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

[Eugenio Perez Martin <eperezma@redhat.com>]

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?

> +        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
>