Re: [RFC v2 05/14] virtio-net: detect VirtIODevice config buffer mid-migration change

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

On 3/24/26 6:48 AM, Eugenio Perez Martin wrote:
> On Fri, Mar 20, 2026 at 3:21 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>>
>> This patch saves the config buffer and its length of a virtio-net
>> device's VirtIODevice to compare with later during the stop-and-copy
>> phase.
>>
>> Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
>> ---
>>   hw/net/virtio-net.c        | 21 +++++++++++++++++++++
>>   include/hw/virtio/virtio.h |  4 ++++
>>   2 files changed, 25 insertions(+)
>>
>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>> index 2733e0130c..ca4385df1a 100644
>> --- a/hw/net/virtio-net.c
>> +++ b/hw/net/virtio-net.c
>> @@ -3871,6 +3871,15 @@ static int virtio_net_early_pre_save(void *opaque)
>>       VirtIODevMigration *vdev_mig = vdev->migration;
>>
>>       vdev_mig->status_early = vdev->status;
>> +
>> +    /* VirtIODevice config buffer snapshot */
>> +    g_free(vdev_mig->config_early);
>> +    vdev_mig->config_len_early = vdev->config_len;
>> +    if (vdev->config_len) {
>> +        vdev_mig->config_early = g_memdup2(vdev->config, vdev->config_len);
>> +    } else {
>> +        vdev_mig->config_early = NULL;
>> +    }
>>       return 0;
>>   }
>>
>> @@ -4150,6 +4159,9 @@ static void virtio_net_device_unrealize(DeviceState *dev)
>>       virtio_cleanup(vdev);
>>
>>       if (n->early_mig) {
>> +        g_free(vdev->migration->config_early);
>> +        vdev->migration->config_early = NULL;
>> +
>>           g_free(vdev->migration);
>>           vdev->migration = NULL;
>>
>> @@ -4251,6 +4263,15 @@ static bool virtio_net_has_delta(VirtIONet *n, VirtIODevice *vdev)
>>           return true;
>>       }
>>
>> +    /* Has the VirtIODevice's config buffer changed? */
>> +    if (vdev->config_len != vdev_mig->config_len_early) {
>> +        return true;
>> +    }
>> +    if (vdev->config_len && memcmp(vdev->config, vdev_mig->config_early,
>> +                                   vdev->config_len) != 0) {
> 
> I'm happy with this but maybe a comparison to the config generation is enough?
> 

A change in vdev->generation would only tell us that 
virtio_notify_config ran. It wouldn't tell us which field changed or 
what exactly needs to be updated.

And although this RFC series currently just aims to detect a change and, 
if found, signal for a full reload, an actual follow-up PATCH series for 
this will handle sending the required change to the destination instead 
of doing this full reload. For that, we'll need actual per-field deltas.

>> +        return true;
>> +    }
>> +
>>       /*
>>        * Always return true for now until we're able to detect all possible
>>        * changes to a VirtIONet device.
>> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
>> index 752c46ce53..9949b94b64 100644
>> --- a/include/hw/virtio/virtio.h
>> +++ b/include/hw/virtio/virtio.h
>> @@ -103,10 +103,14 @@ enum virtio_device_endian {
>>    * struct VirtIODevMigration - Common VirtIODevice migration structure
>>    * @early_load: Flag to indicate an early virtio_load for the device.
>>    * @status_early: Device status at the time it was sent early.
>> + * @config_len_early: Length of the config buffer at the time it was sent early.
>> + * @config_early: Config buffer at the time it was sent early.
>>    */
>>   typedef struct VirtIODevMigration {
>>       bool early_load;
>>       uint8_t status_early;
>> +    size_t config_len_early;
>> +    uint8_t *config_early;
>>   } VirtIODevMigration;
>>
>>   /**
>> --
>> 2.51.0
>>
>