Messages in this thread | | | Date | Wed, 11 Oct 2006 11:05:04 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: Dropping NETIF_F_SG since no checksum feature. |
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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?
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