[Xen-devel] Re: [PATCH] Don't allow sharing of tx skbs on xen-netfront

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2011-11-14 at 14:22 -0500, Neil Horman wrote:
> It was pointed out to me recently that the xen-netfront driver can't safely
> support shared skbs on transmit, since, while it doesn't maintain skb state
> directly, it does pass a pointer to the skb to the hypervisor via a list, and
> the hypervisor may expect the contents of the skb to remain stable.  Clearing
> the IFF_TX_SKB_SHARING flag after the call to alloc_etherdev to make it safe.

What are the actual constraints here? The skb is used as a handle to the
skb->data and shinfo (frags) and to complete at the end. It's actually
those which are passed to the hypervisor (effectively the same as
passing those addresses to the h/w for DMA).

Which parts of the skb are expected/allowed to not remain stable?

(Appologies if the above seems naive, I seem to have missed the
introduction of shared tx skbs and IFF_TX_SKB_SHARING)

Ian.

> 
> Signed-off-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
> CC: "David S. Miller" <davem@xxxxxxxxxxxxx>
> CC: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> CC: xen-devel@xxxxxxxxxxxxxxxxxxx
> ---
>  drivers/net/xen-netfront.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 226faab..fb1077b 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -1252,6 +1252,12 @@ static struct net_device * __devinit 
> xennet_create_dev(struct xenbus_device *dev
>               return ERR_PTR(-ENOMEM);
>       }
>  
> +     /*
> +      * Since frames remain on a queue after a return from xennet_start_xmit,
> +      * we can't support tx shared skbs
> +      */
> +     netdev->priv_flags &= ~IFF_TX_SKB_SHARING;
> +
>       np                   = netdev_priv(netdev);
>       np->xbdev            = dev;
>  



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