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

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

On 3/24/26 7:35 AM, Eugenio Perez Martin wrote:
> On Fri, Mar 20, 2026 at 3:21 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>>
>> 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;
> 
> I'm failing to see one thing: If we add a new feature that the guest
> can change during migration and it's only added in the
> VMStateDescription vmstate_virtio_net_device, will this "return false"
> prevent the state from being resent in the stop-and-copy phase?
> 
> Mandating its addition here is ok somehow, but I'm not sure if I'm
> missing something.
> 

Correct. But this is also the same case today. That is, any new 
migratable device state would still need to be added to the appropriate 
VMSD/subsection. For example, if today we added a new member to 
VirtIONet that also should be migrated, it would also need to be added 
to vmstate_virtio_net_device (or the appropriate subsection).

So it's not a new *kind* of maintenance burden, but it is an additional 
place that'd need to be updated.

>>   }
>>
>>   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
>>
>