On Sat, 12 May 2018 02:30:33 -0700
k...@linuxonhyperv.com wrote:

>  int vmbus_post_msg(void *buffer, size_t buflen, bool can_sleep)
>  {
> +     struct vmbus_channel_message_header *hdr;
>       union hv_connection_id conn_id;
>       int ret = 0;
>       int retries = 0;
>       u32 usec = 1;
>  
>       conn_id.asu32 = 0;
> -     conn_id.u.id = VMBUS_MESSAGE_CONNECTION_ID;
> +     conn_id.u.id = vmbus_connection.msg_conn_id;
>  
>       /*
>        * hv_post_message() can have transient failures because of
> @@ -372,6 +400,18 @@ int vmbus_post_msg(void *buffer, size_t buflen, bool 
> can_sleep)
>  
>               switch (ret) {
>               case HV_STATUS_INVALID_CONNECTION_ID:
> +                     /*
> +                      * See vmbus_negotiate_version(): VMBus protocol 5.0
> +                      * requires that we must use
> +                      * VMBUS_MESSAGE_CONNECTION_ID_4 for the Initiate
> +                      * Contact message, but on old hosts that only
> +                      * support VMBus protocol 4.0 or lower, here we get
> +                      * HV_STATUS_INVALID_CONNECTION_ID and we should
> +                      * return an error immediately without retrying.
> +                      */
> +                     hdr = (struct vmbus_channel_message_header *)buffer;

Hate to pick o the details, but buffer is void * so cast is not necessary here.

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