[openib-general] Dropping NETIF_F_SG since no checksum feature.

[Michael S. Tsirkin]

Quoting r. David Miller <davem at davemloft.net>:
> Subject: Re: Dropping NETIF_F_SG since no checksum feature.
> 
> From: "Michael S. Tsirkin" <mst at 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