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    SubjectRe: Dropping NETIF_F_SG since no checksum feature.
    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
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