ping!

On 19.07.2021 17:21, 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;
>   }
>   
> @@ -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)