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Re: [PATCH v0] vhost: make SET_VRING_ADDR, SET_[PROTOCOL_]FEATEURES send


From: Michael S. Tsirkin
Subject: Re: [PATCH v0] vhost: make SET_VRING_ADDR, SET_[PROTOCOL_]FEATEURES send replies
Date: Wed, 7 Jul 2021 06:10:17 -0400

On Fri, Jun 25, 2021 at 11:52:10AM +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_FEATURES 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.

Could you be more explicit please? What kind of race have you
observed? Getting a reply slows down the setup considerably and
should not be done lightly.

Thanks!

> To resolve this issue, this patch makes qemu wait for the commands result
> explicilty if VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated.
> Also, this patch adds the reply waiting for VHOST_USER_SET_PROTOCOL_FEATURES
> command to make the features setting functions work similary.
> 
> Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
> ---
>  hw/virtio/vhost-user.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index ee57abe04526..e47b82adab00 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1105,10 +1105,20 @@ 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) {
> +        msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
> +    }
> +
>      if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
>          return -1;
>      }
>  
> +    if (reply_supported) {
> +        return process_message_reply(dev, &msg);
> +    }
> +
>      return 0;
>  }
>  
> @@ -1297,10 +1307,20 @@ static int vhost_user_set_u64(struct vhost_dev *dev, 
> int request, uint64_t u64)
>          .hdr.size = sizeof(msg.payload.u64),
>      };
>  
> +    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 (reply_supported) {
> +        return process_message_reply(dev, &msg);
> +    }
> +
>      return 0;
>  }
>  
> -- 
> 2.25.1




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