Re: Dropping NETIF_F_SG since no checksum feature.

Michael S. Tsirkin
2006-10-11T09:06:18Z

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 02:13:38 +0200
>
> > Maybe I can patch linux to allow SG without checksum?
> > Dave, maybe you could drop a hint or two on whether this is worthwhile
> > and what are the issues that need addressing to make this work?
> >
> > I imagine it's not just the matter of changing net/core/dev.c :).
>
> You can't, it's a quality of implementation issue. We sendfile()
> pages directly out of the filesystem page cache without any
> blocking of modifications to the page contents, and the only way
> that works is if the card computes the checksum for us.
>
> If we sendfile() a page directly, we must compute a correct checksum
> no matter what the contents. We can't do this on the cpu before the
> data hits the device because another thread of execution can go in and
> modify the page contents which would invalidate the checksum and thus
> invalidating the packet. We cannot allow this.
>
> Blocking modifications is too expensive, so that's not an option
> either.
>

But copying still works fine, does it not?
Dave, could you clarify this please?

ssize_t tcp_sendpage(struct socket *sock, struct page *page, int offset,
size_t size, int flags)
{
ssize_t res;
struct sock *sk = sock->sk;

if (!(sk->sk_route_caps & NETIF_F_SG) ||
!(sk->sk_route_caps & NETIF_F_ALL_CSUM))
return sock_no_sendpage(sock, page, offset, size, flags);

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?

--
MST