Re: [PATCH v2 4/5] vhost-vsock: preserve vhost FD during CPR

[Maciej S. Szmigiero]

On 19.06.2026 12:55, Andrey Drobyshev wrote:

During CPR migration, instead of reopening /dev/vhost-vsock device on
each initialization, let's make sure the already open FD is preserved and
properly reused on a subsequent initialization.

Signed-off-by: Andrey Drobyshev <andrey.drobys...@virtuozzo.com>
---
  hw/virtio/vhost-vsock.c | 25 ++++++++++++++++++++-----
  1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/hw/virtio/vhost-vsock.c b/hw/virtio/vhost-vsock.c
index da244eb1657..ea9a7d2149d 100644
--- a/hw/virtio/vhost-vsock.c
+++ b/hw/virtio/vhost-vsock.c
@@ -20,6 +20,7 @@
  #include "hw/core/qdev-properties.h"
  #include "hw/virtio/vhost-vsock.h"
  #include "monitor/monitor.h"
+#include "migration/cpr.h"

static void vhost_vsock_get_config(VirtIODevice *vdev, uint8_t *config)

  {
@@ -126,6 +127,7 @@ static void vhost_vsock_device_realize(DeviceState *dev, 
Error **errp)
      VHostVSockCommon *vvc = VHOST_VSOCK_COMMON(dev);
      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
      VHostVSock *vsock = VHOST_VSOCK(dev);
+    DeviceState *proxy = qdev_get_parent_bus(DEVICE(vsock))->parent;
      int vhostfd;
      int ret;

@@ -141,7 +143,8 @@ static void vhost_vsock_device_realize(DeviceState *dev, Error **errp)

      }

if (vsock->conf.vhostfd) {

-        vhostfd = monitor_fd_param(monitor_cur(), vsock->conf.vhostfd, errp);
+        vhostfd = cpr_get_fd_param(proxy->id, vsock->conf.vhostfd, 0,

Is this proxy->id *guaranteed* unique in a QEMU instance?

+                                   errp);
          if (vhostfd == -1) {
              error_prepend(errp, "vhost-vsock: unable to parse vhostfd: ");
              return;
@@ -151,10 +154,20 @@ static void vhost_vsock_device_realize(DeviceState *dev, 
Error **errp)
              return;
          }
      } else {
-        vhostfd = open("/dev/vhost-vsock", O_RDWR);
-        if (vhostfd < 0) {
-            error_setg_file_open(errp, errno, "/dev/vhost-vsock");
-            return;
+        if (cpr_is_incoming()) {
+            vhostfd = cpr_find_fd(proxy->id, 0);
+            if (vhostfd < 0) {
+                error_setg(errp, "vhost-vsock: could not restore vhost FD "
+                           "for %s", proxy->id);
+                return;
+            }
+        } else {
+            vhostfd = open("/dev/vhost-vsock", O_RDWR);
+            if (vhostfd < 0) {
+                error_setg_file_open(errp, errno, "/dev/vhost-vsock");
+                return;
+            }
+            cpr_save_fd(proxy->id, 0, vhostfd);
          }

if (!qemu_set_blocking(vhostfd, false, errp)) {

             return;
          }

So if qemu_set_blocking() fails the FD leaks (existing bug) and now also
CPR has this dangling FD saved?

Furthermore, if calls further down vhost_vsock_device_realize() fail
AFAIK vhost_vsock_device_unrealize() won't get called so this FD won't
be removed from CPR - even in something else in vhost stack actually closes
this FD during error cleanup.

@@ -193,10 +206,12 @@ static void vhost_vsock_device_unrealize(DeviceState *dev)
  {
      VHostVSockCommon *vvc = VHOST_VSOCK_COMMON(dev);
      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
+    DeviceState *proxy = qdev_get_parent_bus(dev)->parent;

/* This will stop vhost backend if appropriate. */

      vhost_vsock_set_status(vdev, 0);

+ cpr_delete_fd(proxy->id, 0);

      vhost_dev_cleanup(&vvc->vhost_dev);
      vhost_vsock_common_unrealize(vdev);
  }

Thanks,
Maciej