Re: [PATCH v4 1/7] vhost: recheck dev state in the vhost_migration_log routine

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, Sep 04, 2020 at 12:31:13PM +0300, Dima Stepanov wrote:
> vhost-user devices can get a disconnect in the middle of the VHOST-USER
> handshake on the migration start. If disconnect event happened right
> before sending next VHOST-USER command, then the vhost_dev_set_log()
> call in the vhost_migration_log() function will return error. This error
> will lead to the assert() and close the QEMU migration source process.
> For the vhost-user devices the disconnect event should not break the
> migration process, because:
>   - the device will be in the stopped state, so it will not be changed
>     during migration
>   - if reconnect will be made the migration log will be reinitialized as
>     part of reconnect/init process:
>     #0  vhost_log_global_start (listener=0x563989cf7be0)
>     at hw/virtio/vhost.c:920
>     #1  0x000056398603d8bc in listener_add_address_space (listener=0x563989cf7be0,
>         as=0x563986ea4340 <address_space_memory>)
>     at softmmu/memory.c:2664
>     #2  0x000056398603dd30 in memory_listener_register (listener=0x563989cf7be0,
>         as=0x563986ea4340 <address_space_memory>)
>     at softmmu/memory.c:2740
>     #3  0x0000563985fd6956 in vhost_dev_init (hdev=0x563989cf7bd8,
>         opaque=0x563989cf7e30, backend_type=VHOST_BACKEND_TYPE_USER,
>         busyloop_timeout=0)
>     at hw/virtio/vhost.c:1385
>     #4  0x0000563985f7d0b8 in vhost_user_blk_connect (dev=0x563989cf7990)
>     at hw/block/vhost-user-blk.c:315
>     #5  0x0000563985f7d3f6 in vhost_user_blk_event (opaque=0x563989cf7990,
>         event=CHR_EVENT_OPENED)
>     at hw/block/vhost-user-blk.c:379
> Update the vhost-user-blk device with the internal started_vu field which
> will be used for initialization (vhost_user_blk_start) and clean up
> (vhost_user_blk_stop). This additional flag in the VhostUserBlk structure
> will be used to track whether the device really needs to be stopped and
> cleaned up on a vhost-user level.
> The disconnect event will set the overall VHOST device (not vhost-user) to
> the stopped state, so it can be used by the general vhost_migration_log
> routine.
> Such approach could be propogated to the other vhost-user devices, but
> better idea is just to make the same connect/disconnect code for all the
> vhost-user devices.
> 
> This migration issue was slightly discussed earlier:
>   - https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg01509.html
>   - https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg05241.html
> 
> Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> ---
>  hw/block/vhost-user-blk.c          | 19 ++++++++++++++++---
>  hw/virtio/vhost.c                  | 27 ++++++++++++++++++++++++---
>  include/hw/virtio/vhost-user-blk.h | 10 ++++++++++
>  3 files changed, 50 insertions(+), 6 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 39aec42..a076b1e 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -150,6 +150,7 @@ static int vhost_user_blk_start(VirtIODevice *vdev)
>          error_report("Error starting vhost: %d", -ret);
>          goto err_guest_notifiers;
>      }
> +    s->started_vu = true;
>  
>      /* guest_notifier_mask/pending not used yet, so just unmask
>       * everything here. virtio-pci will do the right thing by
> @@ -175,6 +176,11 @@ static void vhost_user_blk_stop(VirtIODevice *vdev)
>      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
>      int ret;
>  
> +    if (!s->started_vu) {
> +        return;
> +    }
> +    s->started_vu = false;
> +
>      if (!k->set_guest_notifiers) {
>          return;
>      }
> @@ -341,9 +347,7 @@ static void vhost_user_blk_disconnect(DeviceState *dev)
>      }
>      s->connected = false;
>  
> -    if (s->dev.started) {
> -        vhost_user_blk_stop(vdev);
> -    }
> +    vhost_user_blk_stop(vdev);
>  
>      vhost_dev_cleanup(&s->dev);
>  }
> @@ -399,6 +403,15 @@ static void vhost_user_blk_event(void *opaque, QEMUChrEvent event)
>                      NULL, NULL, false);
>              aio_bh_schedule_oneshot(ctx, vhost_user_blk_chr_closed_bh, opaque);
>          }
> +
> +        /*
> +         * Move vhost device to the stopped state. The vhost-user device
> +         * will be clean up and disconnected in BH. This can be useful in
> +         * the vhost migration code. If disconnect was caught there is an
> +         * option for the general vhost code to get the dev state without
> +         * knowing its type (in this case vhost-user).
> +         */
> +        s->dev.started = false;
>          break;
>      case CHR_EVENT_BREAK:
>      case CHR_EVENT_MUX_IN:

Hi Dima,
Is it possible to move this logic into the vhost_*() API so that all
devices benefit from it? This seems like a generic vhost-user issue
rather than a vhost-user-blk issue.

In other words, it would be great if the vhost APIs in QEMU are designed
in a way so that the user doesn't need to think about this.

Stefan

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