diag_stat_inc(DIAG_STAT_X258);
 59	asm volatile(
 60		"	diag	%[refbk],%[rc],0x258\n"
 61		"0:	nopr	%%r7\n"
 62		EX_TABLE(0b, 0b)
 63		: [rc] "+d" (rc)
 64		: [refbk] "a" (&pfault_init_refbk), "m" (pfault_init_refbk)
 65		: "cc");
 66	return rc;
 67}
 68
 69static struct pfault_refbk pfault_fini_refbk = {
 70	.refdiagc = 0x258,
 71	.reffcode = 1,
 72	.refdwlen = 5,
 73	.refversn = 2,
 74};
 75
 76void __pfault_fini(void)
 77{
 78	if (pfault_disable)
 79		return;
 80	diag_stat_inc(DIAG_STAT_X258);
 81	asm volatile(
 82		"	diag	%[refbk],0,0x258\n"
 83		"0:	nopr	%%r7\n"
 84		EX_TABLE(0b, 0b)
 85		:
 86		: [refbk] "a" (&pfault_fini_refbk), "m" (pfault_fini_refbk)
 87		: "cc");
 88}
 89
 90static DEFINE_SPINLOCK(pfault_lock);
 91static LIST_HEAD(pfault_list);
 92
 93#define PF_COMPLETE	0x0080
 94
 95/*
 96 * The mechanism of our pfault code: if Linux is running as guest, runs a user
 97 * space process and the user space process accesses a page that the host has
 98 * paged out we get a pfault interrupt.
 99 *
100 * This allows us, within the guest, to schedule a different process. Without
101 * this mechanism the host would have to suspend the whole virtual cpu until
102 * the page has been paged in.
103 *
104 * So when we get such an interrupt then we set the state of the current task
105 * to uninterruptible and also set the need_resched flag. Both happens within
106 * interrupt context(!). If we later on want to return to user space we
107 * recognize the need_resched flag and then call schedule().  It's not very
108 * obvious how this works...
109 *
110 * Of course we have a lot of additional fun with the completion interrupt (->
111 * host signals that a page of a process has been paged in and the process can
112 * continue to run). This interrupt can arrive on any cpu and, since we have
113 * virtual cpus, actually appear before the interrupt that signals that a page
114 * is missing.
115 */
116static void pfault_interrupt(struct ext_code ext_code,
117			     unsigned int param32, unsigned long param64)
118{
119	struct task_struct *tsk;
120	__u16 subcode;
121	pid_t pid;
122
123	/*
124	 * Get the external interruption subcode & pfault initial/completion
125	 * signal bit. VM stores this in the 'cpu address' field associated
126	 * with the external interrupt.
127	 */
128	subcode = ext_code.subcode;
129	if ((subcode & 0xff00) != __SUBCODE_MASK)
130		return;
131	inc_irq_stat(IRQEXT_PFL);
132	/* Get the token (= pid of the affected task). */
133	pid = param64 & LPP_PID_MASK;
134	rcu_read_lock();
135	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
136	if (tsk)
137		get_task_struct(tsk);
138	rcu_read_unlock();
139	if (!tsk)
140		return;
141	spin_lock(&pfault_lock);
142	if (subcode & PF_COMPLETE) {
143		/* signal bit is set -> a page has been swapped in by VM */
144		if (tsk->thread.pfault_wait == 1) {
145			/*
146			 * Initial interrupt was faster than the completion
147			 * interrupt. pfault_wait is valid. Set pfault_wait
148			 * back to zero and wake up the process. This can
149			 * safely be done because the task is still sleeping
150			 * and can't produce new pfaults.
151			 */
152			tsk->thread.pfault_wait = 0;
153			list_del(&tsk->thread.list);
154			wake_up_process(tsk);
155			put_task_struct(tsk);
156		} else {
157			/*
158			 * Completion interrupt was faster than initial
159			 * interrupt. Set pfault_wait to -1 so the initial
160			 * interrupt doesn't put the task to sleep.
161			 * If the task is not running, ignore the completion
162			 * interrupt since it must be a leftover of a PFAULT
163			 * CANCEL operation which didn't remove all pending
164			 * completion interrupts.
165			 */
166			if (task_is_running(tsk))
167				tsk->thread.pfault_wait = -1;
168		}
169	} else {
170		/* signal bit not set -> a real page is missing. */
171		if (WARN_ON_ONCE(tsk != current))
172			goto out;
173		if (tsk->thread.pfault_wait == 1) {
174			/* Already on the list with a reference: put to sleep */
175			goto block;
176		} else if (tsk->thread.pfault_wait == -1) {
177			/*
178			 * Completion interrupt was faster than the initial
179			 * interrupt (pfault_wait == -1). Set pfault_wait
180			 * back to zero and exit.
181			 */
182			tsk->thread.pfault_wait = 0;
183		} else {
184			/*
185			 * Initial interrupt arrived before completion
186			 * interrupt. Let the task sleep.
187			 * An extra task reference is needed since a different
188			 * cpu may set the task state to TASK_RUNNING again
189			 * before the scheduler is reached.
190			 */
191			get_task_struct(tsk);
192			tsk->thread.pfault_wait = 1;
193			list_add(&tsk->thread.list, &pfault_list);
194block:
195			/*
196			 * Since this must be a userspace fault, there
197			 * is no kernel task state to trample. Rely on the
198			 * return to userspace schedule() to block.
199			 */
200			__set_current_state(TASK_UNINTERRUPTIBLE);
201			set_tsk_need_resched(tsk);
202			set_preempt_need_resched();
203		}
204	}
205out:
206	spin_unlock(&pfault_lock);
207	put_task_struct(tsk);
208}
209
210static int pfault_cpu_dead(unsigned int cpu)
211{
212	struct thread_struct *thread, *next;
213	struct task_struct *tsk;
214
215	spin_lock_irq(&pfault_lock);
216	list_for_each_entry_safe(thread, next, &pfault_list, list) {
217		thread->pfault_wait = 0;
218		list_del(&thread->list);
219		tsk = container_of(thread, struct task_struct, thread);
220		wake_up_process(tsk);
221		put_task_struct(tsk);
222	}
223	spin_unlock_irq(&pfault_lock);
224	return 0;
225}
226
227static int __init pfault_irq_init(void)
228{
229	int rc;
230
231	rc = register_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
232	if (rc)
233		goto out_extint;
234	rc = pfault_init() == 0 ? 0 : -EOPNOTSUPP;
235	if (rc)
236		goto out_pfault;
237	irq_subclass_register(IRQ_SUBCLASS_SERVICE_SIGNAL);
238	cpuhp_setup_state_nocalls(CPUHP_S390_PFAULT_DEAD, "s390/pfault:dead",
239				  NULL, pfault_cpu_dead);
240	return 0;
241
242out_pfault:
243	unregister_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
244out_extint:
245	pfault_disable = 1;
246	return rc;
247}
248early_initcall(pfault_irq_init);