[RFC 6/7] net/tap: experimental support for vhost in local tap migration

[Vladimir Sementsov-Ogievskiy]

Just reinitialize vhost after getting the fd on target.
This significantly increase downtime (0.03s -> 0.06s in my
simple testing). So next step is to rework it, completely migrating
the vhost state to the new qemu without reinitialization.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@yandex-team.ru>
---
 net/tap.c | 35 +++++++++++++++++------------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/net/tap.c b/net/tap.c
index 4d3cdc0662..7a4de07025 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -84,6 +84,9 @@ typedef struct TAPState {
     unsigned host_vnet_hdr_len;
     Notifier exit;
     bool local_incoming;
+    bool need_vhost;
+    char *vhost_fdname;
+    uint32_t vhost_busyloop_timeout;
 } TAPState;
 
 static void launch_script(const char *setup_script, const char *ifname,
@@ -98,8 +101,7 @@ static int net_tap_fd_init_common(const Netdev *netdev, 
NetClientState *peer,
                                   const char *downscript,
                                   const char *vhostfdname,
                                   int vnet_hdr, int fd, Error **errp);
-static int net_tap_setup_vhost(TAPState *s, const NetdevTapOptions *tap,
-                               const char *vhostfdname, Error **errp);
+static int net_tap_setup_vhost(TAPState *s, Error **errp);
 
 static void tap_update_fd_handler(TAPState *s)
 {
@@ -384,6 +386,7 @@ static void tap_cleanup(NetClientState *nc)
         g_free(s->vhost_net);
         s->vhost_net = NULL;
     }
+    g_free(s->vhost_fdname);
 
     qemu_purge_queued_packets(nc);
 
@@ -794,10 +797,8 @@ int tap_load(NetClientState *nc, QEMUFile *f)
     qemu_get_be32s(f, &s->host_vnet_hdr_len);
 
     tap_read_poll(s, true);
-    s->exit.notify = tap_exit_notify;
-    qemu_add_exit_notifier(&s->exit);
 
-    return 0;
+    return net_tap_setup_vhost(s, NULL);
 }
 
 static int net_tap_fd_init_common(const Netdev *netdev, NetClientState *peer,
@@ -874,8 +875,12 @@ static int net_tap_fd_init_common(const Netdev *netdev, 
NetClientState *peer,
         }
     }
 
+    s->need_vhost = tap->has_vhost ? tap->vhost :
+        vhostfdname || (tap->has_vhostforce && tap->vhostforce);
+    s->vhost_fdname = g_strdup(vhostfdname);
+    s->vhost_busyloop_timeout = tap->has_poll_us ? tap->poll_us : 0;
     if (!local_incoming) {
-        ret = net_tap_setup_vhost(s, tap, vhostfdname, errp);
+        ret = net_tap_setup_vhost(s, errp);
         if (ret < 0) {
             goto failed;
         }
@@ -888,28 +893,22 @@ failed:
     return -1;
 }
 
-static int net_tap_setup_vhost(TAPState *s, const NetdevTapOptions *tap,
-                               const char *vhostfdname, Error **errp)
+static int net_tap_setup_vhost(TAPState *s, Error **errp)
 {
-    if (tap->has_vhost ? tap->vhost :
-        vhostfdname || (tap->has_vhostforce && tap->vhostforce)) {
+    if (s->need_vhost) {
         VhostNetOptions options;
         int vhostfd;
 
         options.backend_type = VHOST_BACKEND_TYPE_KERNEL;
         options.net_backend = &s->nc;
-        if (tap->has_poll_us) {
-            options.busyloop_timeout = tap->poll_us;
-        } else {
-            options.busyloop_timeout = 0;
-        }
+        options.busyloop_timeout = s->vhost_busyloop_timeout;
 
-        if (vhostfdname) {
-            vhostfd = monitor_fd_param(monitor_cur(), vhostfdname, errp);
+        if (s->vhost_fdname) {
+            vhostfd = monitor_fd_param(monitor_cur(), s->vhost_fdname, errp);
             if (vhostfd == -1) {
                 return -1;
             }
-            if (!set_fd_nonblocking(vhostfd, vhostfdname, errp)) {
+            if (!set_fd_nonblocking(vhostfd, s->vhost_fdname, errp)) {
                 return -1;
             }
         } else {
-- 
2.48.1