[QEMU HCI-8.0 PATCH v2 5/5] vhost-blk: preserve the uevent socket across cpr-exec

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

qemu-update uses cpr-exec migration: QEMU re-execs itself in place.
A plain monitor fdset descriptor like our uevent socket does not
survive that (unlike the cpr_save_fd()'d tap/vhost fds), so without
help the resized-device notifications would silently stop until the
next full VM start.

Preserve it the same way the net backends do: on cold boot
cpr_save_fd() the socket under the device's canonical path, and on
the re-exec'd (incoming) side adopt it with cpr_find_fd() instead of
reopening a command line fd that no longer resolves. The management
layer passes a placeholder for the "ueventfd" property in that case.
Drop the saved descriptor on teardown so it does not outlive the
device.

https://virtuozzo.atlassian.net/browse/VSTOR-143437
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 hw/block/vhost-blk.c | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/hw/block/vhost-blk.c b/hw/block/vhost-blk.c
index 7cb842a859..eca12356ae 100644
--- a/hw/block/vhost-blk.c
+++ b/hw/block/vhost-blk.c
@@ -28,6 +28,7 @@
 #include <sys/ioctl.h>
 #include <linux/fs.h>
 #include <linux/netlink.h>
+#include "migration/cpr.h"
 #include "system/runstate.h"
 
 static int vhost_blk_start(VirtIODevice *vdev)
@@ -364,20 +365,36 @@ static bool vhost_blk_uevent_check(int fd, const char *src, Error **errp)
 
 static bool vhost_blk_uevent_attach(VHostBlk *s, Error **errp)
 {
+    g_autofree char *cpr_name = NULL;
+
     if (!s->conf.ueventfd) {
         return true;
     }
 
-    s->uevent_fd = qemu_open(s->conf.ueventfd, O_RDWR, errp);
-    if (s->uevent_fd < 0) {
-        error_prepend(errp, "vhost-blk: unable to open uevent socket: ");
-        return false;
-    }
+    cpr_name = object_get_canonical_path(OBJECT(s));
 
-    if (!vhost_blk_uevent_check(s->uevent_fd, s->conf.ueventfd, errp)) {
-        qemu_close(s->uevent_fd);
-        s->uevent_fd = -1;
-        return false;
+    if (cpr_is_incoming()) {
+        s->uevent_fd = cpr_find_fd(cpr_name, 0);
+        if (s->uevent_fd < 0) {
+            error_setg(errp,
+                       "vhost-blk: no preserved uevent socket to restore");
+            return false;
+        }
+    } else {
+        s->uevent_fd = qemu_open(s->conf.ueventfd, O_RDWR, errp);
+        if (s->uevent_fd < 0) {
+            error_prepend(errp, "vhost-blk: unable to open uevent socket: ");
+            return false;
+        }
+
+        if (!vhost_blk_uevent_check(s->uevent_fd, s->conf.ueventfd, errp)) {
+            qemu_close(s->uevent_fd);
+            s->uevent_fd = -1;
+            return false;
+        }
+
+        /* Preserve the socket across a future cpr-exec qemu-update. */
+        cpr_save_fd(cpr_name, 0, s->uevent_fd);
     }
 
     s->resize_bh = qemu_bh_new(vhost_blk_resize_bh, s);
@@ -387,10 +404,14 @@ static bool vhost_blk_uevent_attach(VHostBlk *s, Error **errp)
 
 static void vhost_blk_uevent_detach(VHostBlk *s)
 {
+    g_autofree char *cpr_name = NULL;
+
     if (s->uevent_fd < 0) {
         return;
     }
 
+    cpr_name = object_get_canonical_path(OBJECT(s));
+    cpr_delete_fd(cpr_name, 0);
     qemu_set_fd_handler(s->uevent_fd, NULL, NULL, NULL);
     qemu_bh_delete(s->resize_bh);
     s->resize_bh = NULL;
-- 
2.43.5