[Qemu-devel] [PATCH] net: Fix link state inter-dependencies between NIC and backend

[Vladislav Yasevich]

Commit 02d38fcb2caa4454cf4ed728d5908c3cc9ba47be
    net: Update netdev peer on link change

introduced a link state dependency between the backend
netdev and the nic.  If the admin turned off the link
on the backend, the nic link state was set to down because
the link had no access to the network at all.  This created
some inconsitet behavior for someone who wanted to play
around the links states.
 1) Turning off the nic link and then turning on the backend
    link (even if it was already on) would turn on the nic link
    again.
 2) Turning off the backend link and then turning on the nic
    link would turn on the link in the VM, but would not change
    the backend state resulting in a broken/unreachable network.

This patch attempts to correct these behaviors.  The patch treats
the nic-backend relationship as two ends of a wire.  Each end tracks
the administratively set link state in addition to actual link
state.  Thus is is possible to set link down at each end effectively
emulating plugging/unplugging the wire at either end.  The patch
continues to preserve the old behavior where setting just
nic side down does NOT impact the backend.

Signed-off-by: Vladislav Yasevich <address@hidden>
---
 include/net/net.h |  1 +
 net/net.c         | 25 +++++++++++++++++++++++--
 roms/ipxe         |  2 +-
 roms/openbios     |  2 +-
 roms/seabios      |  2 +-
 5 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/include/net/net.h b/include/net/net.h
index 50ffcb9..8d04861 100644
--- a/include/net/net.h
+++ b/include/net/net.h
@@ -78,6 +78,7 @@ typedef struct NetClientInfo {
 struct NetClientState {
     NetClientInfo *info;
     int link_down;
+    int admin_link_down;
     QTAILQ_ENTRY(NetClientState) next;
     NetClientState *peer;
     NetQueue *incoming_queue;
diff --git a/net/net.c b/net/net.c
index 0be084d..be8de61 100644
--- a/net/net.c
+++ b/net/net.c
@@ -1158,6 +1158,26 @@ void qmp_set_link(const char *name, bool up, Error 
**errp)
     }
     nc = ncs[0];
 
+    /* Always set the administratively requested link state */
+    nc->admin_link_down = !up;
+
+    /* If link status will not change, don't bother doing the work */
+    if (nc->link_down == !up) {
+        return;
+    }
+
+    /* If this is a NIC device and the user is trying to turn the link on,
+     * look at our peer link state first.
+     * If the peer link state is down, then the peer was unpluged and
+     * turning on the link state on this NIC will result in a broken network.
+     * Report an error to the user.
+     */
+    if (nc->info->type == NET_CLIENT_OPTIONS_KIND_NIC && up &&
+        nc->peer && nc->peer->link_down) {
+        error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, name);
+        return;
+    }
+
     for (i = 0; i < queues; i++) {
         ncs[i]->link_down = !up;
     }
@@ -1166,8 +1186,9 @@ void qmp_set_link(const char *name, bool up, Error **errp)
         nc->info->link_status_changed(nc);
     }
 
-    if (nc->peer) {
-        /* Change peer link only if the peer is NIC and then notify peer.
+    if (nc->peer && !nc->peer->admin_link_down) {
+        /* Change peer link only if the peer is a NIC that has not been forced
+         * down and then notify peer.
          * If the peer is a HUBPORT or a backend, we do not change the
          * link status.
          *
diff --git a/roms/ipxe b/roms/ipxe
index 35c5379..5de37e1 160000
--- a/roms/ipxe
+++ b/roms/ipxe
@@ -1 +1 @@
-Subproject commit 35c5379760aa1fea5e38f7a78b090f92bb7813ee
+Subproject commit 5de37e124fd21c8f918f3fe26fc33a764709ead4
diff --git a/roms/openbios b/roms/openbios
index 5d3db90..d9e38ba 160000
--- a/roms/openbios
+++ b/roms/openbios
@@ -1 +1 @@
-Subproject commit 5d3db901435ef5a114c9d89461dd0f6d1ef1d44f
+Subproject commit d9e38ba2ffd2d2cdfa840ea9bc7dd4a64472f2c7
diff --git a/roms/seabios b/roms/seabios
index 4adadbd..67d1fbe 160000
--- a/roms/seabios
+++ b/roms/seabios
@@ -1 +1 @@
-Subproject commit 4adadbde6904807de2e990c0af839ad0cc977806
+Subproject commit 67d1fbef0f630e1e823f137d1bae7fa5790bcf4e
-- 
1.9.3