Re: [PATCH] replay: improve determinism of virtio-net

[Michael S. Tsirkin]

On Mon, May 17, 2021 at 04:04:20PM +0300, Pavel Dovgalyuk wrote:
> virtio-net device uses bottom halves for callbacks.
> These callbacks should be deterministic, because they affect VM state.
> This patch replaces BH invocations with corresponding replay functions,
> making them deterministic in record/replay mode.
> This patch also disables guest announce timers for record/replay,
> because they break correct loadvm in deterministic mode.
> 
> Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>

Seems to make sense but clearly Jason's area.
Jason?


> ---
>  hw/net/virtio-net.c |   13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 6b7e8dd04e..e876363236 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -44,6 +44,7 @@
>  #include "hw/pci/pci.h"
>  #include "net_rx_pkt.h"
>  #include "hw/virtio/vhost.h"
> +#include "sysemu/replay.h"
>  
>  #define VIRTIO_NET_VM_VERSION    11
>  
> @@ -394,7 +395,7 @@ static void virtio_net_set_status(struct VirtIODevice 
> *vdev, uint8_t status)
>                  timer_mod(q->tx_timer,
>                                 qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + 
> n->tx_timeout);
>              } else {
> -                qemu_bh_schedule(q->tx_bh);
> +                replay_bh_schedule_event(q->tx_bh);
>              }
>          } else {
>              if (q->tx_timer) {
> @@ -2546,7 +2547,7 @@ static void virtio_net_handle_tx_bh(VirtIODevice *vdev, 
> VirtQueue *vq)
>          return;
>      }
>      virtio_queue_set_notification(vq, 0);
> -    qemu_bh_schedule(q->tx_bh);
> +    replay_bh_schedule_event(q->tx_bh);
>  }
>  
>  static void virtio_net_tx_timer(void *opaque)
> @@ -2602,7 +2603,7 @@ static void virtio_net_tx_bh(void *opaque)
>      /* If we flush a full burst of packets, assume there are
>       * more coming and immediately reschedule */
>      if (ret >= n->tx_burst) {
> -        qemu_bh_schedule(q->tx_bh);
> +        replay_bh_schedule_event(q->tx_bh);
>          q->tx_waiting = 1;
>          return;
>      }
> @@ -2616,7 +2617,7 @@ static void virtio_net_tx_bh(void *opaque)
>          return;
>      } else if (ret > 0) {
>          virtio_queue_set_notification(q->tx_vq, 0);
> -        qemu_bh_schedule(q->tx_bh);
> +        replay_bh_schedule_event(q->tx_bh);
>          q->tx_waiting = 1;
>      }
>  }
> @@ -3206,6 +3207,10 @@ static void virtio_net_device_realize(DeviceState 
> *dev, Error **errp)
>          n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>      }
>  
> +    if (replay_mode != REPLAY_MODE_NONE) {
> +        n->host_features &= ~(1ULL << VIRTIO_NET_F_GUEST_ANNOUNCE);
> +    }
> +
>      if (n->net_conf.duplex_str) {
>          if (strncmp(n->net_conf.duplex_str, "half", 5) == 0) {
>              n->net_conf.duplex = DUPLEX_HALF;