netdev - Re: [RFC] tcp: use order-3 pages in tcp

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Date:	Fri, 21 Sep 2012 13:04:40 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	rick.jones2@...com, subramanian.vijay@...il.com,
	netdev@...r.kernel.org
Subject: Re: [RFC] tcp: use order-3 pages in tcp_sendmsg()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 21 Sep 2012 18:51:21 +0200

> On Fri, 2012-09-21 at 12:27 -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Fri, 21 Sep 2012 17:48:31 +0200
>> 
>> > There is probably a reason why lo default MTU is 16436 ?
>> 
>> That's what fit into L1 caches back in 1999
> 
> I see ;)
> 
> Nowadays, we even have the NETIF_F_NOCACHE_COPY flag and
> __copy_from_user_nocache()
> 
> Hmm, we can not toggle this flag on loopback yet
> 
> # ethtool -K lo tx-nocache-copy on
> Could not change any device features

It's a silly limitation, in net/core/dev.c:

	/* Turn on no cache copy if HW is doing checksum */
	if (!(dev->flags & IFF_LOOPBACK)) {
		dev->hw_features |= NETIF_F_NOCACHE_COPY;
		if (dev->features & NETIF_F_ALL_CSUM) {
			dev->wanted_features |= NETIF_F_NOCACHE_COPY;
			dev->features |= NETIF_F_NOCACHE_COPY;
		}
	}

Maybe this is probably better done as:

	/* Turn on no cache copy if HW is doing checksum */
	dev->hw_features |= NETIF_F_NOCACHE_COPY;
	if (!(dev->flags & IFF_LOOPBACK)) {
		if (dev->features & NETIF_F_ALL_CSUM) {
			dev->wanted_features |= NETIF_F_NOCACHE_COPY;
			dev->features |= NETIF_F_NOCACHE_COPY;
		}
	}

And then the code matches more closely the comment. :-)
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