chardev/char-hub: implement backend chardev aggregator

This patch implements a new chardev backend `hub` device, which
aggregates input from multiple backend devices and forwards it to a
single frontend device. Additionally, `hub` device takes the output
from the frontend device and sends it back to all the connected
backend devices. This allows for seamless interaction between
different backend devices and a single frontend interface.

The idea of the change is trivial: keep list of backend devices
(up to 4), init them on demand and forward data buffer back and
forth.

The following is QEMU command line example:

   -chardev pty,path=/tmp/pty,id=pty0 \
   -chardev vc,id=vc0 \
   -chardev hub,id=hub0,chardevs.0=pty0,chardevs.1=vc0 \
   -device virtconsole,chardev=hub0 \
   -vnc 0.0.0.0:0

Which creates 2 backend devices: text virtual console (`vc0`) and a
pseudo TTY (`pty0`) connected to the single virtio hvc console with
the backend aggregator (`hub0`) help. `vc0` renders text to an image,
which can be shared over the VNC protocol.  `pty0` is a pseudo TTY
backend which provides biderectional communication to the virtio hvc
console.

'chardevs.N' list syntax is used for the sake of compatibility with
the representation of JSON lists in 'key=val' pairs format of the
util/keyval.c, despite the fact that modern QAPI way of parsing,
namely qobject_input_visitor_new_str(), is not used. Choice of keeping
QAPI list syntax may help to smoothly switch to modern parsing in the
future.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Reviewed-by: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
Message-ID: <20250123085327.965501-3-r.peniaev@gmail.com>

1 files changed, 301 insertions, 0 deletions

diff --git a/chardev/char-hub.c b/chardev/char-hub.c
new file mode 100644
index 0000000000..3a4aae3289
--- /dev/null
+++ b/chardev/char-hub.c
@@ -0,0 +1,301 @@
+/*
+ * QEMU Character Hub Device
+ *
+ * Author: Roman Penyaev <r.peniaev@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu/osdep.h"
+#include "qapi/error.h"
+#include "qemu/option.h"
+#include "chardev/char.h"
+#include "chardev-internal.h"
+
+/*
+ * Character hub device aggregates input from multiple backend devices
+ * and forwards it to a single frontend device. Additionally, hub
+ * device takes the output from the frontend device and sends it back
+ * to all the connected backend devices.
+ */
+
+/*
+ * Write to all backends. Different backend devices accept data with
+ * various rate, so it is quite possible that one device returns less,
+ * then others. In this case we return minimum to the caller,
+ * expecting caller will repeat operation soon. When repeat happens
+ * send to the devices which consume data faster must be avoided
+ * for obvious reasons not to send data, which was already sent.
+ * Called with chr_write_lock held.
+ */
+static int hub_chr_write(Chardev *chr, const uint8_t *buf, int len)
+{
+    HubChardev *d = HUB_CHARDEV(chr);
+    int r, i, ret = len;
+    unsigned int written;
+
+    /* Invalidate index on every write */
+    d->be_eagain_ind = -1;
+
+    for (i = 0; i < d->be_cnt; i++) {
+        if (!d->backends[i].be.chr->be_open) {
+            /* Skip closed backend */
+            continue;
+        }
+        written = d->be_written[i] - d->be_min_written;
+        if (written) {
+            /* Written in the previous call so take into account */
+            ret = MIN(written, ret);
+            continue;
+        }
+        r = qemu_chr_fe_write(&d->backends[i].be, buf, len);
+        if (r < 0) {
+            if (errno == EAGAIN) {
+                /* Set index and expect to be called soon on watch wake up */
+                d->be_eagain_ind = i;
+            }
+            return r;
+        }
+        d->be_written[i] += r;
+        ret = MIN(r, ret);
+    }
+    d->be_min_written += ret;
+
+
+    return ret;
+}
+
+static int hub_chr_can_read(void *opaque)
+{
+    HubCharBackend *backend = opaque;
+    CharBackend *fe = backend->hub->parent.be;
+
+    if (fe && fe->chr_can_read) {
+        return fe->chr_can_read(fe->opaque);
+    }
+
+    return 0;
+}
+
+static void hub_chr_read(void *opaque, const uint8_t *buf, int size)
+{
+    HubCharBackend *backend = opaque;
+    CharBackend *fe = backend->hub->parent.be;
+
+    if (fe && fe->chr_read) {
+        fe->chr_read(fe->opaque, buf, size);
+    }
+}
+
+static void hub_chr_event(void *opaque, QEMUChrEvent event)
+{
+    HubCharBackend *backend = opaque;
+    HubChardev *d = backend->hub;
+    CharBackend *fe = d->parent.be;
+
+    if (event == CHR_EVENT_OPENED) {
+        /*
+         * Catch up with what was already written while this backend
+         * was closed
+         */
+        d->be_written[backend->be_ind] = d->be_min_written;
+
+        if (d->be_event_opened_cnt++) {
+            /* Ignore subsequent open events from other backends */
+            return;
+        }
+    } else if (event == CHR_EVENT_CLOSED) {
+        if (!d->be_event_opened_cnt) {
+            /* Don't go below zero. Probably assert is better */
+            return;
+        }
+        if (--d->be_event_opened_cnt) {
+            /* Serve only the last one close event */
+            return;
+        }
+    }
+
+    if (fe && fe->chr_event) {
+        fe->chr_event(fe->opaque, event);
+    }
+}
+
+static GSource *hub_chr_add_watch(Chardev *s, GIOCondition cond)
+{
+    HubChardev *d = HUB_CHARDEV(s);
+    Chardev *chr;
+    ChardevClass *cc;
+
+    if (d->be_eagain_ind == -1) {
+        return NULL;
+    }
+
+    assert(d->be_eagain_ind < d->be_cnt);
+    chr = qemu_chr_fe_get_driver(&d->backends[d->be_eagain_ind].be);
+    cc = CHARDEV_GET_CLASS(chr);
+    if (!cc->chr_add_watch) {
+        return NULL;
+    }
+
+    return cc->chr_add_watch(chr, cond);
+}
+
+static bool hub_chr_attach_chardev(HubChardev *d, Chardev *chr,
+                                   Error **errp)
+{
+    bool ret;
+
+    if (d->be_cnt >= MAX_HUB) {
+        error_setg(errp, "hub: too many uses of chardevs '%s'"
+                   " (maximum is " stringify(MAX_HUB) ")",
+                   d->parent.label);
+        return false;
+    }
+    ret = qemu_chr_fe_init(&d->backends[d->be_cnt].be, chr, errp);
+    if (ret) {
+        d->backends[d->be_cnt].hub = d;
+        d->backends[d->be_cnt].be_ind = d->be_cnt;
+        d->be_cnt += 1;
+    }
+
+    return ret;
+}
+
+static void char_hub_finalize(Object *obj)
+{
+    HubChardev *d = HUB_CHARDEV(obj);
+    int i;
+
+    for (i = 0; i < d->be_cnt; i++) {
+        qemu_chr_fe_deinit(&d->backends[i].be, false);
+    }
+}
+
+static void hub_chr_update_read_handlers(Chardev *chr)
+{
+    HubChardev *d = HUB_CHARDEV(chr);
+    int i;
+
+    for (i = 0; i < d->be_cnt; i++) {
+        qemu_chr_fe_set_handlers_full(&d->backends[i].be,
+                                      hub_chr_can_read,
+                                      hub_chr_read,
+                                      hub_chr_event,
+                                      NULL,
+                                      &d->backends[i],
+                                      chr->gcontext, true, false);
+    }
+}
+
+static void qemu_chr_open_hub(Chardev *chr,
+                                 ChardevBackend *backend,
+                                 bool *be_opened,
+                                 Error **errp)
+{
+    ChardevHub *hub = backend->u.hub.data;
+    HubChardev *d = HUB_CHARDEV(chr);
+    strList *list = hub->chardevs;
+
+    d->be_eagain_ind = -1;
+
+    if (list == NULL) {
+        error_setg(errp, "hub: 'chardevs' list is not defined");
+        return;
+    }
+
+    while (list) {
+        Chardev *s;
+
+        s = qemu_chr_find(list->value);
+        if (s == NULL) {
+            error_setg(errp, "hub: chardev can't be found by id '%s'",
+                       list->value);
+            return;
+        }
+        if (CHARDEV_IS_HUB(s) || CHARDEV_IS_MUX(s)) {
+            error_setg(errp, "hub: multiplexers and hub devices can't be "
+                       "stacked, check chardev '%s', chardev should not "
+                       "be a hub device or have 'mux=on' enabled",
+                       list->value);
+            return;
+        }
+        if (!hub_chr_attach_chardev(d, s, errp)) {
+            return;
+        }
+        list = list->next;
+    }
+
+    /* Closed until an explicit event from backend */
+    *be_opened = false;
+}
+
+static void qemu_chr_parse_hub(QemuOpts *opts, ChardevBackend *backend,
+                                  Error **errp)
+{
+    ChardevHub *hub;
+    strList **tail;
+    int i;
+
+    backend->type = CHARDEV_BACKEND_KIND_HUB;
+    hub = backend->u.hub.data = g_new0(ChardevHub, 1);
+    qemu_chr_parse_common(opts, qapi_ChardevHub_base(hub));
+
+    tail = &hub->chardevs;
+
+    for (i = 0; i < MAX_HUB; i++) {
+        char optbuf[16];
+        const char *dev;
+
+        snprintf(optbuf, sizeof(optbuf), "chardevs.%u", i);
+        dev = qemu_opt_get(opts, optbuf);
+        if (!dev) {
+            break;
+        }
+
+        QAPI_LIST_APPEND(tail, g_strdup(dev));
+    }
+}
+
+static void char_hub_class_init(ObjectClass *oc, void *data)
+{
+    ChardevClass *cc = CHARDEV_CLASS(oc);
+
+    cc->parse = qemu_chr_parse_hub;
+    cc->open = qemu_chr_open_hub;
+    cc->chr_write = hub_chr_write;
+    cc->chr_add_watch = hub_chr_add_watch;
+    /* We handle events from backends only */
+    cc->chr_be_event = NULL;
+    cc->chr_update_read_handler = hub_chr_update_read_handlers;
+}
+
+static const TypeInfo char_hub_type_info = {
+    .name = TYPE_CHARDEV_HUB,
+    .parent = TYPE_CHARDEV,
+    .class_init = char_hub_class_init,
+    .instance_size = sizeof(HubChardev),
+    .instance_finalize = char_hub_finalize,
+};
+
+static void register_types(void)
+{
+    type_register_static(&char_hub_type_info);
+}
+
+type_init(register_types);