virtio-serial: add enable_backend callback

We should guarantee that RAM will not be modified while VM has a stopped
state, otherwise it can lead to negative consequences during post-copy
migration. In RUN_STATE_FINISH_MIGRATE step, it's expected that RAM on
source side will not be modified as this could lead to non-consistent vm state
on the destination side. Also RAM access during postcopy-ram migration with
enabled release-ram capability can lead to sad consequences.

Let's add enable_backend() callback to avoid undesirable virtioqueue changes
in the guest memory.

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Message-Id: <20170919120733.22020-1-pbutsykin@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/hw/char/virtio-console.c b/hw/char/virtio-console.c
index 198b2a89c0..172c72d06c 100644
--- a/hw/char/virtio-console.c
+++ b/hw/char/virtio-console.c
@@ -187,6 +187,26 @@ static int chr_be_change(void *opaque)
     return 0;
 }
 
+static void virtconsole_enable_backend(VirtIOSerialPort *port, bool enable)
+{
+    VirtConsole *vcon = VIRTIO_CONSOLE(port);
+
+    if (!qemu_chr_fe_backend_connected(&vcon->chr)) {
+        return;
+    }
+
+    if (enable) {
+        VirtIOSerialPortClass *k = VIRTIO_SERIAL_PORT_GET_CLASS(port);
+
+        qemu_chr_fe_set_handlers(&vcon->chr, chr_can_read, chr_read,
+                                 k->is_console ? NULL : chr_event,
+                                 chr_be_change, vcon, NULL, false);
+    } else {
+        qemu_chr_fe_set_handlers(&vcon->chr, NULL, NULL, NULL,
+                                 NULL, NULL, NULL, false);
+    }
+}
+
 static void virtconsole_realize(DeviceState *dev, Error **errp)
 {
     VirtIOSerialPort *port = VIRTIO_SERIAL_PORT(dev);
@@ -258,6 +278,7 @@ static void virtserialport_class_init(ObjectClass *klass, void *data)
     k->unrealize = virtconsole_unrealize;
     k->have_data = flush_buf;
     k->set_guest_connected = set_guest_connected;
+    k->enable_backend = virtconsole_enable_backend;
     k->guest_writable = guest_writable;
     dc->props = virtserialport_properties;
 }