chardev/mux: switch mux frontends management to bitset

Frontends can be attached and detached during run-time (although detach
is not implemented, but will follow). Counter variable of muxes is not
enough for proper attach/detach management, so this patch implements
bitset: if bit is set for the `mux_bitset` variable, then frontend
device can be found in the `backend` array (yes, huge confusion with
backend and frontends names).

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20241014152408.427700-7-r.peniaev@gmail.com>

chardev/char-mux.c

@@ -26,6 +26,7 @@
 #include "qapi/error.h"
 #include "qemu/module.h"
 #include "qemu/option.h"
+#include "qemu/bitops.h"
 #include "chardev/char.h"
 #include "sysemu/block-backend.h"
 #include "qapi/qapi-commands-control.h"
@@ -168,12 +169,19 @@ static int mux_proc_byte(Chardev *chr, MuxChardev *d, int ch)
         case 'b':
             qemu_chr_be_event(chr, CHR_EVENT_BREAK);
             break;
-        case 'c':
+        case 'c': {
-            assert(d->mux_cnt > 0); /* handler registered with first fe */
+            unsigned int bit;
+
+            /* Handler registered with first fe */
+            assert(d->mux_bitset != 0);
             /* Switch to the next registered device */
-            mux_set_focus(chr, (d->focus + 1) % d->mux_cnt);
+            bit = find_next_bit(&d->mux_bitset, MAX_MUX, d->focus + 1);
+            if (bit >= MAX_MUX) {
+                bit = find_next_bit(&d->mux_bitset, MAX_MUX, 0);
+            }
+            mux_set_focus(chr, bit);
             break;
-        case 't':
+        } case 't':
             d->timestamps = !d->timestamps;
             d->timestamps_start = -1;
             d->linestart = false;
@@ -243,15 +251,16 @@ static void mux_chr_read(void *opaque, const uint8_t *buf, int size)
 void mux_chr_send_all_event(Chardev *chr, QEMUChrEvent event)
 {
     MuxChardev *d = MUX_CHARDEV(chr);
-    unsigned int i;
+    int bit;
 
     if (!muxes_opened) {
         return;
     }
 
     /* Send the event to all registered listeners */
-    for (i = 0; i < d->mux_cnt; i++) {
+    bit = -1;
-        mux_chr_send_event(d, i, event);
+    while ((bit = find_next_bit(&d->mux_bitset, MAX_MUX, bit + 1)) < MAX_MUX) {
+        mux_chr_send_event(d, bit, event);
     }
 }
 
@@ -276,10 +285,11 @@ static GSource *mux_chr_add_watch(Chardev *s, GIOCondition cond)
 static void char_mux_finalize(Object *obj)
 {
     MuxChardev *d = MUX_CHARDEV(obj);
-    unsigned int i;
+    int bit;
 
-    for (i = 0; i < d->mux_cnt; i++) {
+    bit = -1;
-        CharBackend *be = d->backends[i];
+    while ((bit = find_next_bit(&d->mux_bitset, MAX_MUX, bit + 1)) < MAX_MUX) {
+        CharBackend *be = d->backends[bit];
         if (be) {
             be->chr = NULL;
         }
@@ -304,7 +314,10 @@ static void mux_chr_update_read_handlers(Chardev *chr)
 bool mux_chr_attach_frontend(MuxChardev *d, CharBackend *b,
                              unsigned int *tag, Error **errp)
 {
-    if (d->mux_cnt >= MAX_MUX) {
+    unsigned int bit;
+
+    bit = find_next_zero_bit(&d->mux_bitset, MAX_MUX, 0);
+    if (bit >= MAX_MUX) {
         error_setg(errp,
                    "too many uses of multiplexed chardev '%s'"
                    " (maximum is " stringify(MAX_MUX) ")",
@@ -312,8 +325,9 @@ bool mux_chr_attach_frontend(MuxChardev *d, CharBackend *b,
         return false;
     }
 
-    d->backends[d->mux_cnt] = b;
+    d->mux_bitset |= (1 << bit);
-    *tag = d->mux_cnt++;
+    d->backends[bit] = b;
+    *tag = bit;
 
     return true;
 }
@@ -322,7 +336,7 @@ void mux_set_focus(Chardev *chr, unsigned int focus)
 {
     MuxChardev *d = MUX_CHARDEV(chr);
 
-    assert(focus < d->mux_cnt);
+    assert(find_next_bit(&d->mux_bitset, MAX_MUX, focus) == focus);
 
     if (d->focus != -1) {
         mux_chr_send_event(d, d->focus, CHR_EVENT_MUX_OUT);

chardev/char.c

@@ -333,7 +333,7 @@ static bool qemu_chr_is_busy(Chardev *s)
 {
     if (CHARDEV_IS_MUX(s)) {
         MuxChardev *d = MUX_CHARDEV(s);
-        return d->mux_cnt > 0;
+        return d->mux_bitset != 0;
     } else {
         return s->be != NULL;
     }

chardev/chardev-internal.h

@@ -37,8 +37,8 @@ struct MuxChardev {
     Chardev parent;
     CharBackend *backends[MAX_MUX];
     CharBackend chr;
+    unsigned long mux_bitset;
     int focus;
-    unsigned int mux_cnt;
     bool term_got_escape;
     /* Intermediate input buffer catches escape sequences even if the
        currently active device is not accepting any input - but only until it