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 */
21 static int pfault_disable;
22
23 static int __init nopfault(char *str)
24 {
25 pfault_disable = 1;
26 return 1;
27 }
28 early_param("nopfault", nopfault);
29
30 struct 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
41 static 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
52 int __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
69 static struct pfault_refbk pfault_fini_refbk = {
70 .refdiagc = 0x258,
71 .reffcode = 1,
72 .refdwlen = 5,
73 .refversn = 2,
74 };
75
76 void __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
90 static DEFINE_SPINLOCK(pfault_lock);
91 static 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 */
116 static 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);
194 block:
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 }
205 out:
206 spin_unlock(&pfault_lock);
207 put_task_struct(tsk);
208 }
209
210 static 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
227 static 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
242 out_pfault:
243 unregister_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
244 out_extint:
245 pfault_disable = 1;
246 return rc;
247 }
248 early_initcall(pfault_irq_init);
249
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