Subject: Re: [Xen-devel] [RFC PATCH 12/13] xen-netfront: implement TX persistent grants

From: David Vrabel <[email-masked]>
Date: Mon, 18 May 2015 16:55:09 +0100

On 12/05/15 18:18, Joao Martins wrote:
> Instead of grant/revoking the buffer related to the skb, it will use
> an already granted page and memcpy  to it. The grants will be mapped
> by xen-netback and reused overtime, but only unmapped when the vif
> disconnects, as opposed to every packet.
> 
> This only happens if the backend supports persistent grants since it
> would, otherwise, introduce the overhead of a memcpy on top of the
> grant map.
[...]
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
[...]
> @@ -1610,7 +1622,10 @@ static int xennet_init_queue(struct netfront_queue *queue)
>  	for (i = 0; i < NET_TX_RING_SIZE; i++) {
>  		skb_entry_set_link(&queue->tx_skbs[i], i+1);
>  		queue->grant_tx[i].ref = GRANT_INVALID_REF;
> -		queue->grant_tx[i].page = NULL;
> +		if (queue->info->feature_persistent)
> +			queue->grant_tx[i].page = alloc_page(GFP_NOIO);

Need to check for alloc failure here and unwind correctly?

Why NOIO?

> +		else
> +			queue->grant_tx[i].page = NULL;
>  	}
>  
>  	/* Clear out rx_skbs */
>