Subject: Re: [Xen-devel] [PATCH net-next] xen-netfront: always set num queues if possible

From: David Vrabel <[email-masked]>
Date: Tue, 15 Sep 2015 11:59:57 +0100

On 14/09/15 22:28, Charles (Chas) Williams wrote:
> The xen store preserves this information across module invocations.
> If you insmod netfront with two queues and later insmod again with one
> queue, the backend will still believe you asked for two queues.

Can you rewrite the commit message to be clearer?

"If netfront connects with 2 (or more) queues and then reconnects with
only 1 queue it fails to delete or rewrite the multi-queue-num-queues
key and netback will try to use the wrong number of queues.

Always write the num-queues field if the backend has multi-queue support."

> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -1819,11 +1819,7 @@ again:
>  		goto destroy_ring;
>  	}
>  
> -	if (num_queues == 1) {
> -		err = write_queue_xenstore_keys(&info->queues[0], &xbt, 0); /* flat */
> -		if (err)
> -			goto abort_transaction_no_dev_fatal;
> -	} else {
> +	if (xenbus_exists(xbt, dev->nodename, "multi-queue-num-queues")) {
>  		/* Write the number of queues */
>  		err = xenbus_printf(xbt, dev->nodename, "multi-queue-num-queues",
>  				    "%u", num_queues);

Isn't this broken?  It looks like it won't write the
multi-queue-num-queues key the first time around.

It think this should be conditional on multi-queue-max-queues existing
(which is written by the backend if multi-queue is supported).

David