Diff below can be seen as 3 logical parts that together change the
current *select(2) implementation:

  - Create & destroy a per-thread kqueue in fork1() and exit1().

  - Change the kqueue_scan() interface to keep track of the end point of
    a scan, this is mostly from visa@.

  - Change dopselect() to translate inputs "fd_set" into "struct kevent",
    register them via kqueue_register(), wait for events via kqueue_scan()
    then translate them back.

A visible change is that programs waiting in *select(2) will now sleep
on the "kqread" or "kqsel" channel.

Please test, comment and report back.

Index: kern/kern_event.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_event.c,v
retrieving revision 1.140
diff -u -p -r1.140 kern_event.c
--- kern/kern_event.c   22 Jun 2020 13:14:32 -0000      1.140
+++ kern/kern_event.c   23 Jun 2020 08:44:47 -0000
@@ -64,9 +64,6 @@ void  KQREF(struct kqueue *);
 void   KQRELE(struct kqueue *);
 
 int    kqueue_sleep(struct kqueue *, struct timespec *);
-int    kqueue_scan(struct kqueue *kq, int maxevents,
-                   struct kevent *ulistp, struct timespec *timeout,
-                   struct proc *p, int *retval);
 
 int    kqueue_read(struct file *, struct uio *, int);
 int    kqueue_write(struct file *, struct uio *, int);
@@ -521,6 +518,14 @@ kqueue_alloc(struct filedesc *fdp)
        return (kq);
 }
 
+void
+kqueue_exit(struct proc *p)
+{
+       kqueue_terminate(p, p->p_kq);
+       kqueue_free(p->p_kq);
+       p->p_kq = NULL;
+}
+
 int
 sys_kqueue(struct proc *p, void *v, register_t *retval)
 {
@@ -554,6 +559,7 @@ out:
 int
 sys_kevent(struct proc *p, void *v, register_t *retval)
 {
+       struct kqueue_scan_state scan;
        struct filedesc* fdp = p->p_fd;
        struct sys_kevent_args /* {
                syscallarg(int) fd;
@@ -569,6 +575,7 @@ sys_kevent(struct proc *p, void *v, regi
        struct timespec ts;
        struct timespec *tsp = NULL;
        int i, n, nerrors, error;
+       int ready, total;
        struct kevent kev[KQ_NEVENTS];
 
        if ((fp = fd_getfile(fdp, SCARG(uap, fd))) == NULL)
@@ -597,9 +604,9 @@ sys_kevent(struct proc *p, void *v, regi
        kq = fp->f_data;
        nerrors = 0;
 
-       while (SCARG(uap, nchanges) > 0) {
-               n = SCARG(uap, nchanges) > KQ_NEVENTS ?
-                   KQ_NEVENTS : SCARG(uap, nchanges);
+       while ((n = SCARG(uap, nchanges)) > 0) {
+               if (n > nitems(kev))
+                       n = nitems(kev);
                error = copyin(SCARG(uap, changelist), kev,
                    n * sizeof(struct kevent));
                if (error)
@@ -635,12 +642,39 @@ sys_kevent(struct proc *p, void *v, regi
                goto done;
        }
 
+
        KQREF(kq);
        FRELE(fp, p);
-       error = kqueue_scan(kq, SCARG(uap, nevents), SCARG(uap, eventlist),
-           tsp, p, &n);
+       /*
+        * Collect as many events as we can.  The timeout on successive
+        * loops is disabled (kqueue_scan() becomes non-blocking).
+        */
+       total = 0;
+       error = 0;
+       kqueue_scan_setup(&scan, kq);
+       while ((n = SCARG(uap, nevents) - total) > 0) {
+               if (n > nitems(kev))
+                       n = nitems(kev);
+               ready = kqueue_scan(&scan, n, kev, tsp, p, &error);
+               if (ready == 0)
+                       break;
+               error = copyout(kev, SCARG(uap, eventlist) + total,
+                   sizeof(struct kevent) * ready);
+#ifdef KTRACE
+               if (KTRPOINT(p, KTR_STRUCT))
+                       ktrevent(p, kev, ready);
+#endif
+               total += ready;
+               if (error || ready < n)
+                       break;
+               tsp = &ts;              /* successive loops non-blocking */
+               timespecclear(tsp);
+       }
+       kqueue_scan_finish(&scan);
        KQRELE(kq);
-       *retval = n;
+       if (error == EWOULDBLOCK)
+               error = 0;
+       *retval = total;
        return (error);
 
  done:
@@ -894,32 +928,32 @@ kqueue_sleep(struct kqueue *kq, struct t
        return (error);
 }
 
+/*
+ * Scan the kqueue, blocking if necessary until the target time is reached.
+ * If tsp is NULL we block indefinitely.  If tsp->ts_secs/nsecs are both
+ * 0 we do not block at all.
+ */
 int
-kqueue_scan(struct kqueue *kq, int maxevents, struct kevent *ulistp,
-    struct timespec *tsp, struct proc *p, int *retval)
+kqueue_scan(struct kqueue_scan_state *scan, int maxevents,
+    struct kevent *kevp, struct timespec *tsp, struct proc *p, int *errorp)
 {
-       struct kevent *kevp;
-       struct knote mend, mstart, *kn;
+       struct knote *kn;
+       struct kqueue *kq = scan->kqs_kq;
        int s, count, nkev = 0, error = 0;
-       struct kevent kev[KQ_NEVENTS];
 
        count = maxevents;
        if (count == 0)
                goto done;
-
-       memset(&mstart, 0, sizeof(mstart));
-       memset(&mend, 0, sizeof(mend));
-
 retry:
        if (kq->kq_state & KQ_DYING) {
                error = EBADF;
                goto done;
        }
 
-       kevp = &kev[0];
        s = splhigh();
        if (kq->kq_count == 0) {
-               if (tsp != NULL && !timespecisset(tsp)) {
+               if ((tsp != NULL && !timespecisset(tsp)) ||
+                   scan->kqs_nevent != 0) {
                        splx(s);
                        error = 0;
                        goto done;
@@ -927,7 +961,7 @@ retry:
                kq->kq_state |= KQ_SLEEP;
                error = kqueue_sleep(kq, tsp);
                splx(s);
-               if (error == 0 || error == EWOULDBLOCK)
+               if (error == 0)
                        goto retry;
                /* don't restart after signals... */
                if (error == ERESTART)
@@ -935,27 +969,40 @@ retry:
                goto done;
        }
 
-       mstart.kn_filter = EVFILT_MARKER;
-       mstart.kn_status = KN_PROCESSING;
-       TAILQ_INSERT_HEAD(&kq->kq_head, &mstart, kn_tqe);
-       mend.kn_filter = EVFILT_MARKER;
-       mend.kn_status = KN_PROCESSING;
-       TAILQ_INSERT_TAIL(&kq->kq_head, &mend, kn_tqe);
+       /*
+        * Put the end marker in the queue to limit the scan to the events
+        * that are currently active.  This prevents events from being
+        * recollected if they reactivate during scan.
+        *
+        * If a partial scan has been performed already but no events have
+        * been collected, reposition the end marker to make any new events
+        * reachable.
+        */
+       if (!scan->kqs_queued) {
+               TAILQ_INSERT_TAIL(&kq->kq_head, &scan->kqs_end, kn_tqe);
+               scan->kqs_queued = 1;
+       } else if (scan->kqs_nevent == 0) {
+               TAILQ_REMOVE(&kq->kq_head, &scan->kqs_end, kn_tqe);
+               TAILQ_INSERT_TAIL(&kq->kq_head, &scan->kqs_end, kn_tqe);
+       }
+
+       TAILQ_INSERT_HEAD(&kq->kq_head, &scan->kqs_start, kn_tqe);
        while (count) {
-               kn = TAILQ_NEXT(&mstart, kn_tqe);
+               kn = TAILQ_NEXT(&scan->kqs_start, kn_tqe);
                if (kn->kn_filter == EVFILT_MARKER) {
-                       if (kn == &mend) {
-                               TAILQ_REMOVE(&kq->kq_head, &mend, kn_tqe);
-                               TAILQ_REMOVE(&kq->kq_head, &mstart, kn_tqe);
+                       if (kn == &scan->kqs_end) {
+                               TAILQ_REMOVE(&kq->kq_head, &scan->kqs_start,
+                                   kn_tqe);
                                splx(s);
-                               if (count == maxevents)
+                               if (scan->kqs_nevent == 0)
                                        goto retry;
                                goto done;
                        }
 
                        /* Move start marker past another thread's marker. */
-                       TAILQ_REMOVE(&kq->kq_head, &mstart, kn_tqe);
-                       TAILQ_INSERT_AFTER(&kq->kq_head, kn, &mstart, kn_tqe);
+                       TAILQ_REMOVE(&kq->kq_head, &scan->kqs_start, kn_tqe);
+                       TAILQ_INSERT_AFTER(&kq->kq_head, kn, &scan->kqs_start,
+                           kn_tqe);
                        continue;
                }
 
@@ -983,6 +1030,12 @@ retry:
                *kevp = kn->kn_kevent;
                kevp++;
                nkev++;
+               count--;
+               scan->kqs_nevent++;
+
+               /*
+                * Post-event action on the note
+                */
                if (kn->kn_flags & EV_ONESHOT) {
                        splx(s);
                        kn->kn_fop->f_detach(kn);
@@ -1008,37 +1061,44 @@ retry:
                        knote_release(kn);
                }
                kqueue_check(kq);
-               count--;
-               if (nkev == KQ_NEVENTS) {
-                       splx(s);
-#ifdef KTRACE
-                       if (KTRPOINT(p, KTR_STRUCT))
-                               ktrevent(p, kev, nkev);
-#endif
-                       error = copyout(kev, ulistp,
-                           sizeof(struct kevent) * nkev);
-                       ulistp += nkev;
-                       nkev = 0;
-                       kevp = &kev[0];
-                       s = splhigh();
-                       if (error)
-                               break;
-               }
        }
-       TAILQ_REMOVE(&kq->kq_head, &mend, kn_tqe);
-       TAILQ_REMOVE(&kq->kq_head, &mstart, kn_tqe);
+       TAILQ_REMOVE(&kq->kq_head, &scan->kqs_start, kn_tqe);
        splx(s);
+       if (scan->kqs_nevent == 0)
+               goto retry;
 done:
-       if (nkev != 0) {
-#ifdef KTRACE
-               if (KTRPOINT(p, KTR_STRUCT))
-                       ktrevent(p, kev, nkev);
-#endif
-               error = copyout(kev, ulistp,
-                   sizeof(struct kevent) * nkev);
+       *errorp = error;
+       return (nkev);
+}
+
+void
+kqueue_scan_setup(struct kqueue_scan_state *scan, struct kqueue *kq)
+{
+       memset(scan, 0, sizeof(*scan));
+       scan->kqs_kq = kq;
+       scan->kqs_start.kn_filter = EVFILT_MARKER;
+       scan->kqs_start.kn_status = KN_PROCESSING;
+       scan->kqs_end.kn_filter = EVFILT_MARKER;
+       scan->kqs_end.kn_status = KN_PROCESSING;
+}
+
+void
+kqueue_scan_finish(struct kqueue_scan_state *scan)
+{
+       struct kqueue *kq = scan->kqs_kq;
+       int s;
+
+       KASSERT(scan->kqs_start.kn_filter == EVFILT_MARKER);
+       KASSERT(scan->kqs_start.kn_status == KN_PROCESSING);
+       KASSERT(scan->kqs_end.kn_filter == EVFILT_MARKER);
+       KASSERT(scan->kqs_end.kn_status == KN_PROCESSING);
+
+       if (scan->kqs_queued) {
+               scan->kqs_queued = 0;
+               s = splhigh();
+               TAILQ_REMOVE(&kq->kq_head, &scan->kqs_end, kn_tqe);
+               splx(s);
        }
-       *retval = maxevents - count;
-       return (error);
 }
 
 /*
@@ -1095,7 +1155,7 @@ kqueue_stat(struct file *fp, struct stat
 }
 
 void
-kqueue_terminate(struct proc *p, struct kqueue *kq)
+kqueue_purge(struct proc *p, struct kqueue *kq)
 {
        int i;
 
@@ -1107,6 +1167,12 @@ kqueue_terminate(struct proc *p, struct 
                for (i = 0; i < kq->kq_knhashmask + 1; i++)
                        knote_remove(p, &kq->kq_knhash[i]);
        }
+}
+
+void
+kqueue_terminate(struct proc *p, struct kqueue *kq)
+{
+       kqueue_purge(p, kq);
        kq->kq_state |= KQ_DYING;
        kqueue_wakeup(kq);
 
Index: kern/kern_exit.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_exit.c,v
retrieving revision 1.188
diff -u -p -r1.188 kern_exit.c
--- kern/kern_exit.c    18 Mar 2020 15:48:21 -0000      1.188
+++ kern/kern_exit.c    23 Jun 2020 08:44:07 -0000
@@ -184,6 +184,8 @@ exit1(struct proc *p, int xexit, int xsi
        if ((p->p_flag & P_THREAD) == 0)
                pr->ps_siglist = 0;
 
+       kqueue_exit(p);
+
 #if NKCOV > 0
        kcov_exit(p);
 #endif
Index: kern/kern_fork.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_fork.c,v
retrieving revision 1.225
diff -u -p -r1.225 kern_fork.c
--- kern/kern_fork.c    20 Mar 2020 08:14:07 -0000      1.225
+++ kern/kern_fork.c    23 Jun 2020 08:44:07 -0000
@@ -422,6 +422,8 @@ fork1(struct proc *curp, int flags, void
                newptstat = malloc(sizeof(*newptstat), M_SUBPROC, M_WAITOK);
 
        p->p_tid = alloctid();
+       p->p_kq = kqueue_alloc(p->p_fd);
+       p->p_kq_serial = arc4random();
 
        LIST_INSERT_HEAD(&allproc, p, p_list);
        LIST_INSERT_HEAD(TIDHASH(p->p_tid), p, p_hash);
@@ -553,6 +555,8 @@ thread_fork(struct proc *curp, void *sta
        cpu_fork(curp, p, stack, tcb, child_return, p);
 
        p->p_tid = alloctid();
+       p->p_kq = kqueue_alloc(p->p_fd);
+       p->p_kq_serial = arc4random();
 
        LIST_INSERT_HEAD(&allproc, p, p_list);
        LIST_INSERT_HEAD(TIDHASH(p->p_tid), p, p_hash);
Index: kern/sys_generic.c
===================================================================
RCS file: /cvs/src/sys/kern/sys_generic.c,v
retrieving revision 1.131
diff -u -p -r1.131 sys_generic.c
--- kern/sys_generic.c  20 Mar 2020 04:11:05 -0000      1.131
+++ kern/sys_generic.c  23 Jun 2020 08:45:00 -0000
@@ -55,6 +55,7 @@
 #include <sys/time.h>
 #include <sys/malloc.h>
 #include <sys/poll.h>
+#include <sys/eventvar.h>
 #ifdef KTRACE
 #include <sys/ktrace.h>
 #endif
@@ -66,8 +67,20 @@
 
 #include <uvm/uvm_extern.h>
 
-int selscan(struct proc *, fd_set *, fd_set *, int, int, register_t *);
-void pollscan(struct proc *, struct pollfd *, u_int, register_t *);
+/*
+ * Debug values:
+ *  1 - print implementation errors, things that should not happen.
+ *  2 - print ppoll(2) information, somewhat verbose
+ *  3 - print pselect(2) and ppoll(2) information, very verbose
+ */
+int kqpoll_debug = 0;
+#define DPRINTFN(v, x...) if (kqpoll_debug > v) {              \
+    printf("%s(%d): ", curproc->p_p->ps_comm, curproc->p_tid); \
+    printf(x);                                                 \
+}
+int pselregister(struct proc *, fd_set *, int, int, int *);
+int pselcollect(struct proc *, struct kevent *, fd_set *[]);
+
 int pollout(struct pollfd *, struct pollfd *, u_int);
 int dopselect(struct proc *, int, fd_set *, fd_set *, fd_set *,
     struct timespec *, const sigset_t *, register_t *);
@@ -584,11 +597,11 @@ int
 dopselect(struct proc *p, int nd, fd_set *in, fd_set *ou, fd_set *ex,
     struct timespec *timeout, const sigset_t *sigmask, register_t *retval)
 {
+       struct kqueue_scan_state scan;
        fd_mask bits[6];
        fd_set *pibits[3], *pobits[3];
-       struct timespec elapsed, start, stop;
-       uint64_t nsecs;
-       int s, ncoll, error = 0;
+       struct timespec ts;
+       int error, nevents = 0;
        u_int ni;
 
        if (nd < 0)
@@ -636,43 +649,61 @@ dopselect(struct proc *p, int nd, fd_set
        if (sigmask)
                dosigsuspend(p, *sigmask &~ sigcantmask);
 
-retry:
-       ncoll = nselcoll;
-       atomic_setbits_int(&p->p_flag, P_SELECT);
-       error = selscan(p, pibits[0], pobits[0], nd, ni, retval);
-       if (error || *retval)
+       /* Register kqueue events */
+       if ((error = pselregister(p, pibits[0], nd, ni, &nevents) != 0))
                goto done;
-       if (timeout == NULL || timespecisset(timeout)) {
-               if (timeout != NULL) {
-                       getnanouptime(&start);
+
+       /*
+        * The poll/select family of syscalls has been designed to
+        * block when file descriptors are not available, even if
+        * there's nothing to wait for.
+        */
+       if (nevents == 0) {
+               uint64_t nsecs = INFSLP;
+
+               if (timeout != NULL)
                        nsecs = MIN(TIMESPEC_TO_NSEC(timeout), MAXTSLP);
-               } else
-                       nsecs = INFSLP;
-               s = splhigh();
-               if ((p->p_flag & P_SELECT) == 0 || nselcoll != ncoll) {
-                       splx(s);
-                       goto retry;
-               }
-               atomic_clearbits_int(&p->p_flag, P_SELECT);
-               error = tsleep_nsec(&selwait, PSOCK | PCATCH, "select", nsecs);
-               splx(s);
-               if (timeout != NULL) {
-                       getnanouptime(&stop);
-                       timespecsub(&stop, &start, &elapsed);
-                       timespecsub(timeout, &elapsed, timeout);
-                       if (timeout->tv_sec < 0)
-                               timespecclear(timeout);
-               }
-               if (error == 0 || error == EWOULDBLOCK)
-                       goto retry;
+
+               error = tsleep_nsec(&p->p_kq, PSOCK | PCATCH, "kqsel", nsecs);
+               /* select is not restarted after signals... */
+               if (error == ERESTART)
+                       error = EINTR;
        }
-done:
-       atomic_clearbits_int(&p->p_flag, P_SELECT);
-       /* select is not restarted after signals... */
-       if (error == ERESTART)
-               error = EINTR;
+
+       /* Collect at most `nevents' possibly waiting in kqueue_scan() */
+       kqueue_scan_setup(&scan, p->p_kq);
+       while (nevents > 0) {
+               struct kevent kev[KQ_NEVENTS];
+               int i, ready, count;
+
+               /* Maxium number of events per iteration */
+               count = MIN(nitems(kev), nevents);
+               ready = kqueue_scan(&scan, count, kev, timeout, p, &error);
+#ifdef KTRACE
+               if (KTRPOINT(p, KTR_STRUCT))
+                       ktrevent(p, kev, ready);
+#endif
+               /* Convert back events that are ready. */
+               for (i = 0; i < ready; i++)
+                       *retval += pselcollect(p, &kev[i], pobits);
+
+               /*
+                * Stop if there was an error or if we had enough
+                * place to collect all events that were ready.
+                */
+               if (error || ready < count)
+                       break;
+
+               timeout = &ts;          /* successive loops non-blocking */
+               timespecclear(timeout);
+
+               nevents -= ready;
+       }
+       kqueue_scan_finish(&scan);
+
        if (error == EWOULDBLOCK)
                error = 0;
+ done:
 #define        putbits(name, x) \
        if (name && (error2 = copyout(pobits[x], name, ni))) \
                error = error2;
@@ -694,41 +725,100 @@ done:
 
        if (pibits[0] != (fd_set *)&bits[0])
                free(pibits[0], M_TEMP, 6 * ni);
+
+       kqueue_purge(p, p->p_kq);
+       p->p_kq_serial += nd;
+
        return (error);
 }
 
+/*
+ * Convert fd_set into kqueue events and register them on the
+ * per-thread queue.
+ */
 int
-selscan(struct proc *p, fd_set *ibits, fd_set *obits, int nfd, int ni,
-    register_t *retval)
+pselregister(struct proc *p, fd_set *ibits, int nfd, int ni, int *nregistered)
 {
-       caddr_t cibits = (caddr_t)ibits, cobits = (caddr_t)obits;
-       struct filedesc *fdp = p->p_fd;
-       int msk, i, j, fd;
+       int evf[] = { EVFILT_READ, EVFILT_WRITE, EVFILT_EXCEPT };
+       caddr_t cibits = (caddr_t)ibits;
+       int msk, i, j, fd, nevents = 0, error = 0;
+       struct kevent kev;
        fd_mask bits;
-       struct file *fp;
-       int n = 0;
-       static const int flag[3] = { POLLIN, POLLOUT|POLL_NOHUP, POLLPRI };
 
        for (msk = 0; msk < 3; msk++) {
                fd_set *pibits = (fd_set *)&cibits[msk*ni];
-               fd_set *pobits = (fd_set *)&cobits[msk*ni];
 
                for (i = 0; i < nfd; i += NFDBITS) {
                        bits = pibits->fds_bits[i/NFDBITS];
                        while ((j = ffs(bits)) && (fd = i + --j) < nfd) {
                                bits &= ~(1 << j);
-                               if ((fp = fd_getfile(fdp, fd)) == NULL)
-                                       return (EBADF);
-                               if ((*fp->f_ops->fo_poll)(fp, flag[msk], p)) {
-                                       FD_SET(fd, pobits);
-                                       n++;
+
+                               DPRINTFN(2, "select fd %d mask %d serial %lu\n",
+                                   fd, msk, p->p_kq_serial);
+                               EV_SET(&kev, fd, evf[msk],
+                                   EV_ADD|EV_ENABLE|__EV_POLL, 0, 0,
+                                   (void *)(p->p_kq_serial));
+#ifdef KTRACE
+                               if (KTRPOINT(p, KTR_STRUCT))
+                                       ktrevent(p, &kev, 1);
+#endif
+                               error = kqueue_register(p->p_kq, &kev, p);
+                               switch (error) {
+                               case 0:
+                                       nevents++;
+                               case EOPNOTSUPP:/* No underlying kqfilter */
+                               case EINVAL:    /* Unimplemented filter */
+                                       error = 0;
+                                       break;
+                               case ENXIO:     /* Device has been detached */
+                               default:
+                                       goto bad;
                                }
-                               FRELE(fp, p);
                        }
                }
        }
-       *retval = n;
+
+       *nregistered = nevents;
        return (0);
+bad:
+       DPRINTFN(0, "select fd %u filt %d error %d\n", (int)kev.ident,
+           kev.filter, error);
+       return (error);
+}
+
+/*
+ * Convert given kqueue event into corresponding select(2) bit.
+ */
+int
+pselcollect(struct proc *p, struct kevent *kevp, fd_set *pobits[3])
+{
+#ifdef DIAGNOSTIC
+       /* Filter out and lazily delete spurious events */
+       if ((unsigned long)kevp->udata != p->p_kq_serial) {
+               DPRINTFN(0, "select fd %u mismatched serial %lu\n",
+                   (int)kevp->ident, p->p_kq_serial);
+               kevp->flags = EV_DISABLE|EV_DELETE;
+               kqueue_register(p->p_kq, kevp, p);
+               return (0);
+       }
+#endif
+
+       switch (kevp->filter) {
+       case EVFILT_READ:
+               FD_SET(kevp->ident, pobits[0]);
+               break;
+       case EVFILT_WRITE:
+               FD_SET(kevp->ident, pobits[1]);
+               break;
+       case EVFILT_EXCEPT:
+               FD_SET(kevp->ident, pobits[2]);
+               break;
+       default:
+               KASSERT(0);
+       }
+
+       DPRINTFN(2, "select fd %d filt %d\n", (int)kevp->ident, kevp->filter);
+       return (1);
 }
 
 int
Index: sys/event.h
===================================================================
RCS file: /cvs/src/sys/sys/event.h,v
retrieving revision 1.44
diff -u -p -r1.44 event.h
--- sys/event.h 22 Jun 2020 13:14:32 -0000      1.44
+++ sys/event.h 23 Jun 2020 08:44:47 -0000
@@ -200,7 +200,18 @@ struct knote {
 #define kn_fp          kn_ptr.p_fp
 };
 
+struct kqueue_scan_state {
+       struct kqueue   *kqs_kq;                /* kqueue of this scan */
+       struct knote     kqs_start;             /* start marker */
+       struct knote     kqs_end;               /* end marker */
+       int              kqs_nevent;            /* number of events collected */
+       int              kqs_queued;            /* if set, end marker is
+                                                * in queue */
+};
+
 struct proc;
+struct filedesc;
+struct timespec;
 
 extern const struct filterops sig_filtops;
 extern const struct filterops dead_filtops;
@@ -210,8 +221,15 @@ extern void        knote_activate(struct knote 
 extern void    knote_remove(struct proc *p, struct knlist *list);
 extern void    knote_fdclose(struct proc *p, int fd);
 extern void    knote_processexit(struct proc *);
+extern struct  kqueue *kqueue_alloc(struct filedesc *);
+extern void    kqueue_exit(struct proc *);
 extern int     kqueue_register(struct kqueue *kq,
                    struct kevent *kev, struct proc *p);
+extern void    kqueue_scan_setup(struct kqueue_scan_state *, struct kqueue *);
+extern void    kqueue_scan_finish(struct kqueue_scan_state *);
+extern int     kqueue_scan(struct kqueue_scan_state *, int, struct kevent *,
+                   struct timespec *, struct proc *, int *);
+extern void    kqueue_purge(struct proc *, struct kqueue *);
 extern int     filt_seltrue(struct knote *kn, long hint);
 extern int     seltrue_kqfilter(dev_t, struct knote *);
 extern void    klist_insert(struct klist *, struct knote *);
Index: sys/proc.h
===================================================================
RCS file: /cvs/src/sys/sys/proc.h,v
retrieving revision 1.295
diff -u -p -r1.295 proc.h
--- sys/proc.h  28 Apr 2020 08:29:40 -0000      1.295
+++ sys/proc.h  23 Jun 2020 08:44:07 -0000
@@ -315,6 +315,7 @@ struct process {
 
 struct kcov_dev;
 struct lock_list_entry;
+struct kqueue;
 
 struct p_inentry {
        u_long   ie_serial;
@@ -377,6 +378,8 @@ struct proc {
        struct  plimit  *p_limit;       /* [l] read ref. of p_p->ps_limit */
        struct  kcov_dev *p_kd;         /* kcov device handle */
        struct  lock_list_entry *p_sleeplocks;  /* WITNESS lock tracking */ 
+       struct  kqueue *p_kq;           /* for select/poll */
+       unsigned long p_kq_serial;      /* for select/poll */
 
        int      p_siglist;             /* Signals arrived but not delivered. */
 

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