netdev - Re: [PATCH net-next v7 4/9] xen-netback: Introduce TX grant mapping

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Date:	Thu, 13 Mar 2014 10:17:54 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	<wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next v7 4/9] xen-netback: Introduce TX grant mapping

Pulling out this one comment for the attention on the core Xen/Linux
maintainers.

On Thu, 2014-03-06 at 21:48 +0000, Zoltan Kiss wrote:
[...]
> @@ -343,8 +347,26 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
>  	vif->pending_prod = MAX_PENDING_REQS;
>  	for (i = 0; i < MAX_PENDING_REQS; i++)
>  		vif->pending_ring[i] = i;
> -	for (i = 0; i < MAX_PENDING_REQS; i++)
> -		vif->mmap_pages[i] = NULL;
> +	spin_lock_init(&vif->callback_lock);
> +	spin_lock_init(&vif->response_lock);
> +	/* If ballooning is disabled, this will consume real memory, so you
> +	 * better enable it. The long term solution would be to use just a
> +	 * bunch of valid page descriptors, without dependency on ballooning
> +	 */

I wonder if we ought to enforce this via Kconfig? i.e. making
CONFIG_XEN_BACKEND (or the individual backends) depend on BALLOON (or
select?) or by making CONFIG_XEN_BALLOON non-optional etc.

IIRC David V was looking into a solution involving auto hotplugging a
new region to use for this case, but then I guess
CONFIG_XEN_BALLOON_MEMORY_HOTPLUG would equally need to be enabled.

> +	err = alloc_xenballooned_pages(MAX_PENDING_REQS,
> +				       vif->mmap_pages,
> +				       false);
[...]

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