Re: [PATCH v2] vhost: make SET_VRING_ADDR, SET_FEATURES send replies

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 19, 2021 at 05:21:38PM +0300, Denis Plotnikov wrote:
> On vhost-user-blk migration, qemu normally sends a number of commands
> to enable logging if VHOST_USER_PROTOCOL_F_LOG_SHMFD is negotiated.
> Qemu sends VHOST_USER_SET_FEATURES to enable buffers logging and
> VHOST_USER_SET_VRING_ADDR per each started ring to enable "used ring"
> data logging.
> The issue is that qemu doesn't wait for reply from the vhost daemon
> for these commands which may result in races between qemu expectation
> of logging starting and actual login starting in vhost daemon.
> 
> The race can appear as follows: on migration setup, qemu enables dirty page
> logging by sending VHOST_USER_SET_FEATURES. The command doesn't arrive to a
> vhost-user-blk daemon immediately and the daemon needs some time to turn the
> logging on internally. If qemu doesn't wait for reply, after sending the
> command, qemu may start migrate memory pages to a destination. At this time,
> the logging may not be actually turned on in the daemon but some guest pages,
> which the daemon is about to write to, may have already been transferred
> without logging to the destination. Since the logging wasn't turned on,
> those pages won't be transferred again as dirty. So we may end up with
> corrupted data on the destination.
> The same scenario is applicable for "used ring" data logging, which is
> turned on with VHOST_USER_SET_VRING_ADDR command.
> 
> To resolve this issue, this patch makes qemu wait for the commands result
> explicilty if VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated and
> logging is enabled.
> 
> Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
> ---
> v1 -> v2:
>   * send reply only when logging is enabled [mst]
> 
> v0 -> v1:
>   * send reply for SET_VRING_ADDR, SET_FEATURES only [mst]
>   
>  hw/virtio/vhost-user.c | 37 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 34 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index ee57abe04526..133588b3961e 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1095,6 +1095,11 @@ static int vhost_user_set_mem_table(struct vhost_dev *dev,
>      return 0;
>  }
>  
> +static bool log_enabled(uint64_t features)
> +{
> +    return !!(features & (0x1ULL << VHOST_F_LOG_ALL));
> +}
> +
>  static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>                                       struct vhost_vring_addr *addr)
>  {
> @@ -1105,10 +1110,21 @@ static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>          .hdr.size = sizeof(msg.payload.addr),
>      };
>  
> +    bool reply_supported = virtio_has_feature(dev->protocol_features,
> +                                              VHOST_USER_PROTOCOL_F_REPLY_ACK);
> +
> +    if (reply_supported && log_enabled(msg.hdr.flags)) {
> +        msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
> +    }
> +
>      if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
>          return -1;
>      }
>  
> +    if (msg.hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
> +        return process_message_reply(dev, &msg);
> +    }
> +
>      return 0;
>  }
>

OK this is good, but the problem is that we then still have a race
if VHOST_USER_PROTOCOL_F_REPLY_ACK is not set. Bummer.

Let's send VHOST_USER_GET_FEATURES in this case to flush out outstanding
messages?

  
> @@ -1288,7 +1304,8 @@ static int vhost_user_set_vring_call(struct vhost_dev *dev,
>      return vhost_set_vring_file(dev, VHOST_USER_SET_VRING_CALL, file);
>  }
>  
> -static int vhost_user_set_u64(struct vhost_dev *dev, int request, uint64_t u64)
> +static int vhost_user_set_u64(struct vhost_dev *dev, int request, uint64_t u64,
> +                              bool need_reply)
>  {
>      VhostUserMsg msg = {
>          .hdr.request = request,
> @@ -1297,23 +1314,37 @@ static int vhost_user_set_u64(struct vhost_dev *dev, int request, uint64_t u64)
>          .hdr.size = sizeof(msg.payload.u64),
>      };
>  
> +    if (need_reply) {
> +        bool reply_supported = virtio_has_feature(dev->protocol_features,
> +                                          VHOST_USER_PROTOCOL_F_REPLY_ACK);
> +        if (reply_supported) {
> +            msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
> +        }
> +    }
> +
>      if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
>          return -1;
>      }
>  
> +    if (msg.hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
> +        return process_message_reply(dev, &msg);
> +    }
> +
>      return 0;
>  }
>  
>  static int vhost_user_set_features(struct vhost_dev *dev,
>                                     uint64_t features)
>  {
> -    return vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES, features);
> +    return vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES, features,
> +                              log_enabled(features));
>  }
>  
>  static int vhost_user_set_protocol_features(struct vhost_dev *dev,
>                                              uint64_t features)
>  {
> -    return vhost_user_set_u64(dev, VHOST_USER_SET_PROTOCOL_FEATURES, features);
> +    return vhost_user_set_u64(dev, VHOST_USER_SET_PROTOCOL_FEATURES, features,
> +                              false);
>  }
>  
>  static int vhost_user_get_u64(struct vhost_dev *dev, int request, uint64_t *u64)
> -- 
> 2.25.1