Hi, On Wed, Oct 11, 2006 at 11:05:04AM +0200, Michael S. Tsirkin wrote: > Quoting r. David Miller <[EMAIL PROTECTED]>: > > Subject: Re: Dropping NETIF_F_SG since no checksum feature. > > > > From: "Michael S. Tsirkin" <[EMAIL PROTECTED]> > > 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. > > >
I would argue that SG does make sense without checksum for protocols that don't need/use a checksum. DECnet for example could do zero-copy without caring about the checksum since it doesn't have one. One of these days I'll get around to writing that bit of code :-) > 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? > I agree with that analysis, Steve. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html