[Qemu-devel] [PATCH] virtio-serial: add enable_backend callback

[Pavel Butsykin]

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 <address@hidden>
---
 hw/char/virtio-console.c          | 21 +++++++++++++++++++++
 hw/char/virtio-serial-bus.c       |  7 +++++++
 include/hw/virtio/virtio-serial.h |  3 +++
 3 files changed, 31 insertions(+)

diff --git a/hw/char/virtio-console.c b/hw/char/virtio-console.c
index 0cb1668c8a..b55905892e 100644
--- a/hw/char/virtio-console.c
+++ b/hw/char/virtio-console.c
@@ -163,6 +163,26 @@ static void chr_event(void *opaque, int event)
     }
 }
 
+static void virtconsole_enable_backend(VirtIOSerialPort *port, bool enable)
+{
+    VirtConsole *vcon = VIRTIO_CONSOLE(port);
+
+    if (!qemu_chr_fe_get_driver(&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,
+                                 vcon, NULL, false);
+    } else {
+        qemu_chr_fe_set_handlers(&vcon->chr, NULL, NULL,
+                                 NULL, NULL, NULL, false);
+    }
+}
+
 static void virtconsole_realize(DeviceState *dev, Error **errp)
 {
     VirtIOSerialPort *port = VIRTIO_SERIAL_PORT(dev);
@@ -233,6 +253,7 @@ 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;
 }
diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
index f5bc173844..f0f18c8e7c 100644
--- a/hw/char/virtio-serial-bus.c
+++ b/hw/char/virtio-serial-bus.c
@@ -637,6 +637,13 @@ static void set_status(VirtIODevice *vdev, uint8_t status)
     if (!(status & VIRTIO_CONFIG_S_DRIVER_OK)) {
         guest_reset(vser);
     }
+
+    QTAILQ_FOREACH(port, &vser->ports, next) {
+        VirtIOSerialPortClass *vsc = VIRTIO_SERIAL_PORT_GET_CLASS(port);
+        if (vsc->enable_backend) {
+            vsc->enable_backend(port, vdev->vm_running);
+        }
+    }
 }
 
 static void vser_reset(VirtIODevice *vdev)
diff --git a/include/hw/virtio/virtio-serial.h 
b/include/hw/virtio/virtio-serial.h
index b19c44727f..12657a9f39 100644
--- a/include/hw/virtio/virtio-serial.h
+++ b/include/hw/virtio/virtio-serial.h
@@ -58,6 +58,9 @@ typedef struct VirtIOSerialPortClass {
         /* Guest opened/closed device. */
     void (*set_guest_connected)(VirtIOSerialPort *port, int guest_connected);
 
+    /* Enable/disable backend for virtio serial port */
+    void (*enable_backend)(VirtIOSerialPort *port, bool enable);
+
         /* Guest is now ready to accept data (virtqueues set up). */
     void (*guest_ready)(VirtIOSerialPort *port);
 
-- 
2.13.0