Re: [Xen-devel] [PATCH] netback: allow arbitrary mtu size until frontend connects

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 06.02.11 at 14:42, Olaf Hering <olaf@xxxxxxxxx> wrote:
> Allow arbitrary mtu size until the frontend is connected.  Once the
> connection is established, adjust mtu by checking if the backend
> supports the 'feature-sg'.  If the backend does support it, keep the
> current mtu. Otherwise set it to the default value, which is 1500.
> 
> This helps the vif-bridge hotplug script to set the mtu size to 9000
> while bringing up the guest.

Isn't this functionally the same as
http://xenbits.xen.org/XCP/linux-2.6.32.pq.hg?file/043b76e4943c/xen-netback-Allow-setting-of-large-MTU-before-rings.patch
or its pv-ops parent 
https://git.kernel.org/?p=linux/kernel/git/jeremy/xen.git;a=commitdiff;h=bee2eec2355c4bf4e149a426d5e30527162de566
(in which case it would seem preferable to pull in that change)?

Ian, looking at that patch of yours again, the adjustment to
netbk_set_sg() seems a little odd: Why is it necessary to
reduce dev->mtu unconditionally (i.e. independent of the
value of "data") here?

Jan

> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> 
> ---
>  drivers/xen/netback/common.h    |    8 ++++++++
>  drivers/xen/netback/interface.c |    8 +++++++-
>  drivers/xen/netback/xenbus.c    |    9 +++++++++
>  3 files changed, 24 insertions(+), 1 deletion(-)
> 
> --- linux-2.6.18-xen.hg.orig/drivers/xen/netback/common.h
> +++ linux-2.6.18-xen.hg/drivers/xen/netback/common.h
> @@ -82,6 +82,8 @@ typedef struct netif_st {
>       u8 can_queue:1; /* can queue packets for receiver? */
>       u8 copying_receiver:1;  /* copy packets to receiver?       */
>  
> +     u8 frontend_connected:1; /* is frontend already connected? */
> +
>       /* Allow netif_be_start_xmit() to peek ahead in the rx request ring. */
>       RING_IDX rx_req_cons_peek;
>  
> @@ -217,4 +219,10 @@ static inline int netbk_can_sg(struct ne
>       return netif->features & NETIF_F_SG;
>  }
>  
> +static inline int netbk_frontend_connected(struct net_device *dev)
> +{
> +     netif_t *netif = netdev_priv(dev);
> +     return !!netif->frontend_connected;
> +}
> +
>  #endif /* __NETIF__BACKEND__COMMON_H__ */
> --- linux-2.6.18-xen.hg.orig/drivers/xen/netback/interface.c
> +++ linux-2.6.18-xen.hg/drivers/xen/netback/interface.c
> @@ -83,9 +83,13 @@ static int net_close(struct net_device *
>       return 0;
>  }
>  
> +/* accept the max value if frontend is not yet connected */
>  static int netbk_change_mtu(struct net_device *dev, int mtu)
>  {
> -     int max = netbk_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
> +     int max = 65535 - ETH_HLEN;
> +
> +     if (netbk_frontend_connected(dev))
> +             max = netbk_can_sg(dev) ? max : ETH_DATA_LEN;
>  
>       if (mtu > max)
>               return -EINVAL;
> @@ -359,6 +363,8 @@ err_rx:
>  
>  void netif_disconnect(netif_t *netif)
>  {
> +     netif->frontend_connected = 0;
> +
>       if (netback_carrier_ok(netif)) {
>               rtnl_lock();
>               netback_carrier_off(netif);
> --- linux-2.6.18-xen.hg.orig/drivers/xen/netback/xenbus.c
> +++ linux-2.6.18-xen.hg/drivers/xen/netback/xenbus.c
> @@ -329,6 +329,13 @@ static int xen_net_read_mac(struct xenbu
>       return 0;
>  }
>  
> +static void xen_net_adjust_mtu(netif_t *netif)
> +{
> +     if (!netif->dev->change_mtu)
> +             return;
> +     netif->dev->change_mtu(netif->dev, netif->dev->mtu);
> +}
> +
>  static void connect(struct backend_info *be)
>  {
>       int err;
> @@ -338,6 +345,8 @@ static void connect(struct backend_info
>       if (err)
>               return;
>  
> +     be->netif->frontend_connected = 1;
> +     xen_net_adjust_mtu(be->netif);
>       err = xen_net_read_mac(dev, be->netif->fe_dev_addr);
>       if (err) {
>               xenbus_dev_fatal(dev, err, "parsing %s/mac", dev->nodename);
> 
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