root/arch/s390/mm/fault.c

DEFINITIONS

1. fault_init
2. get_fault_type
3. bad_address
4. dump_pagetable
5. dump_fault_info
6. report_user_fault
7. do_sigsegv
8. s390_search_extables
9. do_no_context
10. do_low_address
11. do_sigbus
12. signal_return
13. do_fault_error
14. do_exception
15. do_protection_exception
16. do_dat_exception
17. nopfault
18. pfault_init
19. pfault_fini
20. pfault_interrupt
21. pfault_cpu_dead
22. pfault_irq_init

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *  S390 version
   4  *    Copyright IBM Corp. 1999
   5  *    Author(s): Hartmut Penner (hp@de.ibm.com)
   6  *               Ulrich Weigand (uweigand@de.ibm.com)
   7  *
   8  *  Derived from "arch/i386/mm/fault.c"
   9  *    Copyright (C) 1995  Linus Torvalds
  10  */
  11 
  12 #include <linux/kernel_stat.h>
  13 #include <linux/perf_event.h>
  14 #include <linux/signal.h>
  15 #include <linux/sched.h>
  16 #include <linux/sched/debug.h>
  17 #include <linux/kernel.h>
  18 #include <linux/errno.h>
  19 #include <linux/string.h>
  20 #include <linux/types.h>
  21 #include <linux/ptrace.h>
  22 #include <linux/mman.h>
  23 #include <linux/mm.h>
  24 #include <linux/compat.h>
  25 #include <linux/smp.h>
  26 #include <linux/kdebug.h>
  27 #include <linux/init.h>
  28 #include <linux/console.h>
  29 #include <linux/extable.h>
  30 #include <linux/hardirq.h>
  31 #include <linux/kprobes.h>
  32 #include <linux/uaccess.h>
  33 #include <linux/hugetlb.h>
  34 #include <asm/asm-offsets.h>
  35 #include <asm/diag.h>
  36 #include <asm/pgtable.h>
  37 #include <asm/gmap.h>
  38 #include <asm/irq.h>
  39 #include <asm/mmu_context.h>
  40 #include <asm/facility.h>
  41 #include "../kernel/entry.h"
  42 
  43 #define __FAIL_ADDR_MASK -4096L
  44 #define __SUBCODE_MASK 0x0600
  45 #define __PF_RES_FIELD 0x8000000000000000ULL
  46 
  47 #define VM_FAULT_BADCONTEXT     0x010000
  48 #define VM_FAULT_BADMAP         0x020000
  49 #define VM_FAULT_BADACCESS      0x040000
  50 #define VM_FAULT_SIGNAL         0x080000
  51 #define VM_FAULT_PFAULT         0x100000
  52 
  53 enum fault_type {
  54         KERNEL_FAULT,
  55         USER_FAULT,
  56         VDSO_FAULT,
  57         GMAP_FAULT,
  58 };
  59 
  60 static unsigned long store_indication __read_mostly;
  61 
  62 static int __init fault_init(void)
  63 {
  64         if (test_facility(75))
  65                 store_indication = 0xc00;
  66         return 0;
  67 }
  68 early_initcall(fault_init);
  69 
  70 /*
  71  * Find out which address space caused the exception.
  72  */
  73 static enum fault_type get_fault_type(struct pt_regs *regs)
  74 {
  75         unsigned long trans_exc_code;
  76 
  77         trans_exc_code = regs->int_parm_long & 3;
  78         if (likely(trans_exc_code == 0)) {
  79                 /* primary space exception */
  80                 if (IS_ENABLED(CONFIG_PGSTE) &&
  81                     test_pt_regs_flag(regs, PIF_GUEST_FAULT))
  82                         return GMAP_FAULT;
  83                 if (current->thread.mm_segment == USER_DS)
  84                         return USER_FAULT;
  85                 return KERNEL_FAULT;
  86         }
  87         if (trans_exc_code == 2) {
  88                 /* secondary space exception */
  89                 if (current->thread.mm_segment & 1) {
  90                         if (current->thread.mm_segment == USER_DS_SACF)
  91                                 return USER_FAULT;
  92                         return KERNEL_FAULT;
  93                 }
  94                 return VDSO_FAULT;
  95         }
  96         if (trans_exc_code == 1) {
  97                 /* access register mode, not used in the kernel */
  98                 return USER_FAULT;
  99         }
 100         /* home space exception -> access via kernel ASCE */
 101         return KERNEL_FAULT;
 102 }
 103 
 104 static int bad_address(void *p)
 105 {
 106         unsigned long dummy;
 107 
 108         return probe_kernel_address((unsigned long *)p, dummy);
 109 }
 110 
 111 static void dump_pagetable(unsigned long asce, unsigned long address)
 112 {
 113         unsigned long *table = __va(asce & _ASCE_ORIGIN);
 114 
 115         pr_alert("AS:%016lx ", asce);
 116         switch (asce & _ASCE_TYPE_MASK) {
 117         case _ASCE_TYPE_REGION1:
 118                 table += (address & _REGION1_INDEX) >> _REGION1_SHIFT;
 119                 if (bad_address(table))
 120                         goto bad;
 121                 pr_cont("R1:%016lx ", *table);
 122                 if (*table & _REGION_ENTRY_INVALID)
 123                         goto out;
 124                 table = (unsigned long *)(*table & _REGION_ENTRY_ORIGIN);
 125                 /* fallthrough */
 126         case _ASCE_TYPE_REGION2:
 127                 table += (address & _REGION2_INDEX) >> _REGION2_SHIFT;
 128                 if (bad_address(table))
 129                         goto bad;
 130                 pr_cont("R2:%016lx ", *table);
 131                 if (*table & _REGION_ENTRY_INVALID)
 132                         goto out;
 133                 table = (unsigned long *)(*table & _REGION_ENTRY_ORIGIN);
 134                 /* fallthrough */
 135         case _ASCE_TYPE_REGION3:
 136                 table += (address & _REGION3_INDEX) >> _REGION3_SHIFT;
 137                 if (bad_address(table))
 138                         goto bad;
 139                 pr_cont("R3:%016lx ", *table);
 140                 if (*table & (_REGION_ENTRY_INVALID | _REGION3_ENTRY_LARGE))
 141                         goto out;
 142                 table = (unsigned long *)(*table & _REGION_ENTRY_ORIGIN);
 143                 /* fallthrough */
 144         case _ASCE_TYPE_SEGMENT:
 145                 table += (address & _SEGMENT_INDEX) >> _SEGMENT_SHIFT;
 146                 if (bad_address(table))
 147                         goto bad;
 148                 pr_cont("S:%016lx ", *table);
 149                 if (*table & (_SEGMENT_ENTRY_INVALID | _SEGMENT_ENTRY_LARGE))
 150                         goto out;
 151                 table = (unsigned long *)(*table & _SEGMENT_ENTRY_ORIGIN);
 152         }
 153         table += (address & _PAGE_INDEX) >> _PAGE_SHIFT;
 154         if (bad_address(table))
 155                 goto bad;
 156         pr_cont("P:%016lx ", *table);
 157 out:
 158         pr_cont("\n");
 159         return;
 160 bad:
 161         pr_cont("BAD\n");
 162 }
 163 
 164 static void dump_fault_info(struct pt_regs *regs)
 165 {
 166         unsigned long asce;
 167 
 168         pr_alert("Failing address: %016lx TEID: %016lx\n",
 169                  regs->int_parm_long & __FAIL_ADDR_MASK, regs->int_parm_long);
 170         pr_alert("Fault in ");
 171         switch (regs->int_parm_long & 3) {
 172         case 3:
 173                 pr_cont("home space ");
 174                 break;
 175         case 2:
 176                 pr_cont("secondary space ");
 177                 break;
 178         case 1:
 179                 pr_cont("access register ");
 180                 break;
 181         case 0:
 182                 pr_cont("primary space ");
 183                 break;
 184         }
 185         pr_cont("mode while using ");
 186         switch (get_fault_type(regs)) {
 187         case USER_FAULT:
 188                 asce = S390_lowcore.user_asce;
 189                 pr_cont("user ");
 190                 break;
 191         case VDSO_FAULT:
 192                 asce = S390_lowcore.vdso_asce;
 193                 pr_cont("vdso ");
 194                 break;
 195         case GMAP_FAULT:
 196                 asce = ((struct gmap *) S390_lowcore.gmap)->asce;
 197                 pr_cont("gmap ");
 198                 break;
 199         case KERNEL_FAULT:
 200                 asce = S390_lowcore.kernel_asce;
 201                 pr_cont("kernel ");
 202                 break;
 203         default:
 204                 unreachable();
 205         }
 206         pr_cont("ASCE.\n");
 207         dump_pagetable(asce, regs->int_parm_long & __FAIL_ADDR_MASK);
 208 }
 209 
 210 int show_unhandled_signals = 1;
 211 
 212 void report_user_fault(struct pt_regs *regs, long signr, int is_mm_fault)
 213 {
 214         if ((task_pid_nr(current) > 1) && !show_unhandled_signals)
 215                 return;
 216         if (!unhandled_signal(current, signr))
 217                 return;
 218         if (!printk_ratelimit())
 219                 return;
 220         printk(KERN_ALERT "User process fault: interruption code %04x ilc:%d ",
 221                regs->int_code & 0xffff, regs->int_code >> 17);
 222         print_vma_addr(KERN_CONT "in ", regs->psw.addr);
 223         printk(KERN_CONT "\n");
 224         if (is_mm_fault)
 225                 dump_fault_info(regs);
 226         show_regs(regs);
 227 }
 228 
 229 /*
 230  * Send SIGSEGV to task.  This is an external routine
 231  * to keep the stack usage of do_page_fault small.
 232  */
 233 static noinline void do_sigsegv(struct pt_regs *regs, int si_code)
 234 {
 235         report_user_fault(regs, SIGSEGV, 1);
 236         force_sig_fault(SIGSEGV, si_code,
 237                         (void __user *)(regs->int_parm_long & __FAIL_ADDR_MASK));
 238 }
 239 
 240 const struct exception_table_entry *s390_search_extables(unsigned long addr)
 241 {
 242         const struct exception_table_entry *fixup;
 243 
 244         fixup = search_extable(__start_dma_ex_table,
 245                                __stop_dma_ex_table - __start_dma_ex_table,
 246                                addr);
 247         if (!fixup)
 248                 fixup = search_exception_tables(addr);
 249         return fixup;
 250 }
 251 
 252 static noinline void do_no_context(struct pt_regs *regs)
 253 {
 254         const struct exception_table_entry *fixup;
 255 
 256         /* Are we prepared to handle this kernel fault?  */
 257         fixup = s390_search_extables(regs->psw.addr);
 258         if (fixup) {
 259                 regs->psw.addr = extable_fixup(fixup);
 260                 return;
 261         }
 262 
 263         /*
 264          * Oops. The kernel tried to access some bad page. We'll have to
 265          * terminate things with extreme prejudice.
 266          */
 267         if (get_fault_type(regs) == KERNEL_FAULT)
 268                 printk(KERN_ALERT "Unable to handle kernel pointer dereference"
 269                        " in virtual kernel address space\n");
 270         else
 271                 printk(KERN_ALERT "Unable to handle kernel paging request"
 272                        " in virtual user address space\n");
 273         dump_fault_info(regs);
 274         die(regs, "Oops");
 275         do_exit(SIGKILL);
 276 }
 277 
 278 static noinline void do_low_address(struct pt_regs *regs)
 279 {
 280         /* Low-address protection hit in kernel mode means
 281            NULL pointer write access in kernel mode.  */
 282         if (regs->psw.mask & PSW_MASK_PSTATE) {
 283                 /* Low-address protection hit in user mode 'cannot happen'. */
 284                 die (regs, "Low-address protection");
 285                 do_exit(SIGKILL);
 286         }
 287 
 288         do_no_context(regs);
 289 }
 290 
 291 static noinline void do_sigbus(struct pt_regs *regs)
 292 {
 293         /*
 294          * Send a sigbus, regardless of whether we were in kernel
 295          * or user mode.
 296          */
 297         force_sig_fault(SIGBUS, BUS_ADRERR,
 298                         (void __user *)(regs->int_parm_long & __FAIL_ADDR_MASK));
 299 }
 300 
 301 static noinline int signal_return(struct pt_regs *regs)
 302 {
 303         u16 instruction;
 304         int rc;
 305 
 306         rc = __get_user(instruction, (u16 __user *) regs->psw.addr);
 307         if (rc)
 308                 return rc;
 309         if (instruction == 0x0a77) {
 310                 set_pt_regs_flag(regs, PIF_SYSCALL);
 311                 regs->int_code = 0x00040077;
 312                 return 0;
 313         } else if (instruction == 0x0aad) {
 314                 set_pt_regs_flag(regs, PIF_SYSCALL);
 315                 regs->int_code = 0x000400ad;
 316                 return 0;
 317         }
 318         return -EACCES;
 319 }
 320 
 321 static noinline void do_fault_error(struct pt_regs *regs, int access,
 322                                         vm_fault_t fault)
 323 {
 324         int si_code;
 325 
 326         switch (fault) {
 327         case VM_FAULT_BADACCESS:
 328                 if (access == VM_EXEC && signal_return(regs) == 0)
 329                         break;
 330                 /* fallthrough */
 331         case VM_FAULT_BADMAP:
 332                 /* Bad memory access. Check if it is kernel or user space. */
 333                 if (user_mode(regs)) {
 334                         /* User mode accesses just cause a SIGSEGV */
 335                         si_code = (fault == VM_FAULT_BADMAP) ?
 336                                 SEGV_MAPERR : SEGV_ACCERR;
 337                         do_sigsegv(regs, si_code);
 338                         break;
 339                 }
 340                 /* fallthrough */
 341         case VM_FAULT_BADCONTEXT:
 342                 /* fallthrough */
 343         case VM_FAULT_PFAULT:
 344                 do_no_context(regs);
 345                 break;
 346         case VM_FAULT_SIGNAL:
 347                 if (!user_mode(regs))
 348                         do_no_context(regs);
 349                 break;
 350         default: /* fault & VM_FAULT_ERROR */
 351                 if (fault & VM_FAULT_OOM) {
 352                         if (!user_mode(regs))
 353                                 do_no_context(regs);
 354                         else
 355                                 pagefault_out_of_memory();
 356                 } else if (fault & VM_FAULT_SIGSEGV) {
 357                         /* Kernel mode? Handle exceptions or die */
 358                         if (!user_mode(regs))
 359                                 do_no_context(regs);
 360                         else
 361                                 do_sigsegv(regs, SEGV_MAPERR);
 362                 } else if (fault & VM_FAULT_SIGBUS) {
 363                         /* Kernel mode? Handle exceptions or die */
 364                         if (!user_mode(regs))
 365                                 do_no_context(regs);
 366                         else
 367                                 do_sigbus(regs);
 368                 } else
 369                         BUG();
 370                 break;
 371         }
 372 }
 373 
 374 /*
 375  * This routine handles page faults.  It determines the address,
 376  * and the problem, and then passes it off to one of the appropriate
 377  * routines.
 378  *
 379  * interruption code (int_code):
 380  *   04       Protection           ->  Write-Protection  (suprression)
 381  *   10       Segment translation  ->  Not present       (nullification)
 382  *   11       Page translation     ->  Not present       (nullification)
 383  *   3b       Region third trans.  ->  Not present       (nullification)
 384  */
 385 static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 386 {
 387         struct gmap *gmap;
 388         struct task_struct *tsk;
 389         struct mm_struct *mm;
 390         struct vm_area_struct *vma;
 391         enum fault_type type;
 392         unsigned long trans_exc_code;
 393         unsigned long address;
 394         unsigned int flags;
 395         vm_fault_t fault;
 396 
 397         tsk = current;
 398         /*
 399          * The instruction that caused the program check has
 400          * been nullified. Don't signal single step via SIGTRAP.
 401          */
 402         clear_pt_regs_flag(regs, PIF_PER_TRAP);
 403 
 404         if (kprobe_page_fault(regs, 14))
 405                 return 0;
 406 
 407         mm = tsk->mm;
 408         trans_exc_code = regs->int_parm_long;
 409 
 410         /*
 411          * Verify that the fault happened in user space, that
 412          * we are not in an interrupt and that there is a 
 413          * user context.
 414          */
 415         fault = VM_FAULT_BADCONTEXT;
 416         type = get_fault_type(regs);
 417         switch (type) {
 418         case KERNEL_FAULT:
 419                 goto out;
 420         case VDSO_FAULT:
 421                 fault = VM_FAULT_BADMAP;
 422                 goto out;
 423         case USER_FAULT:
 424         case GMAP_FAULT:
 425                 if (faulthandler_disabled() || !mm)
 426                         goto out;
 427                 break;
 428         }
 429 
 430         address = trans_exc_code & __FAIL_ADDR_MASK;
 431         perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
 432         flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 433         if (user_mode(regs))
 434                 flags |= FAULT_FLAG_USER;
 435         if (access == VM_WRITE || (trans_exc_code & store_indication) == 0x400)
 436                 flags |= FAULT_FLAG_WRITE;
 437         down_read(&mm->mmap_sem);
 438 
 439         gmap = NULL;
 440         if (IS_ENABLED(CONFIG_PGSTE) && type == GMAP_FAULT) {
 441                 gmap = (struct gmap *) S390_lowcore.gmap;
 442                 current->thread.gmap_addr = address;
 443                 current->thread.gmap_write_flag = !!(flags & FAULT_FLAG_WRITE);
 444                 current->thread.gmap_int_code = regs->int_code & 0xffff;
 445                 address = __gmap_translate(gmap, address);
 446                 if (address == -EFAULT) {
 447                         fault = VM_FAULT_BADMAP;
 448                         goto out_up;
 449                 }
 450                 if (gmap->pfault_enabled)
 451                         flags |= FAULT_FLAG_RETRY_NOWAIT;
 452         }
 453 
 454 retry:
 455         fault = VM_FAULT_BADMAP;
 456         vma = find_vma(mm, address);
 457         if (!vma)
 458                 goto out_up;
 459 
 460         if (unlikely(vma->vm_start > address)) {
 461                 if (!(vma->vm_flags & VM_GROWSDOWN))
 462                         goto out_up;
 463                 if (expand_stack(vma, address))
 464                         goto out_up;
 465         }
 466 
 467         /*
 468          * Ok, we have a good vm_area for this memory access, so
 469          * we can handle it..
 470          */
 471         fault = VM_FAULT_BADACCESS;
 472         if (unlikely(!(vma->vm_flags & access)))
 473                 goto out_up;
 474 
 475         if (is_vm_hugetlb_page(vma))
 476                 address &= HPAGE_MASK;
 477         /*
 478          * If for any reason at all we couldn't handle the fault,
 479          * make sure we exit gracefully rather than endlessly redo
 480          * the fault.
 481          */
 482         fault = handle_mm_fault(vma, address, flags);
 483         /* No reason to continue if interrupted by SIGKILL. */
 484         if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
 485                 fault = VM_FAULT_SIGNAL;
 486                 if (flags & FAULT_FLAG_RETRY_NOWAIT)
 487                         goto out_up;
 488                 goto out;
 489         }
 490         if (unlikely(fault & VM_FAULT_ERROR))
 491                 goto out_up;
 492 
 493         /*
 494          * Major/minor page fault accounting is only done on the
 495          * initial attempt. If we go through a retry, it is extremely
 496          * likely that the page will be found in page cache at that point.
 497          */
 498         if (flags & FAULT_FLAG_ALLOW_RETRY) {
 499                 if (fault & VM_FAULT_MAJOR) {
 500                         tsk->maj_flt++;
 501                         perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
 502                                       regs, address);
 503                 } else {
 504                         tsk->min_flt++;
 505                         perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
 506                                       regs, address);
 507                 }
 508                 if (fault & VM_FAULT_RETRY) {
 509                         if (IS_ENABLED(CONFIG_PGSTE) && gmap &&
 510                             (flags & FAULT_FLAG_RETRY_NOWAIT)) {
 511                                 /* FAULT_FLAG_RETRY_NOWAIT has been set,
 512                                  * mmap_sem has not been released */
 513                                 current->thread.gmap_pfault = 1;
 514                                 fault = VM_FAULT_PFAULT;
 515                                 goto out_up;
 516                         }
 517                         /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
 518                          * of starvation. */
 519                         flags &= ~(FAULT_FLAG_ALLOW_RETRY |
 520                                    FAULT_FLAG_RETRY_NOWAIT);
 521                         flags |= FAULT_FLAG_TRIED;
 522                         down_read(&mm->mmap_sem);
 523                         goto retry;
 524                 }
 525         }
 526         if (IS_ENABLED(CONFIG_PGSTE) && gmap) {
 527                 address =  __gmap_link(gmap, current->thread.gmap_addr,
 528                                        address);
 529                 if (address == -EFAULT) {
 530                         fault = VM_FAULT_BADMAP;
 531                         goto out_up;
 532                 }
 533                 if (address == -ENOMEM) {
 534                         fault = VM_FAULT_OOM;
 535                         goto out_up;
 536                 }
 537         }
 538         fault = 0;
 539 out_up:
 540         up_read(&mm->mmap_sem);
 541 out:
 542         return fault;
 543 }
 544 
 545 void do_protection_exception(struct pt_regs *regs)
 546 {
 547         unsigned long trans_exc_code;
 548         int access;
 549         vm_fault_t fault;
 550 
 551         trans_exc_code = regs->int_parm_long;
 552         /*
 553          * Protection exceptions are suppressing, decrement psw address.
 554          * The exception to this rule are aborted transactions, for these
 555          * the PSW already points to the correct location.
 556          */
 557         if (!(regs->int_code & 0x200))
 558                 regs->psw.addr = __rewind_psw(regs->psw, regs->int_code >> 16);
 559         /*
 560          * Check for low-address protection.  This needs to be treated
 561          * as a special case because the translation exception code
 562          * field is not guaranteed to contain valid data in this case.
 563          */
 564         if (unlikely(!(trans_exc_code & 4))) {
 565                 do_low_address(regs);
 566                 return;
 567         }
 568         if (unlikely(MACHINE_HAS_NX && (trans_exc_code & 0x80))) {
 569                 regs->int_parm_long = (trans_exc_code & ~PAGE_MASK) |
 570                                         (regs->psw.addr & PAGE_MASK);
 571                 access = VM_EXEC;
 572                 fault = VM_FAULT_BADACCESS;
 573         } else {
 574                 access = VM_WRITE;
 575                 fault = do_exception(regs, access);
 576         }
 577         if (unlikely(fault))
 578                 do_fault_error(regs, access, fault);
 579 }
 580 NOKPROBE_SYMBOL(do_protection_exception);
 581 
 582 void do_dat_exception(struct pt_regs *regs)
 583 {
 584         int access;
 585         vm_fault_t fault;
 586 
 587         access = VM_READ | VM_EXEC | VM_WRITE;
 588         fault = do_exception(regs, access);
 589         if (unlikely(fault))
 590                 do_fault_error(regs, access, fault);
 591 }
 592 NOKPROBE_SYMBOL(do_dat_exception);
 593 
 594 #ifdef CONFIG_PFAULT 
 595 /*
 596  * 'pfault' pseudo page faults routines.
 597  */
 598 static int pfault_disable;
 599 
 600 static int __init nopfault(char *str)
 601 {
 602         pfault_disable = 1;
 603         return 1;
 604 }
 605 
 606 __setup("nopfault", nopfault);
 607 
 608 struct pfault_refbk {
 609         u16 refdiagc;
 610         u16 reffcode;
 611         u16 refdwlen;
 612         u16 refversn;
 613         u64 refgaddr;
 614         u64 refselmk;
 615         u64 refcmpmk;
 616         u64 reserved;
 617 } __attribute__ ((packed, aligned(8)));
 618 
 619 static struct pfault_refbk pfault_init_refbk = {
 620         .refdiagc = 0x258,
 621         .reffcode = 0,
 622         .refdwlen = 5,
 623         .refversn = 2,
 624         .refgaddr = __LC_LPP,
 625         .refselmk = 1ULL << 48,
 626         .refcmpmk = 1ULL << 48,
 627         .reserved = __PF_RES_FIELD
 628 };
 629 
 630 int pfault_init(void)
 631 {
 632         int rc;
 633 
 634         if (pfault_disable)
 635                 return -1;
 636         diag_stat_inc(DIAG_STAT_X258);
 637         asm volatile(
 638                 "       diag    %1,%0,0x258\n"
 639                 "0:     j       2f\n"
 640                 "1:     la      %0,8\n"
 641                 "2:\n"
 642                 EX_TABLE(0b,1b)
 643                 : "=d" (rc)
 644                 : "a" (&pfault_init_refbk), "m" (pfault_init_refbk) : "cc");
 645         return rc;
 646 }
 647 
 648 static struct pfault_refbk pfault_fini_refbk = {
 649         .refdiagc = 0x258,
 650         .reffcode = 1,
 651         .refdwlen = 5,
 652         .refversn = 2,
 653 };
 654 
 655 void pfault_fini(void)
 656 {
 657 
 658         if (pfault_disable)
 659                 return;
 660         diag_stat_inc(DIAG_STAT_X258);
 661         asm volatile(
 662                 "       diag    %0,0,0x258\n"
 663                 "0:     nopr    %%r7\n"
 664                 EX_TABLE(0b,0b)
 665                 : : "a" (&pfault_fini_refbk), "m" (pfault_fini_refbk) : "cc");
 666 }
 667 
 668 static DEFINE_SPINLOCK(pfault_lock);
 669 static LIST_HEAD(pfault_list);
 670 
 671 #define PF_COMPLETE     0x0080
 672 
 673 /*
 674  * The mechanism of our pfault code: if Linux is running as guest, runs a user
 675  * space process and the user space process accesses a page that the host has
 676  * paged out we get a pfault interrupt.
 677  *
 678  * This allows us, within the guest, to schedule a different process. Without
 679  * this mechanism the host would have to suspend the whole virtual cpu until
 680  * the page has been paged in.
 681  *
 682  * So when we get such an interrupt then we set the state of the current task
 683  * to uninterruptible and also set the need_resched flag. Both happens within
 684  * interrupt context(!). If we later on want to return to user space we
 685  * recognize the need_resched flag and then call schedule().  It's not very
 686  * obvious how this works...
 687  *
 688  * Of course we have a lot of additional fun with the completion interrupt (->
 689  * host signals that a page of a process has been paged in and the process can
 690  * continue to run). This interrupt can arrive on any cpu and, since we have
 691  * virtual cpus, actually appear before the interrupt that signals that a page
 692  * is missing.
 693  */
 694 static void pfault_interrupt(struct ext_code ext_code,
 695                              unsigned int param32, unsigned long param64)
 696 {
 697         struct task_struct *tsk;
 698         __u16 subcode;
 699         pid_t pid;
 700 
 701         /*
 702          * Get the external interruption subcode & pfault initial/completion
 703          * signal bit. VM stores this in the 'cpu address' field associated
 704          * with the external interrupt.
 705          */
 706         subcode = ext_code.subcode;
 707         if ((subcode & 0xff00) != __SUBCODE_MASK)
 708                 return;
 709         inc_irq_stat(IRQEXT_PFL);
 710         /* Get the token (= pid of the affected task). */
 711         pid = param64 & LPP_PID_MASK;
 712         rcu_read_lock();
 713         tsk = find_task_by_pid_ns(pid, &init_pid_ns);
 714         if (tsk)
 715                 get_task_struct(tsk);
 716         rcu_read_unlock();
 717         if (!tsk)
 718                 return;
 719         spin_lock(&pfault_lock);
 720         if (subcode & PF_COMPLETE) {
 721                 /* signal bit is set -> a page has been swapped in by VM */
 722                 if (tsk->thread.pfault_wait == 1) {
 723                         /* Initial interrupt was faster than the completion
 724                          * interrupt. pfault_wait is valid. Set pfault_wait
 725                          * back to zero and wake up the process. This can
 726                          * safely be done because the task is still sleeping
 727                          * and can't produce new pfaults. */
 728                         tsk->thread.pfault_wait = 0;
 729                         list_del(&tsk->thread.list);
 730                         wake_up_process(tsk);
 731                         put_task_struct(tsk);
 732                 } else {
 733                         /* Completion interrupt was faster than initial
 734                          * interrupt. Set pfault_wait to -1 so the initial
 735                          * interrupt doesn't put the task to sleep.
 736                          * If the task is not running, ignore the completion
 737                          * interrupt since it must be a leftover of a PFAULT
 738                          * CANCEL operation which didn't remove all pending
 739                          * completion interrupts. */
 740                         if (tsk->state == TASK_RUNNING)
 741                                 tsk->thread.pfault_wait = -1;
 742                 }
 743         } else {
 744                 /* signal bit not set -> a real page is missing. */
 745                 if (WARN_ON_ONCE(tsk != current))
 746                         goto out;
 747                 if (tsk->thread.pfault_wait == 1) {
 748                         /* Already on the list with a reference: put to sleep */
 749                         goto block;
 750                 } else if (tsk->thread.pfault_wait == -1) {
 751                         /* Completion interrupt was faster than the initial
 752                          * interrupt (pfault_wait == -1). Set pfault_wait
 753                          * back to zero and exit. */
 754                         tsk->thread.pfault_wait = 0;
 755                 } else {
 756                         /* Initial interrupt arrived before completion
 757                          * interrupt. Let the task sleep.
 758                          * An extra task reference is needed since a different
 759                          * cpu may set the task state to TASK_RUNNING again
 760                          * before the scheduler is reached. */
 761                         get_task_struct(tsk);
 762                         tsk->thread.pfault_wait = 1;
 763                         list_add(&tsk->thread.list, &pfault_list);
 764 block:
 765                         /* Since this must be a userspace fault, there
 766                          * is no kernel task state to trample. Rely on the
 767                          * return to userspace schedule() to block. */
 768                         __set_current_state(TASK_UNINTERRUPTIBLE);
 769                         set_tsk_need_resched(tsk);
 770                         set_preempt_need_resched();
 771                 }
 772         }
 773 out:
 774         spin_unlock(&pfault_lock);
 775         put_task_struct(tsk);
 776 }
 777 
 778 static int pfault_cpu_dead(unsigned int cpu)
 779 {
 780         struct thread_struct *thread, *next;
 781         struct task_struct *tsk;
 782 
 783         spin_lock_irq(&pfault_lock);
 784         list_for_each_entry_safe(thread, next, &pfault_list, list) {
 785                 thread->pfault_wait = 0;
 786                 list_del(&thread->list);
 787                 tsk = container_of(thread, struct task_struct, thread);
 788                 wake_up_process(tsk);
 789                 put_task_struct(tsk);
 790         }
 791         spin_unlock_irq(&pfault_lock);
 792         return 0;
 793 }
 794 
 795 static int __init pfault_irq_init(void)
 796 {
 797         int rc;
 798 
 799         rc = register_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
 800         if (rc)
 801                 goto out_extint;
 802         rc = pfault_init() == 0 ? 0 : -EOPNOTSUPP;
 803         if (rc)
 804                 goto out_pfault;
 805         irq_subclass_register(IRQ_SUBCLASS_SERVICE_SIGNAL);
 806         cpuhp_setup_state_nocalls(CPUHP_S390_PFAULT_DEAD, "s390/pfault:dead",
 807                                   NULL, pfault_cpu_dead);
 808         return 0;
 809 
 810 out_pfault:
 811         unregister_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
 812 out_extint:
 813         pfault_disable = 1;
 814         return rc;
 815 }
 816 early_initcall(pfault_irq_init);
 817 
 818 #endif /* CONFIG_PFAULT */