[RFC v2 13/14] virtio-net: detect pending Tx work for VQs mid-migration changes

[Jonah Palmer <jonah.palmer@oracle.com>]

Track per-queue pending Tx work state during early migration time.

This includes a snapshot of pending Tx state for active queue pairs,
checking for deltas at the end of migration, and freeing the snapshot
buffer during device unrealize.

With this final delta signal in place, drop the temporary always-true
fallback in virtio_net_has_delta and return false when no deltas are
found.

Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
---
 hw/net/virtio-net.c            | 26 +++++++++++++++++++++-----
 include/hw/virtio/virtio-net.h |  2 ++
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 3ee49a043a..483a43be4f 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3943,6 +3943,14 @@ static int virtio_net_early_pre_save(void *opaque)
     vnet_mig->mq_early = n->multiqueue;
     vnet_mig->queue_pairs_early = n->curr_queue_pairs;
 
+    /* Tx waiting snapshot for active queue pairs */
+    if (!vnet_mig->tx_waiting_early) {
+        vnet_mig->tx_waiting_early = g_new0(uint32_t, n->max_queue_pairs);
+    }
+    for (int i = 0; i < n->curr_queue_pairs; i++) {
+        vnet_mig->tx_waiting_early[i] = n->vqs[i].tx_waiting;
+    }
+
     /* RSS state snapshot */
     vnet_mig->rss_enabled_early = n->rss_data.enabled;
     vnet_mig->rss_redirect_early = n->rss_data.redirect;
@@ -4254,6 +4262,8 @@ static void virtio_net_device_unrealize(DeviceState *dev)
         n->migration->mtable_macs_early = NULL;
         g_free(n->migration->vlans_early);
         n->migration->vlans_early = NULL;
+        g_free(n->migration->tx_waiting_early);
+        n->migration->tx_waiting_early = NULL;
         g_free(n->migration->rss_indirections_table_early);
         n->migration->rss_indirections_table_early = NULL;
         g_free(n->migration);
@@ -4412,6 +4422,16 @@ static bool virtio_net_has_delta(VirtIONet *n, VirtIODevice *vdev)
         return true;
     }
 
+    /* Has any active queue's tx_waiting changed? */
+    if (!vnet_mig->tx_waiting_early) {
+        return true;
+    }
+    for (int i = 0; i < n->curr_queue_pairs; i++) {
+        if (n->vqs[i].tx_waiting != vnet_mig->tx_waiting_early[i]) {
+            return true;
+        }
+    }
+
     /* Has the VirtIONet's RSS state changed? */
     if (n->rss_data.enabled != vnet_mig->rss_enabled_early ||
         n->rss_data.redirect != vnet_mig->rss_redirect_early ||
@@ -4439,11 +4459,7 @@ static bool virtio_net_has_delta(VirtIONet *n, VirtIODevice *vdev)
         }
     }
 
-    /*
-     * Always return true for now until we're able to detect all possible
-     * changes to a VirtIONet device.
-     */
-    return true;
+    return false;
 }
 
 static bool virtio_net_needed(void *opaque)
diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index 88074a0976..dbbacc83bb 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -180,6 +180,7 @@ typedef struct VirtIONetQueue {
  * @guest_offloads_early: Guest offloads snapshot.
  * @mq_early: Multiqueue state snapshot.
  * @queue_pairs_early: Queue pairs snapshot.
+ * @tx_waiting_early: Per-queue pending-Tx snapshot.
  * @rss_enabled_early: RSS enabled flag.
  * @rss_redirect_early: RSS redirect flag.
  * @rss_populate_hash_early: RSS populate hash flag.
@@ -201,6 +202,7 @@ typedef struct VirtIONetMigration {
     uint64_t guest_offloads_early;
     int mq_early;
     uint16_t queue_pairs_early;
+    uint32_t *tx_waiting_early;
     bool rss_enabled_early;
     bool rss_redirect_early;
     bool rss_populate_hash_early;
-- 
2.51.0