Use atomic ops on the set of used segment registers

Each pmap sets a bit in usedsr to claim 16 unique VSIDs for its
segment registers. Use atomic_cas_uint to set this bit (checking that
the other cpu didn't steal it) and atomic_clearbits_int to clear it.
Stop using splvm.

ok miod@

diff --git a/sys/arch/powerpc/powerpc/pmap.c b/sys/arch/powerpc/powerpc/pmap.c
index f27adad..66e5dc2 100644 (file)
--- a/sys/arch/powerpc/powerpc/pmap.c
+++ b/sys/arch/powerpc/powerpc/pmap.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: pmap.c,v 1.177 2022/09/10 20:35:28 miod Exp $ */
+/*     $OpenBSD: pmap.c,v 1.178 2023/01/10 21:27:12 gkoehler Exp $ */
 
 /*
  * Copyright (c) 2015 Martin Pieuchot
@@ -1094,8 +1094,9 @@ int pmap_id_avail = 0;
 pmap_t
 pmap_create(void)
 {
-       int i, k, try, tblidx, tbloff;
-       int s, seg;
+       u_int bits;
+       int first, i, k, try, tblidx, tbloff;
+       int seg;
        pmap_t pm;
 
        pm = pool_get(&pmap_pmap_pool, PR_WAITOK|PR_ZERO);
@@ -1107,23 +1108,21 @@ pmap_create(void)
         * Allocate segment registers for this pmap.
         * Try not to reuse pmap ids, to spread the hash table usage.
         */
+       first = pmap_id_avail;
 again:
        for (i = 0; i < NPMAPS; i++) {
-               try = pmap_id_avail + i;
+               try = first + i;
                try = try % NPMAPS; /* truncate back into bounds */
                tblidx = try / (8 * sizeof usedsr[0]);
                tbloff = try % (8 * sizeof usedsr[0]);
-               if ((usedsr[tblidx] & (1 << tbloff)) == 0) {
-                       /* pmap create lock? */
-                       s = splvm();
-                       if ((usedsr[tblidx] & (1 << tbloff)) == 1) {
-                               /* entry was stolen out from under us, retry */
-                               splx(s); /* pmap create unlock */
+               bits = usedsr[tblidx];
+               if ((bits & (1U << tbloff)) == 0) {
+                       if (atomic_cas_uint(&usedsr[tblidx], bits,
+                           bits | (1U << tbloff)) != bits) {
+                               first = try;
                                goto again;
                        }
-                       usedsr[tblidx] |= (1 << tbloff); 
                        pmap_id_avail = try + 1;
-                       splx(s); /* pmap create unlock */
 
                        seg = try << 4;
                        for (k = 0; k < 16; k++)
@@ -1173,17 +1172,14 @@ void
 pmap_release(pmap_t pm)
 {
        int i, tblidx, tbloff;
-       int s;
 
        pmap_vp_destroy(pm);
        i = (pm->pm_sr[0] & SR_VSID) >> 4;
        tblidx = i / (8  * sizeof usedsr[0]);
        tbloff = i % (8  * sizeof usedsr[0]);
 
-       /* LOCK? */
-       s = splvm();
-       usedsr[tblidx] &= ~(1 << tbloff);
-       splx(s);
+       /* powerpc can do atomic cas, clearbits on same word. */
+       atomic_clearbits_int(&usedsr[tblidx], 1U << tbloff);
 }
 
 void