netdev - Re: [PATCH net-next] xen-netfront: convert to GRO API and advertise this feature

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Date:	Sun, 22 Sep 2013 22:58:27 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Anirban Chakraborty <abchak@...iper.net>
Cc:	Wei Liu <wei.liu2@...rix.com>,
	"<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
	"<xen-devel@...ts.xen.org>" <xen-devel@...ts.xen.org>,
	Ian Campbell <ian.campbell@...rix.com>
Subject: Re: [PATCH net-next] xen-netfront: convert to GRO API and advertise
 this feature

On Sun, 2013-09-22 at 23:09 +0000, Anirban Chakraborty wrote:
> On Sep 22, 2013, at 7:55 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > On Sat, 2013-09-21 at 17:05 +0100, Wei Liu wrote:
> >> Anirban was seeing netfront received MTU size packets, which downgraded
> >> throughput. The following patch makes netfront use GRO API which
> >> improves throughput for that case.
> > 
> >> -	netdev->hw_features	= NETIF_F_IP_CSUM | NETIF_F_SG | NETIF_F_TSO;
> >> +	netdev->hw_features	= NETIF_F_IP_CSUM | NETIF_F_SG | NETIF_F_TSO |
> >> +				  NETIF_F_GRO;
> > 
> > 
> > This part is not needed.
> 
> Shouldn't the flag be set? In dev_gro_receive() we do check if this flag is set or not:
> 
>         if (!(skb->dev->features & NETIF_F_GRO) || netpoll_rx_on(skb))
>                goto normal;

Drivers do not set NETIF_F_GRO themselves, they do not need to.

Look at other drivers which are GRO ready : NETIF_F_GRO is enabled by
default by core networking stack, in register_netdevice()


dev->hw_features |= NETIF_F_SOFT_FEATURES;
dev->features |= NETIF_F_SOFT_FEATURES;



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