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/asm-offsets.h>
13#include <asm/pfault.h>
14#include <asm/diag.h>
15
16#define __SUBCODE_MASK 0x0600
17#define __PF_RES_FIELD 0x8000000000000000UL
18
19/*
20 * 'pfault' pseudo page faults routines.
21 */
22static int pfault_disable;
23
24static int __init nopfault(char *str)
25{
26 pfault_disable = 1;
27 return 1;
28}
29early_param("nopfault", nopfault);
30
31struct pfault_refbk {
32 u16 refdiagc;
33 u16 reffcode;
34 u16 refdwlen;
35 u16 refversn;
36 u64 refgaddr;
37 u64 refselmk;
38 u64 refcmpmk;
39 u64 reserved;
40};
41
42static struct pfault_refbk pfault_init_refbk = {
43 .refdiagc = 0x258,
44 .reffcode = 0,
45 .refdwlen = 5,
46 .refversn = 2,
47 .refgaddr = __LC_LPP,
48 .refselmk = 1UL << 48,
49 .refcmpmk = 1UL << 48,
50 .reserved = __PF_RES_FIELD
51};
52
53int __pfault_init(void)
54{
55 int rc = -EOPNOTSUPP;
56
57 if (pfault_disable)
58 return rc;
59 diag_stat_inc(DIAG_STAT_X258);
60 asm_inline volatile(
61 " diag %[refbk],%[rc],0x258\n"
62 "0: nopr %%r7\n"
63 EX_TABLE(0b, 0b)
64 : [rc] "+d" (rc)
65 : [refbk] "a" (&pfault_init_refbk), "m" (pfault_init_refbk)
66 : "cc");
67 return rc;
68}
69
70static struct pfault_refbk pfault_fini_refbk = {
71 .refdiagc = 0x258,
72 .reffcode = 1,
73 .refdwlen = 5,
74 .refversn = 2,
75};
76
77void __pfault_fini(void)
78{
79 if (pfault_disable)
80 return;
81 diag_stat_inc(DIAG_STAT_X258);
82 asm_inline volatile(
83 " diag %[refbk],0,0x258\n"
84 "0: nopr %%r7\n"
85 EX_TABLE(0b, 0b)
86 :
87 : [refbk] "a" (&pfault_fini_refbk), "m" (pfault_fini_refbk)
88 : "cc");
89}
90
91static DEFINE_SPINLOCK(pfault_lock);
92static LIST_HEAD(pfault_list);
93
94#define PF_COMPLETE 0x0080
95
96/*
97 * The mechanism of our pfault code: if Linux is running as guest, runs a user
98 * space process and the user space process accesses a page that the host has
99 * paged out we get a pfault interrupt.
100 *
101 * This allows us, within the guest, to schedule a different process. Without
102 * this mechanism the host would have to suspend the whole virtual cpu until
103 * the page has been paged in.
104 *
105 * So when we get such an interrupt then we set the state of the current task
106 * to uninterruptible and also set the need_resched flag. Both happens within
107 * interrupt context(!). If we later on want to return to user space we
108 * recognize the need_resched flag and then call schedule(). It's not very
109 * obvious how this works...
110 *
111 * Of course we have a lot of additional fun with the completion interrupt (->
112 * host signals that a page of a process has been paged in and the process can
113 * continue to run). This interrupt can arrive on any cpu and, since we have
114 * virtual cpus, actually appear before the interrupt that signals that a page
115 * is missing.
116 */
117static void pfault_interrupt(struct ext_code ext_code,
118 unsigned int param32, unsigned long param64)
119{
120 struct task_struct *tsk;
121 __u16 subcode;
122 pid_t pid;
123
124 /*
125 * Get the external interruption subcode & pfault initial/completion
126 * signal bit. VM stores this in the 'cpu address' field associated
127 * with the external interrupt.
128 */
129 subcode = ext_code.subcode;
130 if ((subcode & 0xff00) != __SUBCODE_MASK)
131 return;
132 inc_irq_stat(IRQEXT_PFL);
133 /* Get the token (= pid of the affected task). */
134 pid = param64 & LPP_PID_MASK;
135 rcu_read_lock();
136 tsk = find_task_by_pid_ns(nr: pid, ns: &init_pid_ns);
137 if (tsk)
138 get_task_struct(t: tsk);
139 rcu_read_unlock();
140 if (!tsk)
141 return;
142 spin_lock(lock: &pfault_lock);
143 if (subcode & PF_COMPLETE) {
144 /* signal bit is set -> a page has been swapped in by VM */
145 if (tsk->thread.pfault_wait == 1) {
146 /*
147 * Initial interrupt was faster than the completion
148 * interrupt. pfault_wait is valid. Set pfault_wait
149 * back to zero and wake up the process. This can
150 * safely be done because the task is still sleeping
151 * and can't produce new pfaults.
152 */
153 tsk->thread.pfault_wait = 0;
154 list_del(entry: &tsk->thread.list);
155 wake_up_process(tsk);
156 put_task_struct(t: tsk);
157 } else {
158 /*
159 * Completion interrupt was faster than initial
160 * interrupt. Set pfault_wait to -1 so the initial
161 * interrupt doesn't put the task to sleep.
162 * If the task is not running, ignore the completion
163 * interrupt since it must be a leftover of a PFAULT
164 * CANCEL operation which didn't remove all pending
165 * completion interrupts.
166 */
167 if (task_is_running(tsk))
168 tsk->thread.pfault_wait = -1;
169 }
170 } else {
171 /* signal bit not set -> a real page is missing. */
172 if (WARN_ON_ONCE(tsk != current))
173 goto out;
174 if (tsk->thread.pfault_wait == 1) {
175 /* Already on the list with a reference: put to sleep */
176 goto block;
177 } else if (tsk->thread.pfault_wait == -1) {
178 /*
179 * Completion interrupt was faster than the initial
180 * interrupt (pfault_wait == -1). Set pfault_wait
181 * back to zero and exit.
182 */
183 tsk->thread.pfault_wait = 0;
184 } else {
185 /*
186 * Initial interrupt arrived before completion
187 * interrupt. Let the task sleep.
188 * An extra task reference is needed since a different
189 * cpu may set the task state to TASK_RUNNING again
190 * before the scheduler is reached.
191 */
192 get_task_struct(t: tsk);
193 tsk->thread.pfault_wait = 1;
194 list_add(new: &tsk->thread.list, head: &pfault_list);
195block:
196 /*
197 * Since this must be a userspace fault, there
198 * is no kernel task state to trample. Rely on the
199 * return to userspace schedule() to block.
200 */
201 __set_current_state(TASK_UNINTERRUPTIBLE);
202 set_need_resched_current();
203 }
204 }
205out:
206 spin_unlock(lock: &pfault_lock);
207 put_task_struct(t: 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(lock: &pfault_lock);
216 list_for_each_entry_safe(thread, next, &pfault_list, list) {
217 thread->pfault_wait = 0;
218 list_del(entry: &thread->list);
219 tsk = container_of(thread, struct task_struct, thread);
220 wake_up_process(tsk);
221 put_task_struct(t: tsk);
222 }
223 spin_unlock_irq(lock: &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(state: CPUHP_S390_PFAULT_DEAD, name: "s390/pfault:dead",
239 NULL, teardown: 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);
249

source code of linux/arch/s390/mm/pfault.c