Re: Dropping NETIF_F_SG since no checksum feature.

Michael S. Tsirkin
2006-10-11T09:47:31Z

Quoting r. David Miller <davem@davemloft.net>:
> Subject: Re: Dropping NETIF_F_SG since no checksum feature.
>
> From: "Michael S. Tsirkin" <mst@mellanox.co.il>
> Date: Wed, 11 Oct 2006 11:05:04 +0200
>
> > So, it seems that if I set NETIF_F_SG but clear NETIF_F_ALL_CSUM,
> > data will be copied over rather than sent directly.
> > So why does dev.c have to force set NETIF_F_SG to off then?
>
> Because it's more efficient to copy into a linear destination
> buffer of an SKB than page sub-chunks when doing checksum+copy.
>

Thanks for the explanation.
Obviously its true as long as you can allocate the skb that big.
I think you won't realistically be able to get 64K in a
linear SKB on a busy system, though, is not that right?

OTOH, having large MTU (e.g. 64K) helps performance a lot since it
reduces receive side processing overhead.

So, if I understand what you are saying correctly,
things do work correctly (just slower for small skb) if NETIF_F_SG is set bug
clear, it seems that all we need to do is drop the following in dev.c:

/* Fix illegal SG+CSUM combinations. */
if ((dev->features & NETIF_F_SG) &&
!(dev->features & NETIF_F_ALL_CSUM)) {
printk(KERN_NOTICE "%s: Dropping NETIF_F_SG since no checksum feature.\n",
dev->name);
dev->features &= ~NETIF_F_SG;
}

is that right?

--
MST